The Raw Materials

What will we cover?
  • What Data is
  • What Variables are
  • Data Types and what to do with them
  • Defining our own data types

Introduction

In any creative activity we need three basic ingredients: tools, materials and techniques. For example when I paint the tools are my brushes, pencils and palettes. The techniques are things like Ďwashesí, wet on wet, blending, spraying etc. Finally the materials are the paints, paper and water. Similarly when I program, my tools are the programming languages, operating systems and hardware. The techniques are the programming constructs that we discussed in the previous section and the material is the data that I manipulate. In this chapter we look at the materials of programming.

This is quite a long section and by its nature you might find it a bit dry, the good news is that you donít need to read it all at once. The chapter starts off by looking at the most basic data types available, then moves on to how we handle collections of items and finally looks at some more advanced material. It should be possible to drop out of the chapter after the collections material, cover a couple of the following chapters and then come back to this one as we start to use the more advanced bits.

Data

Data is one of those terms that everyone uses but few really understand. My dictionary defines it as:

"facts or figures from which conclusions can be inferred; information"

That's not too much help but at least gives a starting point. Letís see if we can clarify things by looking at how data is used in programming terms. Data is the ďstuffĒ, the raw information, that your program manipulates. Without data a program cannot perform any useful function. Programs manipulate data in many ways, often depending on the type of the data. Each data type also has a number of operations - things that you can do to it. For example weíve seen that we can add numbers together. Addition is an operation on the number type of data. Data comes in many types and weíll look at each of the most common types and the operations available for that type:

Variables

Data is stored in the memory of your computer. You can liken this to the big wall full of boxes used in mail rooms to sort the mail. You can put a letter in any box but unless the boxes are labeled with the destination address itís pretty meaningless. Variables are the labels on the boxes in your computer's memory.

Knowing what data looks like is fine so far as it goes but to manipulate it we need to be able to access it and thatís what variables are used for. In programming terms we can create instances of data types and assign them to variables. A variable is a reference to a specific area somewhere in the computers memory. These areas hold the data. In some computer languages a variable must match the type of data that it points to. Any attempt to assign the wrong type of data to such a variable will cause an error. Some programmers prefer this type of system, known as static typing because it can prevent some subtle bugs which are hard to detect.

Variable names follow certain rules dependent on the programming language. Every language has its own rules about which characters are allowed or not allowed. Some languages, including Python and JavaScript, take notice of the case and are therefore called case sensitive languages, others, like VBScript don't care. Case sensitive languages require a little bit more care from the programmer to avoid mistakes, but a consistent approach to naming variables will help a lot. One common style which we will use a lot is to start variable names with a lower case letter and use a capital letter for each first letter of subsequent words in the name, like this:

aVeryLongVariableNameWithCapitalisedStyle

We won't discuss the specific rules about which characters are legal in our languages but if you consistently use a style like that shown you shouldn't have too many problems.

In Python a variable takes the type of the data assigned to it. It will keep that type and you will be warned if you try to mix data in strange ways - like trying to add a string to a number. (Recall the example error message? It was an example of just that kind of error.) We can change the type of data that a variable points to by reassigning the variable.

>>> q = 7         # q is now a number
>>> print q
7
>>> q = "Seven"   # reassign q to a string
>>> print q
Seven

Note that q was set to point to the number 7 initially. It maintained that value until we made it point at the character string "Seven". Thus, Python variables maintain the type of whatever they point to, but we can change what they point to simply by reassigning the variable. At that point the original data is 'lost' and Python will erase it from memory (unless another variable points at it too) this is known as garbage collection.

Garbage collection can be likened to the mail room clerk who comes round once in a while and removes any packets that are in boxes with no labels. If he can't find an owner or address on the packets he throws them in the garbage. Letís take a look at some examples of data types and see how all of this fits together.

VBScript and JavaScript variables

Both JavaScript and VBScript introduce a subtle variation in the way we use variables. Both languages prefer that variables be declared before being used. This is a common feature of compiled languages and of strictly typed languages. There is a big advantage in doing this in that if a spelling error is made when using a variable the translator can detect that an unknown variable has been used and flag an error. The disadvantage is, of course, some extra typing required by the programmer.

VBScript

In VBScript the declaration of a variable is done via the Dim statement, which is short for Dimension. This is a throwback to VBScript's early roots in BASIC and in turn to Assembler languages before that. In those languages you had to tell the assembler how much memory a variable would use - its dimensions. The abbreviation has carried through from there.

A variable declaration in VBScript looks like this:

Dim aVariable

Once declared we can proceed to assign values to it just like we did in Python. We can declare several variables in the one Dim statement by listing them separated by commas:

Dim aVariable, another, aThird

Assignment then looks like this:

aVariable = 42
another = "This is a nice short sentence."
aThird = 3.14159

There is another keyword, Let that you may occasionally see. This is another throwback to BASIC and because it's not really needed you very rarely see it. In case you do, it's used like this:

Let aVariable = 22

I will not be using Let in this tutor.

JavaScript

In JavaScript you can pre-declare variables with the var keyword and, like VBScript, you can list several variables in a single var statement:

var aVariable, another, aThird;

JavaScript also allows you to initialize (or define) the variables as part of the var statement. Like this:

var aVariable = 42;
var another = "A short phrase", aThird = 3.14159;

This saves a little typing but otherwise is no different to VBScript's two step approach to variables. You can also declare and initialise JavaScript vartiables without using var, in the same way that you do in Python:

aVariable = 42;

But JavaScript afficianados consider it good practice to use the var statement, so I will do so in this tutor.

Hopefully this brief look at VBScript and JavaScript variables has demonstrated the difference between declaration and definition of variables. Python variables are declared by defining them.

