Arrays

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Notes

1-Dimensional Arrays

An **array** is an ordered, static set of elements stored in a fixed-size memory location.
An array can only store **one data type** (e.g., all integers or all strings).
In CIE pseudocode, **indexing starts at 1**; in Python, indexing starts at 0 (zero-indexed).
Declare a 1D array in pseudocode: `DECLARE scores: ARRAY[1:5] OF INTEGER` creates an array with 5 elements (indices 1–5).
Assign a value: `scores[4] ← "Red"` (pseudocode) or `scores[4] = "Red"` (Python).
Access an element: `MyArray[1]` (pseudocode) or `array[0]` (Python).
Modify an element: `MyArray[index] ← newValue` (pseudocode) or `array[index] = newValue` (Python).
Use `len(array)` in Python to get array length; pseudocode requires storing the length in a variable.

2-Dimensional Arrays

A **2D array** extends a 1D array by adding another dimension, visualised as a **table** with rows and columns.
In pseudocode, declare: `DECLARE arr: ARRAY[1:3, 1:2] OF STRING` (3 rows, 2 columns).
In Python, initialise as a list of lists: `arr = [["Alice", 25], ["Bob", 30]]`.
Access an element: `arr[row, col]` (pseudocode) or `arr[row][col]` (Python).
Assign a value: `players[3,1] ← "Holly"` (pseudocode) or `players[2][0] = "Holly"` (Python).
Iterate using **nested loops**: outer loop for rows, inner loop for columns.
**Examiner tip**: Always check the question's example to see whether the order is `[row][col]` or `[col][row]`.

Common Operations on Arrays

**Iterating** over a 1D array: use a single `FOR` loop from 1 to length (pseudocode) or `for element in array:` (Python).
**Iterating** over a 2D array: use nested `FOR` loops (pseudocode) or nested `for` loops (Python).
**Length** of a 1D array: in Python, `len(array)`; in pseudocode, store the declared size in a variable.
**Modifying** elements: assign a new value to a specific index.
**Accessing** elements: use the index (starting at 1 in pseudocode, 0 in Python).

Pseudocode vs Python Syntax

**Declaration**: `DECLARE arr: ARRAY[1:5] OF INTEGER` (pseudocode) vs `arr = [0]×5` or `arr = []` (Python).
**Indexing**: pseudocode uses `arr[1]` for first element; Python uses `arr[0]`.
**Assignment**: `arr[1] ← 10` (pseudocode) vs `arr[0] = 10` (Python).
**2D indexing**: `arr[1,2]` (pseudocode) vs `arr[0][1]` (Python).
**Iteration**: `FOR i ← 1 TO 5` (pseudocode) vs `for i in range(5):` (Python).

Worked Example: 2D Array Access

Given a 2D array `minsWatched` with rows (days) and columns (children), access specific values.
Example: `minsWatched[0][3]` gives Elias's minutes on Monday (row 0, col 3).
To output Lyla's minutes on Tuesday: `print(minsWatched[1][1])` (row 1, col 1).
To output Harry's minutes on Friday: `print(minsWatched[4][2])` (row 4, col 2).
To output Quinn's minutes on Wednesday: `print(minsWatched[2][0])` (row 2, col 0).

A 1D array of 5 integers with indices 1–5 (pseudocode).

A 2D array with 4 rows (players) and 2 columns (name, score).

