Concepts of Data Structures and Arrays (AQA GCSE Computer Science): Revision Notes

Data structures - Concepts of data structures and arrays

What are data structures and arrays?

A data structure is like a filing system for your computer programmes. Just as you might organise your school papers in different folders, programmers need ways to organise and store multiple pieces of information together.

An array is one of the most important types of data structure. Think of it as a row of numbered boxes where you can store different pieces of data. Instead of creating separate variables for each piece of information (like student1, student2, student3), you can store all related data together under one name.

Understanding 1-dimensional arrays

A 1-dimensional array (or 1D array) is the simplest type of array. Picture it as a single row of storage compartments, each with its own address number.

Index values - the key to finding your data

Every item in an array has an index value. This is simply a number that tells you exactly where to find that piece of data.

In this example, you can see how the array stores four student names. Notice that 'Fletcher' is at index 0, 'Imogen' is at index 1, and so on.

Creating and working with arrays

When you create an array, you need to understand two important characteristics:

• Arrays have a fixed number of items that's decided when you create them
• Every item in the array must be the same data type (all numbers, or all text, but not mixed)

Let's look at a practical example with numbers:

This array contains 8 numbers, with indices running from 0 to 7. If you wanted to access the value 10, you'd use index 3.

Using FOR loops with arrays

Arrays become really powerful when you combine them with FOR loops. This allows you to process every item in the array automatically, without having to write separate code for each element.

total ← 0
FOR i ← 0 TO 7
    total ← total + scores[i]
ENDFOR
OUTPUT total

Programming examples in different languages

Different programming languages have their own ways of working with arrays, but the core concepts remain the same.

Python uses something called lists instead of traditional arrays. Lists are actually more flexible - they can contain different data types and can change size while your programme runs:

scores = [5,7,0,10,8,3,7,3]
total = 0
for x in range(8):
    total = total + scores[x]
print(total)

VB.Net uses this syntax:

Dim scores As Integer() = {5,7,0,10,8,3,7,3}
Dim total As Integer = 0
For x = 0 To 7
    total = total + scores(x)
Next x
Console.WriteLine(total)

C# looks like this:

int[] scores = new int[] {5,7,0,10,8,3,7,3};
int total = 0;
for (int x = 0; x < 8; x++)
{
    total = total + scores[x];
}
Console.WriteLine(total);

FOR EACH loops - an easier way

Many programming languages offer FOR EACH loops, which make working with arrays even simpler. Instead of worrying about index numbers, you can process each item directly:

Python:

scores = [5,7,0,10,8,3,7,3]
total = 0
for item in scores:
    total = total + item
print(total)

Understanding 2-dimensional arrays

A 2-dimensional array (or 2D array) is like a table or spreadsheet with rows and columns. Instead of just one row of data, you have multiple rows stacked on top of each other.

This 2D array shows game scores arranged in a table format. To access any specific value, you need two index numbers - one for the row and one for the column. For example, to get the value 99, you'd use scores[1][1].

Accessing 2D array elements

To work with every element in a 2D array, you need nested loops - that's one loop inside another loop:

FOR x ← 0 TO 2
    FOR y ← 0 TO 3
        OUTPUT scores[x][y]
    ENDFOR
ENDFOR

2D arrays in different programming languages

Python handles 2D arrays using lists within lists:

for x in range(3):
    for y in range(4):
        print(scores[x][y])

VB.Net uses this approach:

For x = 0 To 2
    For y = 0 To 3
        Console.WriteLine(scores(x,y))
    Next y
Next x

C# syntax looks like:

for (int x = 0; x < 3; x++)
{
    for (int y = 0; y < 4; y++)
    {
        Console.WriteLine(scores[x,y]);
    }
}

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