Primitive Data Types

Primitive data types are so called because they are the most basic types of data we can manipulate. More complex data types are really combinations of the primitive types. These are the building blocks upon which all the other types are built, the very foundation of computing. They include letters, numbers and something called a boolean type.

Character Strings

We've already seen these. They are literally any string or sequence of characters that can be printed on your screen. (In fact there can even be non-printable control characters too).

In Python, strings can be represented in several ways:

With single quotes:

'Here is a string'

With double quotes:

"Here is a very similar string"

With triple double quotes:

""" Here is a very long string that can
    if we wish span several lines and Python will
    preserve the lines as we type them..."""

One special use of the latter form is to build in documentation for Python functions that we create ourselves - we'll see this later.

You can access the individual characters in a string by treating it as an array of characters (see arrays below). There are also usually some operations provided by the programming language to help you manipulate strings - find a sub string, join two strings, copy one to another etc.

It is worth pointing out that some languages have a separate type for characters themselves, that is for a single character. In this case strings are literally just collections of these character values. Python by contrast just uses a string of length 1 to store an individual character, no special syntax is required.

String Operators

There are a number of operations that can be performed on strings. Some of these are built in to Python but many others are provided by modules that you must import (as we did with sys in the Simple Sequences section).

String operators

Operator Description
S1 + S2 Concatenation of S1 and S2
S1 * N N repetitions of S1

We can see these in action in the following examples:

>>> print 'Again and ' + 'again'    # string concatenation
Again and again
>>> print 'Repeat ' * 3             # string repetition
Repeat Repeat Repeat
>>> print 'Again ' + ('and again ' * 3)  # combine '+' and '*'
Again and again and again and again

We can also assign character strings to variables:

>>> s1 = 'Again '
>>> s2 = 'and again '
>>> print s1 + (s2 * 3)
Again and again and again and again

Notice that the last two examples produced the same output.

There are lots of other things we can do with strings but we'll look at those in more detail in a later topic after we've gained a bit more basic knowledge.

VBScript String Variables

In VBScript all variables are called variants, that is they can hold any type of data and VBScript tries to convert it to the appropriate type as needed. Thus you may assign a number to a variable but if you use it as a string VBScript will try to convert it for you. In practice this is similar to what Python's print command does but extended to any VBScript command. You can give VBScript a hint that you want a numeric value treated as a string by enclosing it in double quotes:

<script language="VBScript">
MyString = "42"
MsgBox MyString
</script>

We can join VBScript strings together, a process known as concatenation, using the & operator:

<script language="VBScript">
MyString = "Hello" & "World"
MsgBox MyString
</script>

JavaScript Strings

JavaScript strings are enclosed in either single or double quotes. In JavaScript you must declare variables before we use them. This is easily done using the var keyword. Thus to declare and define two string variables in JavaScript we do this:

<script type="text/javascript">
var aString, another;
aString = "Hello ";
another = "World";
document.write(aString + another)
</script>

Finally JavaScript also allows us to create String objects. We will discuss objects a little later in this topic but for now just think of String objects as being strings with some extra features. The main difference is that we create them slightly differently:

<script type="text/javascript">
var aStringObj, anotherObj;
aStringObj = String("Hello ");
anotherObj = String("World");
document.write(aStringObj + anotherObj);
</script>

Integers

Integers are whole numbers from a large negative value through to a large positive value. Thatís an important point to remember. Normally we donít think of numbers being restricted in size but on a computer there are upper and lower limits. The size of this upper limit is known as MAXINT and depends on the number of bits used on your computer to represent a number. On most current computers and programming languages it's 32 bits so MAXINT is around 2 billion (however VBScript is limited to about +/-32000).

Numbers with positive and negative values are known as signed integers. You can also get unsigned integers which are restricted to positive numbers, including zero. This means there is a bigger maximum number available of around 2 * MAXINT or 4 billion on a 32 bit computer since we can use the space previously used for representing negative numbers to represent more positive numbers.

Because integers are restricted in size to MAXINT adding two integers together where the total is greater than MAXINT causes the total to be wrong. On some systems/languages the wrong value is just returned as is (usually with some kind of secret flag raised that you can test if you think it might have been set). Normally an error condition is raised and either your program can handle the error or the program will exit. VBScript and JavaScript both adopt this latter approach. Recent versions of Python are a little different in that from version 2.3 onwards Python will automatically convert an integer into something called a Long Integer, which is a Python specific feature allowing virtually unlimited size integers. We don't get these for free of course, they come at the cost of much slower processing speed - but at least you know your calculations will complete, eventually. And of course speed in computer terms is relative, unless you are doing a lot of processing of these long integers you probably won't notice the difference! You can tell a long integer because Python prints it with a training 'L', like this:

>>> 1234567 * 3456789
>>> 4267637625363L

Note that we didn't use the print statement here, if we had the 'L' would be hidden. Python has two ways of displaying results, the printed version is usually prettier, i.e. easier to read, but the plain value as used here sometimes has more detail. Try typing in the examples in the previous topic without the print statements and see how many subtle differences in presentation you can spot. In general I will use the print statement, partly because most languages insist on it and I'm trying to get you used to good general practice not just Python's cozy way of doing things.

