One- and two-dimensional data structures

The built-in structures

Each structure has a different purpose.

Records, sets and dictionaries

• A record groups related fields about one item. The PLS describes a record as a list of mixed-type fields, such as ['Ana', 17, True]; a dictionary ({'name': 'Ana', 'score': 71}), a tuple or a class (see the OOP lessons) also work. A list of records is a table.
• A one-item tuple needs a trailing comma: (5,) is a tuple, but (5) is just the int 5.
• Sets test membership fast and support | (union), & (intersection), - (difference) and ^ (symmetric difference). s.remove(x) raises KeyError if x is missing; s.discard(x) does nothing.
• A dictionary looks values up by key. d.get(key, default) avoids a KeyError when the key may be missing; d.keys(), d.values() and d.items() let you loop.

a = {1, 2, 3, 4}
b = {3, 4, 5}
print(a | b, a & b, a - b, a ^ b)
stock = {"pen": 12, "ruler": 0}
print(stock.get("pen"), stock.get("glue", "none"))

Output:

{1, 2, 3, 4, 5} {3, 4} {1, 2} {1, 2, 5}
12 none

Sorting

• items.sort() sorts a list in place and returns None; sorted(items) returns a new list and leaves the original unchanged. Both accept reverse=True and key=function.
• The default order is case sensitive: every capital letter comes before every lower-case letter. For a case-insensitive sort pass the function itself, key=str.lower, with no brackets. (The PLS prints this as key=str.lower(), but in real Python the brackets call the method with no string and raise TypeError.)

words = ["bat", "Cat", "ant"]
print(sorted(words), sorted(words, key=str.lower), words)
scores = [64, 88, 71]
scores.sort(reverse=True)
print(scores)
try:
    sorted(words, key=str.lower())
except TypeError:
    print("TypeError: key=str.lower() calls the method too early")

Output:

['Cat', 'ant', 'bat'] ['ant', 'bat', 'Cat'] ['bat', 'Cat', 'ant']
[88, 71, 64]
TypeError: key=str.lower() calls the method too early

Two-dimensional lists

A 2D list is a list of rows. grid[r][c] is row r, column c. len(grid) is the number of rows; len(grid[0]) the number of columns. Use nested loops, or a comprehension, to process every cell.

grid = [[1, 2, 3],
        [4, 5, 6]]
print(grid[1][0], len(grid), len(grid[0]))
col_sums = [sum(row[c] for row in grid) for c in range(3)]
print(col_sums)

Output:

4 2 3
[5, 7, 9]

The aliasing trap

[[0] * 3] * 2 copies a reference to the same row twice, so changing one row changes both. Build each row separately with a comprehension.

bad = [[0] * 3] * 2
good = [[0] * 3 for _ in range(2)]
bad[0][0] = 9
good[0][0] = 9
print(bad)
print(good)

Output:

[[9, 0, 0], [9, 0, 0]]
[[9, 0, 0], [0, 0, 0]]

Worked example

Scores for 3 students in 4 tests are stored in a 2D list. Print each student's total and the class's highest single score.

1. ```python scores = [[55, 61, 70, 48], [80, 72, 91, 85], [66, 59, 73, 70]] for number, row in enumerate(scores, start=1): print("Student", number, "total", sum(row)) print("Highest:", max(max(row) for row in scores)) ```
2. Output: ``` Student 1 total 234 Student 2 total 328 Student 3 total 268 Highest: 91 ```

Answer: Loop over rows for per-student totals; take the maximum of each row's maximum for the overall highest.

Check yourself

1. What does this program print? ```python t = [[2, 4, 6], [8, 10, 12], [14, 16, 18]] print(t[2][1], t[0][2]) ```
   Show answer

   16 6. t[2] is [14, 16, 18], so t[2][1] = 16. t[0] is [2, 4, 6], so t[0][2] = 6. Output: `16 6`

2. What does this program print? ```python t = [[2, 4, 6], [8, 10, 12]] print(sum(t[r][1] for r in range(len(t)))) ```
   Show answer

   14. Column 1 holds 4 and 10, so the sum is `14`.

3. What does this program print? ```python rows = [[0] * 2] * 2 rows[1][0] = 5 print(rows) ```
   Show answer

   [[5, 0], [5, 0]]. Both rows are the same list object, so setting rows[1][0] also changes rows[0][0]. Output: `[[5, 0], [5, 0]]`

4. What does this program print? ```python a = {2, 4, 6, 8} b = {6, 8, 10} print(sorted(a ^ b)) ```
   Show answer

   [2, 4, 10]. Symmetric difference keeps items in exactly one set: 2, 4 and 10. Output: `[2, 4, 10]`

5. What does this program print? ```python ages = {"Ali": 17, "Bea": 18} print(ages.get("Cal", 0) + ages["Bea"]) ```
   Show answer

   18. 'Cal' is missing, so get returns the default 0; 0 + 18 = `18`.

6. A program stores the fixed coordinates of a map's four corners. Which structure is most suitable?
   • A set, because coordinates are unique
   • A dictionary with the coordinates as values only
   • A tuple of tuples, because the values must not change
   • A string of comma-separated numbers
   Show answer

   A tuple of tuples, because the values must not change. Tuples are immutable, so the corners cannot be changed by accident, and they keep their order.

7. What does this program print? ```python q = [5, 7, 9] q.pop(0) q.append(1) print(q) ```
   • [7, 9, 1]
   • [5, 7, 9, 1]
   • [7, 9]
   • [5, 7, 1]
   Show answer

   [7, 9, 1]. pop(0) removes the first item (5); append(1) adds to the end. Output: `[7, 9, 1]`

8. What does this program print? ```python fruit = ["pear", "Plum", "apple"] print(sorted(fruit)[0], sorted(fruit, key=str.lower)[0]) ```
   Show answer

   Plum apple. The default sort is case sensitive, so the capital P of Plum comes before every lower-case letter; with key=str.lower the words are compared as 'pear', 'plum' and 'apple'. Output: `Plum apple`

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Computer Science specification, Issue 1 (March 2026)
• Pearson Edexcel IAL Computer Science Unit 4 Programming Language Subset (PLS)
• Pearson Edexcel IAL Computer Science sample assessment materials (Unit 3 and Unit 4)

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