Practice questions

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1.What is an array?
Easy
- AAn ordered, static set of elements in a fixed-size memory location
- BAn unordered collection of elements that can change size dynamically
- CA single variable that stores multiple data types
- DA data structure that can only store characters
2.In CIE pseudocode, what is the starting index of an array?
Easy
- A0
- B1
- CDepends on the programming language
- DThe index is always specified by the programmer
3.Which of the following correctly declares a 1D array in CIE pseudocode?
Easy
- ADECLARE scores: ARRAY[1:5] OF INTEGER
- BDECLARE scores: ARRAY[0:4] OF INTEGER
- Cscores = [1,2,3,4,5]
- DARRAY scores[5]
4.What does the following CIE pseudocode output? DECLARE MyArray : ARRAY[1:3] OF INTEGER MyArray[1] ← 5 MyArray[2] ← 10 MyArray[3] ← 15 OUTPUT MyArray[2]
Easy
- A5
- B10
- C15
- DError
5.A 2D array is declared as DECLARE table: ARRAY[1:4, 1:3] OF INTEGER. How many elements does it contain?
Medium
- A7
- B12
- C4
- D3
6.In CIE pseudocode, what is the correct way to assign the value 'Red' to the fourth element of a 1D array called colours?
Medium
- Acolours[4] ← 'Red'
- Bcolours[3] ← 'Red'
- Ccolours[4] = 'Red'
- Dcolours[0] ← 'Red'
7.Consider the following 2D array in CIE pseudocode: DECLARE grid : ARRAY[1:2, 1:2] OF INTEGER grid[1,1] ← 1 grid[1,2] ← 2 grid[2,1] ← 3 grid[2,2] ← 4 What is the value of grid[2,1]?
Medium
- A1
- B2
- C3
- D4
8.In CIE pseudocode, which loop structure is typically used to iterate through all elements of a 1D array?
Medium
- AFOR...NEXT loop
- BWHILE loop
- CREPEAT...UNTIL loop
- DIF...THEN statement