Arithmetic Operators

We've already seen most of the arithmetic operators that you need in the 'Simple Sequences' section, however to recap:

Python Arithmetic Operators

Operator Example Description
M + N Addition of M and N
M - N Subtraction of N from M
M * N Multiplication of M and N
M / N Division, either integer or floating point result depending on the types of M and N. If either M or N are real numbers(see below) the result will be real.
M % N Modulo: find the remainder of M divided by N
M**N Exponentiation: M to the power N

We havenít seen the last one before so letís look at an example of creating some integer variables and using the exponentiation operator:

>>> i1 = 2     # create an integer and assign it to i1
>>> i2 = 4
>>> i3 = i1**i2  # assign the result of 2 to the power 4 to i3
>>> print i3
16
>>> print 2**4  # confirm the result
16

Shortcut operators

One very common operation that is carried out while programming is incrementing a variable's value. Thus if we have a variable called x with avalue of 42 and we want to increase its value to 43 we can do it like this:

>>> x = 42
>>> print x
>>> x = x + 1
>>> print x

Notice the line

x = x + 1

This is not sensible in mathematics but in programming it is. What it means is that x takes on the previous value of x plus 1. If you have done a lot of math this might take a bit of getting used to, but basically the equal sign in this case could be read as becomes. So that it reads: x becomes x + 1.

Now it turns out that this type of operation is so common in practice that Python (and JavaScript) provides a shortcut operator to save some typing:

>>> x += 1
>>> print x

This means exactly the same as the previous assignment statement but is shorter. And for consistency similar shortcuts exist for the other arithmetic operators:

Shortcut Operators

Operator Example Description
M += N M = M + N
M -= N M = M - N
M *= N M = M * N
M /= N M = M / N
M %= N M = M % N

VBScript Integers

As I said earlier VBScript integers are limited to a lower value of MAXINT corresponding to a 16 bit value, namely about +/- 32000. If you need an integer bigger than that you can use a long integer which is the same size as a standard Python integer. There is also a byte type which is an 8 bit number with a maximum size of 255. In practice you will usually find the standard integer type sufficient.

All the usual arithmetic operators are supported.

JavaScript Numbers

It will be no surprise to discover that JavaScript too has a numeric type. It too is an object as we'll describe later and its called a Number, original eh? :-)

A JavaScript number can also be Not a Number or NaN. This is a special version of the Number object which represents invalid numbers, usually the result of some operation which is mathematically impossible. The point of NaN is that it allows us to check for certain kinds of error without actually breaking the program. JavaScript also has special number versions to represent positive and negative infinity, a rare feature in a programming language. JavaScript number objects can be either integers or real numbers, which we look at next.

Real Numbers

These include fractions. (I'm using the OED definition of fraction here. Some US correspondents tell me the US term fraction means something more specific. I simply mean any number that is not a whole number). They can represent very large numbers, much bigger than MAXINT, but with less precision. That is to say that 2 real numbers which should be identical may not seem to be when compared by the computer. This is because the computer only approximates some of the lowest details. Thus 5.0 could be represented by the computer as 4.9999999.... or 5.000000....01. These approximations are close enough for most purposes but occasionally they become important! If you get a funny result when using real numbers, bear this in mind.

Real numbers, also known as Floating Point numbers have the same operations as integers with the addition of the capability to truncate the number to an integer value.

Python, VBScript and JavaScript all support real numbers. In Python we create them by simply specifying a number with a decimal point in it, as we saw in the simple sequences topic. In VBScript and JavaScript there is no clear distinction between integers and real numbers, just use them and mostly the language will pretty much sort itself out OK.

Complex or Imaginary Numbers

If you have a scientific or mathematical background you may be wondering about complex numbers? If you haven't you may not even have heard of complex numbers, in which case you can safely jump to the next heading because you don't need them! Anyhow some programming languages, including Python, provide built in support for the complex type while others provide a library of functions which can operate on complex numbers. And before you ask, the same applies to matrices too.

In Python a complex number is represented as:

(real+imaginaryj)

Thus a simple complex number addition looks like:

>>> M = (2+4j)
>>> N = (7+6j)
>>> print M + N
(9+10j)

All of the integer operations also apply to complex numbers.

Neither VBScript nor JavaScript offer support for complex numbers.

Boolean Values - True and False

This strange sounding type is named after a 19th century mathematician, George Boole who studied logic. Like the heading says, this type has only 2 values - either true or false. Some languages support Boolean values directly, others use a convention whereby some numeric value (often 0) represents false and another (often 1 or -1) represents true. Up until version 2.2 Python did this, however since version 2.3 Python supports Boolean values directly, using the values True and False.

Boolean values are sometimes known as "truth values" because they are used to test whether something is true or not. For example if you write a program to backup all the files in a directory you might backup each file then ask the operating system for the name of the next file. If there are no more files to save it will return an empty string. You can then test to see if the name is an empty string and store the result as a boolean value (True if it is empty, False if it isn't). You'll see how we would use that result later on in the course.

Boolean (or Logical) Operators

Operator Example Description Effect
A and B AND True if A,B are both True, False otherwise.
A or B OR True if either or both of A,B are true. False if both A and B are false
A == B Equality True if A is equal to B
A != B
or
A <> B
Inequality True if A is NOT equal to B.
not B Negation True if B is not True

Note: the last one operates on a single value, the others all compare two values.

VBScript, like Python has a Boolean type with the values True and False.

JavaScript also supports a Boolean type but this time the values are true and false (note, with a lowercase first letter).

Finally the different languages have slightly different names for the Boolean type internally, in Python it is bool, in VBScript and JavaScript it is Boolean. Most of the time you won't need to worry about that because we tend not to create variables of Boolean types but simply use the results in tests.

Collections

Computer science has built a whole discipline around studying collections and their various behaviors. Sometimes collections are called containers. In this section we will look first of all at the collections supported in Python, VBScript and JavaScript, then weíll conclude with a brief summary of some other collection types you might come across in other languages.

List

We are all familiar with lists in everyday life. A list is just a sequence of items. We can add items to a list or remove items from the list. Usually, where the list is written paper we can't insert items in the middle of a list only at the end. However if the list is in electronic format - in a word processor say - then we can insert items anywhere in the list.

We can also search a list to check whether something is already in the list or not. But you have to find the item you need by stepping through the list from front to back checking each item to see if it's the item you want. Lists are a fundamental collection type found in many modern programming languages.

