Variables, Constants and Data Structures

The purpose of any IT system is to process data, whatever form that data takes (numbers, text, images, sounds, etc.). One of the most fundamental features of any programming language, therefore, is the ability to store the data in a structured format.

Variables

The most simple form of storage is called a variable. It's an area of memory that stores one item of data, such as a number or a character. They have two purposes - the programmer is able to choose the names of the variables, making programming easier, and also, you can write programs or functions that will work with any values. If you're familiar with spreadsheets already, you can think of variables as being like the cells, which you can then use in formulae regardless of the values they contain. All procedural programming languages, such as C, BASIC and Pascal, have variables, although they may support different types and let you manipulate them in different ways.

Some languages are strongly typed (see below), whereas others aren't typed at all. Some require that you declare a variable before you use it, and others let you go straight in and define a variable's value without declaring it first.

Declaring a variable gives the variable a name, and, in most programming languages, gives it a type - in effect it creates the container that stores your value. See the Type section for examples of declarations.  Note that some languages, such as Python, don't require you to declare variables - the type is set when they are defined.

When you define a variable, you are simply giving it a value.

Type

Most procedural programming languages support some sort of typing - that is variables can only store one type of value. Anyone who has created a database will be familiar with this idea - each field in an Access database is also given a type, be it number, text, memo, date, etc. The types supported will vary from language to language, but will include some, or all, of the following (and maybe more!):

These types may go by different names in different languages, for example an integer in one might be a short in another, or a single in one might be a float elsewhere. Notice that the Python version isn't really the same - in Python there is no need to declare types, the type is selected automatically according to the value being stored and can change if a different type of value is stored.

dim name as string
dim age as integer
dim height as single
dim sex as string * 1
dim married as boolean

char*	name;
int	age;
float	height;
char	sex;
int	married;

name = "Dave"
age = 21
height = 1.8
sex = "M"
married = True

Note that there is no string type in C, but that you can use a char pointer (char* - I'm not going to go into the complexities of pointers and references here!), and nor is there a Boolean type. I've used a fixed-length string in the BASIC example to represent a single character (either M or F).

BASIC also allows you to declare the type of a variable by using a suffix on its name, e.g. $ for strings, % for integers, etc.. In the above example, I could have defined name$ and age%, and the variables would have been set to set to a string and an integer automatically.

See how variables and types are used in Python on the Advanced ICT YouTube channel.

Some languages, such as JavaScript, are not typed, but still have a declaration statement, e.g.:

var name, age, height, sex, married;

This might sound easier, but can lead to confusing results when it can't decide whether your variables are numbers or strings. For example, say you have two variables, a = 123 and b = 456 - what would you expect c = a + b to give you? The + operator can also be used to concatenate strings, so the value of c will depend on the context. Sometimes c will be 579, and sometimes it might be 123456! To be sure, you'd have to use something like c = (a * 1) + (b * 1) (for addition) or c = a + "" + b (for string concatenation).

Scope

A variable can't always be used throughout the whole of your program - they have something called a scope which determines where a value can be read or changed.

Global variables are variables that can be used through your program - that is, the scope of a global variable is the entire application. Most variables, however, will be local - local variables can only be used in the function (or procedure) in which they were declared, or any other function called by that function. Scope, therefore, is hierarchical, and generally only applies downwards (from the main body of the program, to the functions it calls, and from functions down to further sub-functions).

This means that if you give a variable at the top of your program, you can't declare another variable with the same name in a function. However, if you declare a variable in one function, you can declare another variable with the same name in another function, and they will effectively be different variables and can have different variables.

Finally, by default a variable declared in a function only exists for the time that the function is running - each time you call the function, the variable is re-declared and reset. If this isn't what you want, you can declare your variable as a static - this means that its value will persist after the function finishes, and its value will be the same next time the function is run. Static variables still have a type (e.g. integer, float, etc.).

Arrays

Sometimes you might want to have variables to represent collections of similar items or objects - for example, the six balls picked by a lottery simulator. You could create six separate integer variables, called something like first_ball, second_ball,third_ball, etc., but it would be better to use an array.

An array is a collection of variables of the same type, with the same name, differentiated by a numbered index. In the lottery example, we could use an array of integers called ball, and refer to the individual balls as ball(0), ball(1), ball(2), etc. Note that the index usual starts at zero rather than one. What's useful about arrays is that you can use the same code to process all of the items in the array using a loop.