More questions

9.Given the following CIE pseudocode, what is the output? DECLARE arr : ARRAY[1:4] OF INTEGER arr[1] ← 2 arr[2] ← 4 arr[3] ← 6 arr[4] ← 8 DECLARE i : INTEGER FOR i ← 1 TO 4 arr[i] ← arr[i]×2 NEXT i OUTPUT arr[3]
Hard
- A6
- B8
- C12
- D16
10.A 2D array is used to store the names and scores of 5 players. In CIE pseudocode, which declaration is correct?
Hard
- ADECLARE players : ARRAY[1:5, 1:2] OF STRING
- BDECLARE players : ARRAY[1:2, 1:5] OF STRING
- CDECLARE players : ARRAY[1:5] OF STRING
- DDECLARE players : ARRAY[1:5, 1:2] OF INTEGER
11.In CIE pseudocode, what is the output of the following code? DECLARE matrix : ARRAY[1:2, 1:3] OF INTEGER matrix[1,1] ← 1 matrix[1,2] ← 2 matrix[1,3] ← 3 matrix[2,1] ← 4 matrix[2,2] ← 5 matrix[2,3] ← 6 OUTPUT matrix[2,2]
Hard
- A2
- B3
- C5
- D6
12.Which of the following statements about arrays is FALSE?
Hard
- AAn array can only store one data type
- BIn Python, array indexes start at 0
- CA 2D array can be visualised as a table with rows and columns
- DIn CIE pseudocode, the length of an array can be obtained using the len function
13.A program stores the temperatures recorded each day for a week in an array called 'temps'. The array is declared as: DECLARE temps : ARRAY[1:7] OF REAL Which of the following correctly assigns the temperature 23.5 to the third day?
Easy
- Atemps[3] ← 23.5
- Btemps[2] ← 23.5
- Ctemps[4] ← 23.5
- Dtemps[0] ← 23.5
14.A 2D array 'grid' is declared as: DECLARE grid : ARRAY[1:3, 1:4] OF INTEGER How many elements does this array contain?
Medium
- A12
- B7
- C16
- D9
15.Consider the following CIE pseudocode: DECLARE scores : ARRAY[1:5] OF INTEGER scores[1] ← 10 scores[2] ← 20 scores[3] ← 30 scores[4] ← 40 scores[5] ← 50 DECLARE i : INTEGER DECLARE total : INTEGER total ← 0 FOR i ← 1 TO 5 IF scores[i] MOD 3 = 0 THEN total ← total + scores[i] ENDIF NEXT i OUTPUT total What is the output?
Hard
- A30
- B10
- C20
- D60
16.A teacher stores the test scores of 30 students in a 1D array called 'testScores'. Which of the following pseudocode snippets correctly calculates the average score?
Medium
- Atotal ← 0 FOR i ← 1 TO 30 total ← total + testScores[i] NEXT i average ← total / 30
- Btotal ← 0 FOR i ← 0 TO 29 total ← total + testScores[i] NEXT i average ← total / 30
- Ctotal ← 0 FOR i ← 1 TO 30 total ← total + testScores[i] NEXT i average ← total / 29
- Dtotal ← 0 FOR i ← 1 TO 30 total ← total + testScores[1] NEXT i average ← total / 30
17.A 2D array 'matrix' is declared as: DECLARE matrix : ARRAY[1:2, 1:3] OF INTEGER matrix[1,1] ← 1 matrix[1,2] ← 2 matrix[1,3] ← 3 matrix[2,1] ← 4 matrix[2,2] ← 5 matrix[2,3] ← 6 What is the value of matrix[2,1] + matrix[1,2]?
Hard
- A6
- B3
- C5
- D7
18.Which of the following correctly declares a 2D array in CIE pseudocode that can store the names and ages of 10 people?
Easy
- ADECLARE people : ARRAY[1:10, 1:2] OF STRING
- BDECLARE people : ARRAY[1:2, 1:10] OF STRING
- CDECLARE people : ARRAY[1:10, 1:2] OF INTEGER
- DDECLARE people : ARRAY[0:9, 0:1] OF STRING
19.The following table shows the number of books read by three students over four weeks, stored in a 2D array 'books'. Week1 Week2 Week3 Week4 Student1 3 5 2 4 Student2 4 6 3 5 Student3 2 4 1 3 In CIE pseudocode, if the array is declared as DECLARE books : ARRAY[1:3, 1:4] OF INTEGER, which expression gives the number of books read by Student2 in Week3?
Medium
- Abooks[2,3]
- Bbooks[3,2]
- Cbooks[1,2]
- Dbooks[2,4]
20.A program uses a 1D array 'numbers' of size 10. Which of the following pseudocode snippets correctly finds the largest value in the array?