Python lists are built into the language. They can do all the basic list operations we discussed above and in addition have the ability to index the elements inside the list. By indexing I mean that we can refer to a list element by its sequence number (assuming the first element starts at zero!).

In VBScript there are no lists as such but other collection types which we discuss later can simulate their features.

In JavaScript there are no lists as such but almost everything you need to do with a list can be done using a JavaScript array which is another collection type that we discuss a little later.

List operations

Python provides many operations on collections. Nearly all of them apply to Lists and a subset apply to other collection types, including strings which are just a special type of list - a list of characters. To create and access a list in Python we use square brackets. You can create an empty list by using a pair of square brackets with nothing inside, or create a list with contents by separating the values with commas inside the brackets:

>>> aList = []
>>> another = [1,2,3]
>>> print another
[1, 2, 3]

We can access the individual elements using an index number, where the first element is 0, inside square brackets. For example to access the third element, which will be index number 2 since we start from zero, we do this:

>>> print another[2]
3

We can also change the values of the elements of a list in a similar fashion:

>>> another[2] = 7
>>> print another
[1, 2, 7]

Notice that the third element (index 2) changed from 3 to 7.

You can use negative index numbers to access members from the end of the list. This is most commonly done using -1 to get the last item:

>>> print another[-1]
7

We can add new elements to the end of a list using the append() operator:

>>> aList.append(42)
>>> print aList
[42]

We can even hold one list inside another, thus if we append our second list to the first:

>>> aList.append(another)
>>> print aList
[42, [1, 2, 7]]

Notice how the result is a list of two elements but the second element is itself a list (as shown by the []ís around it). We can now access the element 7 by using a double index:

>>> print aList[1][2]
7

The first index, 1, extracts the second element which is in turn a list. The second index, 2, extracts the third element of the sublist.

This nesting of lists one inside the other is extremely useful since it effectively allows us to build tables of data, like this:

>>> row1 = [1,2,3]
>>> row2 = ['a','b','c']
>>> table = [row1, row2]
>>> print table
[ [1,2,3], ['a','b','c'] ]
>>> element2 = table[0][1]

We could use this to create an address book where each entry was a list of name and address details. For example, here is such an address book with two entries:

>>> addressBook = [
... ['Fred', '9 Some St',' Anytown', '0123456789'],
... ['Rose', '11 Nother St', 'SomePlace', '0987654321']
... ]
>>>

Notice that we constructed the nested list all on one line. That is because Python sees that the number of opening and closing brackets don't match and keeps on reading input until they do. This can be a very effective way of quickly constructing complex data structures while making the overall structure - a list of lists in this case - clear to the reader.

As an exercise try extracting Fred's telephone number - element 3, from the first row - remembering that the indexes start at zero. Also try adding a few new entries of your own using the append() operation described above.

Note that when you exit Python your data will be lost, however you will find out how to preserve it once we reach the topic on files.

The opposite of adding elements is, of course, removing them and to do that we use the del command:

>>> del aList[1]
>>> print aList
[42]

If we want to join two lists together to make one we can use the same concatenation operator Ď+í that we saw for strings:

>>> newList = aList + another
>>> print newList
[42, 1, 2, 7]

Notice that this is slightly different to when we appended the two lists earlier, then there were 2 elements, the second being a list, this time there are 4 elements because the elements of the second list have each, individually been added to newList. This time if we access element 1, instead of getting a sublist, as we did previously, we will only get 1 returned:

>>> print newList[1]
1

We can also apply the multiplication sign as a repetition operator to populate a list with multiples of the same value:

>>> zeroList = [0] * 5
>>> print zeroList
[0, 0, 0, 0, 0]

We can find the index of a particular element in a list using the index() operation, like this:

>>> print [1,3,5,7].index(5)
2
>>> print [1,3,5,7].index(9)
Traceback (most recent call last):
  File "", line 1, in ?
ValueError: list.index(x): x not in list

Notice that trying to find the index of something that's not in the list results in an error. We will look at ways to test whether something is in a list or not in a later topic.

Finally, we can determine the length of a list using the built-in len() function:

>>> print len(aList)
1
>>> print len(newList)
4
>>> print len(zeroList)
5

Neither JavaScript nor VBScript directly support a list type although as we will see later they do have an Array type that can do many of the things that Python's lists can do.

Tuple

Not every language provides a tuple construct but in those that do itís extremely useful. A tuple is really just an arbitrary collection of values which can be treated as a unit. In many ways a tuple is like a list, but with the significant difference that tuples are immutable which is to say that you canít change them nor append to them once created. In Python, tuples are simply represented by parentheses containing a comma separated list of values, like so:

>>> aTuple = (1,3,5)
>>> print aTuple[1]    # use indexing like a list
3
>> aTuple[2] = 7       # error, canít change a tupleís elements
Traceback (innermost last):
  File "", line 1, in ?
        aTuple[2] = 7
TypeError: object doesn't support item assignment

The main things to remember are that while parentheses are used to define the tuple, square brackets are used to index it and you canít change a tuple once its created. Otherwise most of the list operations also apply to tuples.

Finally, although you cannot change a tuple you can effectively add members using the addition operator because this actually creates a new tuple. Like this:

>>> tup1 = (1,2,3)
>>> tup2 = tup1 + (4,) # comma to make it a tuple rather than integer
>>> print tup2
(1,2,3,4)

If we didn't use the trailing comma after the 4 then Python would have interpreted it as the integer 4 inside parentheses, not as a true tuple. But since you can't add integers to tuples it results in an error, so we add the comma to tell Python to treat the parentheses as a tuple. Any time you need to persuade Python that a single entry tuple really is a tuple add a trailing comma as we did here.