Arrays can also be used to implement a simple look-up table. For example, of you wanted to generate a day of the week at random, you could use a string array to contain the days of the week. You could then generate an integer from 0 to 6, and use this as the index to return the day of the week. To do this in Visual BASIC or JavaScript, the code would be:

dim day(7) as integer
dim day_number as integer

day(0) = "Sunday"
day(1) = "Monday"
day(2) = "Tuesday"
day(3) = "Wednesday"
day(4) = "Thursday"
day(5) = "Friday"
day(6) = "Saturday"

day_number = int(rnd() * 7)

msgbox(day(day_number))

day = new Array(7);
var day_number;

day[0] = "Sunday";
day[1] = "Monday";
day[2] = "Tuesday";
day[3] = "Wednesday";
day[4] = "Thursday";
day[5] = "Friday";
day[6] = "Saturday";

day_number = Math.round(Math.random() * 7);

alert(day[day_number]);

from random import randint
day = []
    
day.append("Sunday")
day.append("Monday")
day.append("Tuesday")
day.append("Wednesday")
day.append("Thursday")
day.append("Friday")
day.append("Saturday")
  
day_number = randint(0, 6)
  
print(day[day_number])

Again, the Python version isn't really the same because Python has lists rather than arrays, although the way they are used is quite similar. The main differences are that with an array, all of the elements are of the same type, whereas lists can contain a mixture of types, e.g. integers, floats, strings and Boolean values in the same list. The second difference is that you can't define a new element in a list using its index - hence the use of the .append() method.

In all three languages you can define the list or array with the elements already in it, e.g. (in Python) day = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"].

Early versions of Visual BASIC (and presumably other languages) also allowed you to have arrays of controls (e.g. fields or buttons). This allows you to write one function or procedure that can then apply to any of the objects in the array.

I have written a blog article about how arrays can be used for selection.

NB. In some programming languages (e.g. C or C++), arrays are called vectors, and Python doesn't have arrays but has lists, which can be used in a similar way.

Associative Arrays / Dictionaries

With a normal array, as described above, the indexes are numbers, e.g. day[0] is Sunday. In an associative array with can use a string or type of index, called a key.

In Python, associative arrays are called dictionaries. The array/dictionary consists of pairs of keys and values:

points = {"A":1, "E":1, "I":1, "L":1, "N":1, "O":1, "R":1, "S":1, "T":1, "U":1,"D":2, "G":2, "B":3, "C":3, "M":3, "P":3, "F":4, "H":4, "V":4, "W":4, "Y":4, "K":5, "J":8, "X":8, "Q":10, "Z":10}
word = input("Enter your word: ").upper()
score = 0
for letter in word:
    score += points[letter]
print("The word '"+word+"' scores", score, "points.")

The program above calculates the Scrabble score for a word. Each character is "looked up" in the dictionary using itself as the key - e.g. if you enter the word HELLO, the first step is to look up points("H"), which returns the value 4.

More Complicated Structures

Some types of data may require more complicated structures to store them. Languages such as C++ allow you to create compound data-structures (e.g. classes, unions, structs, etc.) by combining variables. For example, you might want to create a time type, which uses integers to separately store the numbers of minutes and hours. You can then create functions for manipulating your types, e.g. setting, comparing and adding the times, as the standard arithmetic operators cannot be used. This is known as operator overloading.

Examples of other types of data structure include trees, queues, lists and stacks.

Data Abstraction

Your code should be written in such a way that the details of how the data are stored are hidden from the rest of the code - this is known as data abstraction. So, for example, if you have a program that uses a stack, you shouldn't read from, or write to, the stack directly, but you should write and use functions for the purpose, e.g. one called push() to put a value on the stack, and one called pop() to remove values. Not only does this allow you to centralise data-storage functions and reduce repetition of code, but it also allows you to change how a data structur is implemented without having to re-write all your code. If you wanted to change your stack implementation from using an array to using a linked-list, for example, you would only need to change your push() and pop() functions, not all of the functions that used the stack.

Constants

Finally, most programming languages also support the use of constants. These are named values that cannot be changed at run-time (if you're answering an exam question, do not use the word variable!) They are useful because you can refer to mathematical values by name, rather than having to remember them, or you can personalise many parts of your programming by setting a constant to the name of the user at the start of the program. Programming languages may also include many constants of their own, which are predefined. In Visual BASIC, for example, these include values such as pi, true and false, and also the arguments used for certain functions, such as msgbox() types (e.g. vbYesNo), which are actually just integers.

Python doesn't support constants, but you can use variables for the same purpose - the convention is to use names in upper case. In some programming languages, constants are values stored in memory, like variables, but are effectively replaced by their values in a kind of "search and replace" as the program is compiled or interpreted.

With compiled languages, the compiler often works by replacing the names of the constants with their actual value (like Find and Replace in a text editor or word processor) before compiling the code, thereby reducing the amount of memory required for storage and making the code more efficient.