Hard
- Amax ← numbers[1] FOR i ← 2 TO 10 IF numbers[i] > max THEN max ← numbers[i] ENDIF NEXT i
- Bmax ← 0 FOR i ← 1 TO 10 IF numbers[i] > max THEN max ← numbers[i] ENDIF NEXT i
- Cmax ← numbers[1] FOR i ← 1 TO 10 IF numbers[i] > max THEN max ← numbers[i] ENDIF NEXT i
- Dmax ← numbers[10] FOR i ← 1 TO 9 IF numbers[i] > max THEN max ← numbers[i] ENDIF NEXT i
21.In CIE pseudocode, what is the output of the following code? DECLARE arr : ARRAY[1:4] OF INTEGER arr[1] ← 7 arr[2] ← 3 arr[3] ← 9 arr[4] ← 1 OUTPUT arr[2] + arr[4]
Easy
- A4
- B10
- C12
- D8
22.A 2D array 'sales' stores the sales figures for 4 products over 6 months. Which of the following loops correctly calculates the total sales for all products over all months?
Medium
- Atotal ← 0 FOR row ← 1 TO 4 FOR col ← 1 TO 6 total ← total + sales[row, col] NEXT col NEXT row
- Btotal ← 0 FOR col ← 1 TO 6 FOR row ← 1 TO 4 total ← total + sales[col, row] NEXT row NEXT col
- Ctotal ← 0 FOR row ← 1 TO 6 FOR col ← 1 TO 4 total ← total + sales[row, col] NEXT col NEXT row
- Dtotal ← 0 FOR row ← 1 TO 4 FOR col ← 1 TO 6 total ← total + sales[col, row] NEXT col NEXT row
23.Consider the following CIE pseudocode: DECLARE data : ARRAY[1:5] OF INTEGER data[1] ← 2 data[2] ← 5 data[3] ← 8 data[4] ← 11 data[5] ← 14 DECLARE i : INTEGER DECLARE result : INTEGER result ← 0 FOR i ← 1 TO 5 IF data[i] MOD 2 = 1 THEN result ← result + 1 ENDIF NEXT i OUTPUT result What is the output?
Hard
- A2
- B3
- C5
- D1
24.A program uses a 2D array 'board' to represent a tic-tac-toe grid. The array is declared as: DECLARE board : ARRAY[1:3, 1:3] OF CHAR Initially, all elements are set to ' '. Which of the following correctly sets the centre cell to 'X'?
Medium
- Aboard[2,2] ← 'X'
- Bboard[1,1] ← 'X'
- Cboard[3,3] ← 'X'
- Dboard[2,3] ← 'X'
25.In CIE pseudocode, what is the index of the first element in a 1D array?
Easy
- A0
- B1
- C-1
- DIt depends on the array
26.Which of the following correctly declares a 1D array in CIE pseudocode to store the names of 10 students?
Easy
- ADECLARE names : ARRAY[1:10] OF STRING
- BDECLARE names : ARRAY[0:9] OF STRING
- CDECLARE names : ARRAY[10] OF STRING
- DDECLARE names : ARRAY[1..10] OF STRING
27.What is the output of the following CIE pseudocode? DECLARE MyArray : ARRAY[1:3] OF INTEGER MyArray[1] ← 5 MyArray[2] ← 10 MyArray[3] ← 15 OUTPUT MyArray[2]
Easy
- A5
- B10
- C15
- D2
28.In CIE pseudocode, which loop structure is typically used to iterate through all elements of a 1D array of size 5?
Medium
- AFOR i ← 1 TO 5
- BFOR i ← 0 TO 4
- CWHILE i < 5
- DREPEAT...UNTIL i = 5
29.In CIE pseudocode, what is the output of the following code? DECLARE matrix : ARRAY[1:2, 1:3] OF INTEGER matrix[1,1] ← 1 matrix[1,2] ← 2 matrix[1,3] ← 3 matrix[2,1] ← 4 matrix[2,2] ← 5 matrix[2,3] ← 6 OUTPUT matrix[1,2] + matrix[2,1]
Medium
- A3
- B5
- C6
- D7
30.A program stores the temperatures recorded each day for a week in an array called 'temps'. The array is declared as: DECLARE temps : ARRAY[1:7] OF REAL Which pseudocode snippet correctly calculates the average temperature?
Hard
- Atotal ← 0 FOR i ← 1 TO 7 total ← total + temps[i] NEXT i average ← total / 7
- Btotal ← 0 FOR i ← 0 TO 6 total ← total + temps[i] NEXT i average ← total / 7
- Ctotal ← 0 FOR i ← 1 TO 7 total ← total + temps[i] NEXT i average ← total / 5
- Dtotal ← 0 FOR i ← 1 TO 7 total ← temps[i] NEXT i average ← total / 7
31.In CIE pseudocode, what is the starting index of a 1D array?
Easy
- A0
- B1
- CIt depends on the data type
- DThe programmer can choose