Neither VBScript nor JavaScript have any concept of tuples.

Dictionary or Hash

In the same way that a literal dictionary associates a meaning with a word a dictionary type contains a value associated with a key, which may or may not be a string. The value can be retrieved by Ďindexingí the dictionary with the key. Unlike a literal dictionary, the key doesnít need to be a character string (although it often is) but can be any immutable type including numbers and tuples. Similarly the values associated with the keys can have any kind of data type. Dictionaries are usually implemented internally using an advanced programming technique known as a hash table. For that reason a dictionary may sometimes be referred to as a hash. This has nothing to do with drugs! :-)

Because access to the dictionary values is via the key, you can only put in elements with unique keys. Dictionaries are immensely useful structures and are provided as a built-in type in Python although in many other languages you need to use a module or even build your own. We can use dictionaries in lots of ways and we'll see plenty examples later, but for now, here's how to create a dictionary in Python, fill it with some entries and read them back:

>>> dct = {}
>>> dct['boolean'] = "A value which is either true or false"
>>> dct['integer'] = "A whole number"
>>> print dct['boolean']
A value which is either true or false

Notice that we initialize the dictionary with braces, then use square brackets to assign and read the values.

Just as we did with lists we can initialize a dictionary as we create it using the following format:

>>> addressBook = {
... 'Fred' : ['Fred', '9 Some St',' Anytown', '0123456789'],
... 'Rose' : ['Rose', '11 Nother St', 'SomePlace', '0987654321']
... }
>>>

The key and value are separated by a colon and the pairs are separated by commas. This time we have made our address book out of a dictionary which is keyed by name and stores our lists as the values. Rather than work out the numerical index of the entry we want we can just use the name to retrieve all the information, like this:

>>> print addressBook['Rose']
['Rose', '11 Nother St', 'SomePlace', '0987654321']
>>> print addressBook['Fred'][3]
0123456789

In the second case we indexed the returned list to get only the telephone number. By creating some variables and assigning the appropriate index values we can make this much easier to use:

>>> name = 0
>>> street = 1
>>> town = 2
>>> tel = 3

And now we can use those variables to find out Rose's town:

>>> print addressBook['Rose'][town]
SomePlace

Notice that whereas 'Rose' was in quotes because the key is a string, the town is not because it is a variable name and Python will convert it to the index value we assigned, namely 2. At this point our Address Book is beginning to resemble a usable database application, thanks largely to the power of dictionaries. It won't take a lot of extra work to save and restore the data and add a query prompt to allow us to specify the data we want. We will do that as we progress through the other tutorial topics.

Due to their internal structure dictionaries do not support very many of the collection operators that weíve seen so far. None of the concatenation, repetition or appending operations work. To assist us in accessing the dictionary keys there is an operation that we can use, keys(), which returns a list of all the keys in a dictionary. For example to get a list of all the names in our address book we could do:

>>> print addressBook.keys()
['Fred','Rose']

Note however that dictionaries do not store their keys in the order in which they are inserted so you may find the keys appear in a strange order, indeed the order may even change over time. Don't worry about that, you can still use the keys to access your data and the right value will still come out OK.

VBScript Dictionaries

VBScript provides a dictionary object which offers similar facilities to the Python dictionary but the usage is slightly different. To create a VBScript dictionary we have to declare a variable to hold the object, then create the object, finally we can add entries to the new dictionary, like this:

Dim dict     ' Create a variable.
Set dict = CreateObject("Scripting.Dictionary")
dict.Add "a", "Athens" ' Add some keys and items.
dict.Add "b", "Belgrade"
dict.Add "c", "Cairo"

Notice that the CreateObject function specifies that we are creating a "Scripting.Dictionary" object, that is a Dictionary object from the VBScript's Scripting module. Don't worry too much about that for now, we'll discuss it in more depth when we look at objects later in the tutor. Hopefully you can at least recognize and recall the concept of using an object from a module from the simple sequences topic earlier. The other point to notice is that we must use the keyword Set when assigning an object to a variable in VBScript.

Now we access the data like so:

item = dict.Item("c") ' Get the item.
dict.Item("c") = "Casablanca" ' Change the item

There are also operations to remove an item, get a list of all the keys, check that a key exists etc.

Here is complete but simplified version of our address book example in VBScript:

<script type="text/VBScript">
Dim addressBook
Set addressBook = CreateObject("Scripting.Dictionary")
addressBook.Add "Fred", "Fred, 9 Some St, Anytown, 0123456789"
addressBook.Add "Rose", "Rose, 11 Nother St, SomePlace, 0987654321"

MsgBox addressBook.Item("Rose")
</script>

This time, instead of using a list, we have stored all the data as a single string. We then access and print Rose's details in a message box.

JavaScript Dictionaries

JavaScript doesn't really have a dictionary object of its own, although if you are using Internet Explorer you can get access to the VBScript Scripting.Dictionary object discussed above, with all of the same facilities. But since it's really the same object I won't cover it further here. Finally JavaScript arrays can be used very much like dictionaries but we'll discuss that in the array section below.

If you're getting a bit fed up, you can jump to the next chapter at this point. Remember to come back and finish this one when you start to come across types of data we haven't mentioned so far.

Other Collection Types

Array or Vector

The array is one of the earlier collection types in computing history. It is basically a list of items which are indexed for easy and fast retrieval. Usually you have to say up front how many items you want to store. It is this fixed size feature which distinguishes it from the list data type discussed above. Python supports arrays through a module but it is rarely needed because the built in list type can usually be used instead. VBScript and JavaScript both have arrays as a data type, so let's briefly look at how they are used:

VBScript Arrays

In VBScript array is a fixed length collection of data accessed by a numerical index. It is declared and accessed like this:

Dim AnArray(42)    ' A 43! element array
AnArray(0) = 27    ' index starts at 0
AnArray(1) = 49
myVariable = AnArray(1) ' read the value

Note the use of the Dim keyword. This dimensions the variable. This is a way of telling VBScript about the variable, if you start your script with OPTION EXPLICIT VBScript will expect you to Dim any variables you use, which many programming experts believe is good practice and leads to more reliable programs. Also notice that we specify the last valid index, 42 in our example, which means the array actually has 43 elements because it starts at 0.

