Data structures concept

What is a Data Structure?

A data structure is an organised way of storing and managing a collection of related data values so that they can be accessed and processed efficiently. Rather than storing each value in a separate, unrelated variable, a data structure groups the values together under a single name and provides a way to work with all of them systematically.

You are already familiar with individual variables such as score or name. These work well for a single value, but programs often need to handle many related values at once - e.g. the scores of every student in a class, or all the items in a shopping basket. Data structures solve this problem by holding multiple values together in one place.

Common Data Structures at GCSE

AQA requires knowledge of the following data structures:

Structure	Description	Key feature
Array	A fixed-size, ordered collection of values of the same data type	Accessed by index (position number)
List	An ordered collection (often variable in size) of values	Can grow or shrink; used natively in Python
Record	A collection of related fields that may have different data types	Groups attributes of one entity (e.g. a student: name, age, score)

Why Data Structures Matter

The tabs below contrast storing five student scores without a data structure versus using one. Notice how the data-structure approach scales easily and allows iteration over the whole collection.

Each score is stored in its own separate variable. Adding a sixth student means adding another variable. Calculating the total requires listing every variable by name - the code cannot loop or scale.

AQA pseudocode:

score1 ← 72
score2 ← 85
score3 ← 91
score4 ← 68
score5 ← 77

total ← score1 + score2 + score3 + score4 + score5
OUTPUT total

Problem: five separate variables, no loop possible, does not scale.

All five scores are stored together in a single array or list. The total can be found by iterating over the structure - the same loop would work for 5 or 500 scores without any code changes.

# Python -- store 5 scores in a list
scores = [72, 85, 91, 68, 77]

total = 0
for score in scores:          # iterate over every element
    total += score

print("Total:", total)        # 393

' VB.NET -- store 5 scores in an array
Dim scores() As Integer = {72, 85, 91, 68, 77}

Dim total As Integer = 0
For Each score As Integer In scores   ' iterate over every element
    total += score
Next

Console.WriteLine("Total: " & total)  ' 393

// C# -- store 5 scores in an array
int[] scores = { 72, 85, 91, 68, 77 };

int total = 0;
foreach (int score in scores)         // iterate over every element
    total += score;

Console.WriteLine("Total: " + total); // 393

Key Takeaways

A data structure groups related values together under a single name, making them easier to store, access, and process.
Individual variables work for single values; data structures are needed when a program must handle a collection of related values.
Using a data structure allows a loop to process every element without needing to name each value separately - the code scales without rewriting.
Common data structures at GCSE include arrays (fixed size, same type), lists (flexible size), and records (mixed types, one entity).
Elements in an array or list are accessed by their index (position number), which typically starts at 0 in Python and C#.