Data structures: arrays, lists, dictionaries, records, tuples and sets

Arrays and lists

• Array: a fixed-size collection of elements of the same data type, stored in contiguous memory and accessed by an index (starting at 0). Fast direct access; but the size cannot change and every element must be the same type. A two-dimensional array is a grid accessed by [row][column].
• List: an ordered collection whose size can change (items can be appended, inserted and removed). In Python, lists can hold mixed types. Flexible; but inserting or deleting in the middle means moving the later items, and lists use more memory than arrays.

Dictionaries, records, tuples and sets

• Dictionary: a collection of key: value pairs. Keys are unique and are used to look values up directly, for example {'Paris': 2.1, 'Cairo': 10.0}. Fast lookup by key; no index order to rely on for position, and keys cannot repeat.
• Record: a single item made of named fields that can be of different types, for example a student record with name (text), age (integer) and enrolled (Boolean). Several records form a table or file.
• Tuple: an ordered collection that is immutable (cannot be changed after creation), for example a coordinate (3, 4). Safe for fixed data; must be rebuilt to change.
• Set: an unordered collection of unique items, with no index. Fast membership tests and removal of duplicates.

Set operations

For A = [2, 4, 6, 8, 10] and B = [3, 6, 9] (written in order here, although sets are unordered):

Tracing a 2D array

grid = [[3, 8, 1], [6, 2, 9], [4, 7, 5]]

• grid[0][2] is row 0, column 2: 1
• grid[2][1] is row 2, column 1: 7
• After grid[1][0] = grid[1][0] + grid[0][1], the value at [1][0] becomes 14.

Always check whether the question indexes from 0, as Python does.

Worked example

Show the contents of the dictionary after these operations. cities = {'Berlin': 3.6, 'Lagos': 15.4} cities['Lima'] = 10.1 del cities['Berlin'] cities['Lagos'] = 16.0

1. Start: {'Berlin': 3.6, 'Lagos': 15.4}
2. Add Lima: {'Berlin': 3.6, 'Lagos': 15.4, 'Lima': 10.1}
3. Delete Berlin: {'Lagos': 15.4, 'Lima': 10.1}
4. Assigning to an existing key replaces its value: {'Lagos': 16.0, 'Lima': 10.1}

Answer: {'Lagos': 16.0, 'Lima': 10.1}

Check yourself

1. grid = [[3, 8, 1], [6, 2, 9], [4, 7, 5]]. What is grid[1][2]?
   Show answer

   9. Row 1 is [6, 2, 9]; column 2 is 9.

2. grid = [[3, 8, 1], [6, 2, 9], [4, 7, 5]]. What is the sum of row 2?
   Show answer

   16. 4 + 7 + 5 = 16.

3. grid = [[3, 8, 1], [6, 2, 9], [4, 7, 5]]. What is the total of column 0?
   Show answer

   13. 3 + 6 + 4 = 13.

4. A = {1, 3, 5, 7, 9} and B = {3, 4, 5, 6}. List the intersection of A and B in ascending order, separated by commas.
   Show answer

   3, 5. Items in A and also in B: 3, 5.

5. A = {1, 3, 5, 7, 9} and B = {3, 4, 5, 6}. How many items are in the union of A and B?
   Show answer

   7. Union = [1, 3, 4, 5, 6, 7, 9], which has 7 items (duplicates count once).

6. Which data structure cannot be changed after it is created?
   • Tuple
   • List
   • Dictionary
   • Set
   Show answer

   Tuple. Tuples are immutable.

7. Which data structure is best for looking up a country's population from the country's name?
   • Tuple
   • One-dimensional array of integers
   • Dictionary
   • Set
   Show answer

   Dictionary. The name is a unique key mapped to its value.

8. What is a record?
   • An unordered collection of unique values
   • A fixed-size collection of items of one type accessed by index
   • A collection that cannot contain duplicates
   • A single item made of named fields that can have different data types
   Show answer

   A single item made of named fields that can have different data types. For example name (text), age (integer), enrolled (Boolean).

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Computer Science specification (2026)

Last updated 8 November 2026. Spotted a mistake? Tell us.