Notice also that in VBScript we use parentheses to dimension and index the array, not the square brackets used in Python and, as we'll soon see, JavaScript.

As with Python lists we can declare multiple dimensional arrays to model tables of data, for our address book example:

Dim MyTable(2,3)  ' 3 rows, 4 columns
MyTable(0,0) = "Fred"  ' Populate Fred's entry
MyTable(0,1) = "9 Some Street"
MyTable(0,2) = "Anytown"
MyTable(0,3) = "0123456789"
MyTable(1,0) = "Rose"  ' And now Rose...
...and so on...

Unfortunately there is no way to populate the data all in one go as we did with Python's lists, we have to populate each field one by one. If we combine VBScripts dictionary and array capability we get almost the same usability as we did with Python. It looks like this:

<script type="text/VBScript">
Dim addressBook
Set addressBook = CreateObject("Scripting.Dictionary")
Dim Fred(3)
Fred(0) = "Fred"
Fred(1) = "9 Some St"
Fred(2) = "Anytown"
Fred(3) = "0123456789"
addressBook.Add "Fred", Fred

MsgBox addressBook.Item("Fred")(3) ' Print the Phone Number
</script>

The final aspect of VBScript arrays that I want to consider is the fact that they don't need to be fixed in size at all! However this does not mean we can just arbitrarily keep adding elements as we did with our lists, rather we can explicitly resize an array. For this to happen we need to declare a Dynamic array which we do, quite simply by omitting the size, like this:

Dim DynArray()  ' no size specified

To resize it we use the ReDim command, like so:

<script type="text/vbscript">
Dim DynArray()
ReDim DynArray(5)  ' Initial size = 5
DynArray(0) = 42
DynArray(4) = 26
MsgBox "Before: " & DynArray(4)  ' prove that it worked
' Resize to 21 elements keeping the data we already stored
ReDim Preserve DynArray(20)
DynArray(15) = 73
MsgBox "After Preserve: " & DynArray(4) & " " & DynArray(15)' Old and new still there
' Resize to 51 items but lose all data
Redim DynArray(50)
MsgBox "After: " & DynArray(4) & " Oops, Where did it go?"
</script>

As you can see this is not so convenient as a list which adjusts its length automatically, but it does give the programmer more control over how the program behaves. This level of control can, amongst other things improve security since some viruses can exploit dynamically re-sizable data stores.

JavaScript Arrays

Arrays in JavaScript are in many ways a misnomer. They are called arrays but are actually a curious mix of the features of lists, dictionaries and traditional arrays. At the simplest level we can declare a new Array of 10 items of some type, like so:

var items = new Array(10);

We can now populate and access the elements of the array like this:

items[4] = 42;
items[7] = 21;
var aValue = items[4];

However JavaScript arrays are not limited to storing a single type of value, we can assign anything to an array element:

items[9] = "A short string";
var msg = items[9];

Also we can create arrays by providing a list of items, like so:

var moreItems = new Array("one","two","three",4,5,6);
aValue = moreItems[3];
msg = moreItems[0];

Another feature of JavaScript arrays is that we can determine the length through a hidden property called length. We access the length like this:

var size = items.length;

Notice that once again the syntax for this uses an name.property format and is very like calling a function in a Python module but without the parentheses.

As usual, JavaScript arrays start indexing at zero. However JavaScript array indexes are not limited to numbers, we can use strings too, and in this case they become almost identical to dictionaries! We can also extend an array by simply assigning a value to an index beyond the current maximum, we can see these features in use in the following code segment:

items[42] = 7;
moreItems["foo"] = 42;
msg = moreItems["foo"];

Finally, let's look at our address book example again using JavaScript arrays:

<script type="text/javascript">
var addressBook = new Array();
addressBook["Fred"] = "Fred, 9 Some St, Anytown, 0123456789";
addressBook["Rose"] = "Rose, 11 Nother St, SomePlace, 0987654321";

document.write(addressBook.Rose);
</script>

Notice that we access the key as if it were a property like length.

Stack

Think of a stack of trays in a restaurant. A member of staff puts a pile of clean trays on top and these are removed one by one by customers. The trays at the bottom of the stack get used last (and least!). Data stacks work the same way: you push an item onto the stack or pop one off. The item popped is always the last one pushed. This property of stacks is sometimes called Last In First Out or LIFO. One useful property of stacks is that you can reverse a list of items by pushing the list onto the stack then popping it off again. The result will be the reverse of the starting list. Stacks are not built in to Python, VBScript or JavaScript. You have to write some program code to implement the behavior. Lists are usually the best starting point since like stacks they can grow as needed.

Bag

A bag is a collection of items with no specified order and it can contain duplicates. Bags usually have operators to enable you to add, find and remove items. In our languages bags are just lists.

Set

A set has the property of only storing one of each item. You can usually test to see if an item is in a set (membership). Add, remove and retrieve items and join two sets together in various ways corresponding to set theory in math (eg union, intersect etc). VBScript and JavaScript do not implement sets directly but you can approximate the behavior fairly easily using dictionaries.

Since Python version 2.3 sets are supported via the sets module, although this functionality is considered experimental and from version 2.4 will be built in to the Python core language.

The basic usage until then is like this:

>>> import sets
>>> A = sets.Set()  # create an empty set
>>> B = sets.Set([1,2,3]) # a 3 element set
>>> C = sets.Set([3,4,5])
>>> D = sets.Set([6,7,8])
>>> # Now try out some set operations
>>> B.union(C)
Set([1,2,3,4,5])
>>> B.intersection(C)
Set([3])
>>> B.issuperset(sets.Set([2]))
True
>>> sets.Set([3]).issubset(C)
True
>>> C.intersection(D) == A
True

There are quite a number of other set operations but these should be enough for now.

Queue

A queue is rather like a stack except that the first item into a queue is also the first item out. This is known as First In First Out or FIFO behavior. This is usually implemented using a list or array.

There's a whole bunch of other collection types but the ones we have covered are the main ones that you are likely to come across. (And in fact we'll only be using a few of the ones we've discussed in this tutor, but you will see the others mentioned in articles and in programming discussion groups!)

Files

As a computer user you should be very familiar with files - they form very basis of nearly everything we do with computers. It should be no surprise then, to discover that most programming languages provide a special file type of data. However files and the processing of them are so important that I will put off discussing them till later when they get a whole topic to themselves.

Dates and Times

Dates and times are often given dedicated types in programming. At other times they are simply represented as a large number (typically the number of seconds from some arbitrary date/time!). In other cases the data type is what is known as a complex type as described in the next section. This usually makes it easier to extract the month, day, hour etc. We will take a brief look at using the Python time module in a later topic. Both VBScript and JavaScript have their own mechanisms for handling time but I won't be discussing them further.

Complex/User Defined

Sometimes the basic types described above are inadequate even when combined in collections. Sometimes, what we want to do is group several bits of data together then treat it as a single item. An example might be the description of an address:
a house number, a street and a town. Finally there's the post code or zip code.

Most languages allow us to group such information together in a record or structure or with the more modern, object oriented version, a class.

VBScript

In VBScript such a record definition looks like:

Class Address
     Public HsNumber
     Public Street
     Public Town
     Public ZipCode
End Class

The Public keyword simply means that the data is accessible to the rest of the program, it's possible to have Private data too, but we'll discuss that later in the course.

Python

In Python it's only a little different:

>>>class Address:
...   def __init__(self, Hs, St, Town, Zip):
...     self.HsNumber = Hs
...     self.Street = St
...     self.Town = Town
...     self.ZipCode = Zip
...

That may look a little arcane but don't worry Iíll explain what the def __init__(...) and self bits mean in the section on object orientation. One thing to note is that there are two underscores at each end on __init__. This is a Python convention that we will discuss later. Also you need to use the spacing shown above, as we'll explain later Python is a bit picky about spacing. For now just make sure you copy the layout above.

Some people have had problems trying to type this example at the Python prompt. At the end of this chapter you will find a box with more explanation, but you can just wait till we get the full story later in the course if you prefer. If you do try typing it into Python then please make sure you copy the indentation shown. As you'll see later Python is very particular about indentation levels.

The main thing I want you to recognize in all of this is that we have gathered several pieces of data into a single structure.

JavaScript

JavaScript provides a slightly strange name for its structure format, namely function! Now functions are normally associated with operations not collections of data however in JavaScript's case it can cover either. To create our address object in JavaScript we do this:

function Address(Hs,St,Town,Zip)
{
   this.HsNum = Hs;
   this.Street = St;
   this.Town = Town;
   this.ZipCode = Zip;
}

Once again the end result is a group of data items that we can treat as a single unit.

Accessing Complex Types

We can assign a complex data type to a variable too, but to access the individual fields of the type we must use some special access mechanism (which will be defined by the language). Usually this is a dot.

Using VBScript

To consider the case of the address class we defined above we would do this in VBScript:

Dim Addr
Set Addr = New Address

Addr.HsNumber = 7
Addr.Street = "High St"
Addr.Town = "Anytown"
Addr.ZipCode = "123 456"

MsgBox Addr.HsNumber & " " & Addr.Street & " " & Addr.Town

Here we first of all Dimension a new variable, Addr, using Dim then we use the Set keyword to create a new instance of the Address class. Next we assign values to the fields of the new address instance and finally we print out the address in a Message Box.

And in Python

And in Python, assuming you have already typed in the class definition above:

Addr = Address(7,"High St","Anytown","123 456")
print Addr.HsNumber, Addr.Street, Addr.Town

Which creates an instance of our Address type and assigns it to the variable addr. In Python we can pass the field values to the new object when we create it. We then print out the HsNumber and Street fields of the newly created instance using the dot operator. You could, of course, create several new Address instances each with their own individual values of house number, street etc. Why not experiment with this yourself? Can you think of how this could be used in our address book example from earlier in the topic?

JavaScript too

The JavaScript mechanism is very similar to the others but has a couple of twists, as we'll see in a moment. However the basic mechanism is straightforward and the one I recommend you use:

var addr = new Address(7, "High St", "Anytown", "123 456");
document.write(addr.HsNum + " " + addr.Street + " " + addr.Town);

One final mechanism that we can use in JavaScript is to treat the object like a dictionary and use the field name as a key:

document.write( addr['HsNum'] + " " + addr['Street'] + " " +  addr['Town']);

I can't really think of any good reason to use this form other than if you were to be given the field name as a string, perhaps after reading a file or input from the user of your program (we'll see how to do that later too).

User Defined Operators

User defined types can, in some languages, have operations defined too. This is the basis of what is known as object oriented programming. We dedicate a whole section to this topic later but essentially an object is a collection of data elements and the operations associated with that data, wrapped up as a single unit. Python uses objects extensively in its standard library of modules and also allows us as programmers to create our own object types.

Object operations are accessed in the same way as data members of a user defined type, via the dot operator, but otherwise look like functions. These special functions are called methods. We have already seen this with the append() operation of a list. Recall that to use it we must tag the function call onto the variable name:

>>> listObject = []    # an empty list
>>> listObject.append(42) # a method call of the list object
>>> print listObject
[42]

When an object type, known as a class, is provided in a Python module we must import the module (as we did with sys earlier), then prefix the object type with the module name when creating an instance that we can store in a variable (while still using the parentheses, of course). We can then use the variable without using the module name.

We will illustrate this by considering a fictitious module meat which provides a Spam class. We import the module, create an instance of Spam, assigning it the name mySpam and then use mySpam to access its operations and data like so:

>>> import meat
>>> mySpam = meat.Spam()  # create an instance, use module name
>>> mySpam.slice()        # use a Spam operation
>>> print mySpam.ingredients  # access Spam data
{"Pork":"40%", "Ham":"45%", "Fat":"15%"}

In the first line we import the (non-existent!) module meat into the program. In the second line we use the meat module to create an instance of the Spam class - by calling it as if it were a function! In the third line we access one of the Spam class's operations, slice(), treating the object (mySpam) as if it were a module and the operation were in the module. Finally we access some data from within the mySpam object using the same module like syntax.

Other than the need to create an instance, thereís no real difference between using objects provided within modules and functions found within modules. Think of the object name simply as a label which keeps related functions and variables grouped together.

Another way to look at it is that objects represent real world things, to which we as programmers can do things. That view is where the original idea of objects in programs came from: writing computer simulations of real world situations.

Both VBScript and JavaScript work with objects and in fact that's exactly what we have been using in each of the Address examples above. We have defined a class and then created an instance which we assigned to a variable so that we could access the instance's properties. Go back and review the previous sections in terms of what we've just said about classes and objects. Think about how classes provide a mechanism for creating new types of data in our programs by binding together the data and operations of the new type.

Python Specific Operators

In this tutor my primary objective is to teach you to program and although I use Python in the tutor there is no reason why, having read this, you couldnít go out and read about another language and use that instead. Indeed thatís exactly what I expect you to do since no single programming language, even Python, can do everything. However because of that objective I do not teach all of the features of Python but focus on those which can generally be found in other languages too. As a result there are several Python specific features which, while they are quite powerful, I donít describe at all, and that includes special operators. Most programming languages have operations which they support and other languages do not. It is often these 'unique' operators that bring new programming languages into being, and certainly are important factors in determining how popular the language becomes.

For example Python supports such relatively uncommon operations as list slicing ( spam[X:Y] ) for extracting a section (or slice) out from the middle of a list(or string, or tuple) and tuple assignment ( X, Y = 12, 34 ) which allows us to assign multiple variable values at one time.

It also has the facility to perform an operation on every member of a collection using its map() function which we describe in the Functional Programming topic. There are many more, itís often said that "Python comes with the batteries included". For details of how most of these Python specific operations work youíll need to consult the Python documentation.

Finally, itís worth pointing out that although I say they are Python specific, that is not to say that they canít be found in any other languages but rather that they will not all be found in every language. The operators that we cover in the main text are generally available in some form in virtually all modern programming languages.

That concludes our look at the raw materials of programming, letís move onto the more exciting topic of technique and see how we can put these materials to work.

More information on the Address example

Although, as I said earlier, the details of this example are explained later, some readers have found difficulty getting the Python example to work. This note gives a line by line explanation of the Python code. The complete code for the example looks like this:


>>> class Address:
...   def __init__(self, Hs, St, Town, Zip):
...     self.HsNumber = Hs
...     self.Street = St
...     self.Town = Town
...     self.Zip_Code = Zip
...
>>> Addr = Address(7,"High St","Anytown","123 456")
>>> print Addr.HsNumber, Addr.Street

Here is the explanation:

>>> class Address:

The class statement tells Python that we are about to define a new type called, in this case, Address. The colon indicates that any indented lines following will be part of the class definition. The definition will end at the next unindented line. If you are using IDLE you should find that the editor has indented the next line for you, if working at a command line Python prompt in an MS DOS window then you will need to manually indent the lines as shown. Python doesn't care how much you indent by, just so long as it is consistent.

...   def __init__(self, Hs, St, Town, Zip):

The first item within our class is what is known as a method definition. One very important detail is that the name has a double underscore at each end, this is a Python convention for names that it treats as having special significance. This particular method is called __init__ and is a special operation, performed by Python, when we create an instance of our new class, we'll see that shortly. The colon, as before, simply tells Python that the next set of indented lines will be the actual definition of the method.

...     self.HsNumber = Hs

This line plus the next three, all assign values to the internal fields of our object. They are indented from the def statement to tell Python that they constitute the actual definition of the __init__ operation.The blank line tells the Python interpreter that the class definition is finished so that we get the >>> prompt back.

>>> Addr = Address(7,"High St","Anytown","123 456")

This creates a new instance of our Address type and Python uses the __init__ operation defined above to assign the values we provide to the internal fields. The instance is assigned to the Addr variable just like an instance of any other data type would be.

>>> print Addr.HsNumber, Addr.Street

Now we print out the values of two of the internal fields using the dot operator to access them.

As I said we cover all of this in more detail later in the tutorial. The key point to take away is that Python allows us to create our own data types and use them pretty much like the built in ones.


Points to remember
  • Variables refer to data and may need to be declared before being defined.
  • Data comes in many types and the operations you can successfully perform will depend on the type of data you are using.
  • Simple data types include character strings, numbers, Boolean or 'truth' values.
  • Complex data types include collections, files, dates and user defined data types.
  • There are many operators in every programming language and part of learning a new language is becoming familiar with both its data types and the operators available for those types.
  • The same operator (e.g. addition) may be available for different types, but the results may not be identical, or even apparently related!

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