32.Given the following CIE pseudocode, what is the output? DECLARE arr : ARRAY[1:4] OF INTEGER arr[1] ← 2 arr[2] ← 4 arr[3] ← 6 arr[4] ← 8 DECLARE i : INTEGER FOR i ← 1 TO 4 OUTPUT arr[i]×2 NEXT i
Medium
- A2 4 6 8
- B4 8 12 16
- C1 2 3 4
- D8 6 4 2
33.In CIE pseudocode, what is the output of the following code? DECLARE matrix : ARRAY[1:2, 1:3] OF INTEGER matrix[1,1] ← 1 matrix[1,2] ← 2 matrix[1,3] ← 3 matrix[2,1] ← 4 matrix[2,2] ← 5 matrix[2,3] ← 6 OUTPUT matrix[2,2] + matrix[1,3]
Hard
- A8
- B7
- C9
- D10
34.A program stores the temperatures recorded each day for a week in an array called 'temps'. The array is declared as: DECLARE temps : ARRAY[1:7] OF REAL Which of the following correctly outputs the temperature on the third day?
Medium
- AOUTPUT temps[3]
- BOUTPUT temps[2]
- COUTPUT temps[1]
- DOUTPUT temps[4]
35.A 2D array 'temps' stores the temperature readings for 7 days (rows 1–7) at 3 different times each day (columns 1–3). Which expression gives the temperature reading for day 4 at the second time of day?
Medium
- Atemps[4,2]
- Btemps[2,4]
- Ctemps[3,4]
- Dtemps[4,3]
36.A program declares a 2D array as: DECLARE grid : ARRAY[1:4, 1:5] OF INTEGER. The program then sets all elements to 0. How many assignment statements are needed to initialise every element?
Hard
- A20
- B9
- C4
- D5
37.Consider the following CIE pseudocode: DECLARE arr : ARRAY[1:5] OF INTEGER arr[1] ← 3 arr[2] ← 7 arr[3] ← 2 arr[4] ← 9 arr[5] ← 5 DECLARE i, total : INTEGER total ← 0 FOR i ← 1 TO 5 IF arr[i] > 4 THEN total ← total + arr[i] ENDIF NEXT i OUTPUT total What is the output?
Medium
- A21
- B16
- C19
- D26
38.A teacher stores the test scores of 30 students in a 1D array called 'scores'. Which of the following correctly declares this array in CIE pseudocode?
Easy
- ADECLARE scores : ARRAY[1:30] OF INTEGER
- BDECLARE scores : ARRAY[0:29] OF INTEGER
- CDECLARE scores : ARRAY[1:30] OF REAL
- DDECLARE scores : ARRAY[30] OF INTEGER
39.The following table shows the sales of three products over four weeks stored in a 2D array 'sales'. Week1 Week2 Week3 Week4 Product1 120 150 110 130 Product2 200 180 210 190 Product3 90 110 95 105 Which pseudocode correctly calculates the total sales of all products over all weeks?
Hard
- Atotal ← 0 FOR row ← 1 TO 3 FOR col ← 1 TO 4 total ← total + sales[row, col] NEXT col NEXT row
- Btotal ← 0 FOR col ← 1 TO 4 FOR row ← 1 TO 3 total ← total + sales[col, row] NEXT row NEXT col
- Ctotal ← 0 FOR row ← 1 TO 4 FOR col ← 1 TO 3 total ← total + sales[row, col] NEXT col NEXT row
- Dtotal ← 0 FOR row ← 1 TO 3 FOR col ← 1 TO 4 total ← total + sales[col, row] NEXT col NEXT row
40.In CIE pseudocode, what is the output of the following code? DECLARE arr : ARRAY[1:4] OF INTEGER arr[1] ← 5 arr[2] ← 10 arr[3] ← 15 arr[4] ← 20 arr[2] ← arr[1] + arr[3] OUTPUT arr[2]
Medium
- A20
- B15
- C25
- D10

Slides

What is an Array?

An array is an ordered, static set of elements stored in a fixed-size memory location.
An array can only store one data type (e.g., all integers or all strings).
A 1D array is a linear array; elements are accessed by their index.
In CIE pseudocode, indices start at 1. In Python, they start at 0 (zero-indexed).

In CIE pseudocode, what is the index of the first element in an array?

0
1
Depends on the data type
It is not defined

Creating and Accessing 1D Arrays

Declare an array with: DECLARE arrayName : ARRAY[lower:upper] OF dataType
Example: DECLARE scores : ARRAY[1:5] OF INTEGER creates a 5-element integer array.
Access an element: arrayName[index] (e.g., scores[3] gives the third element).
Modify an element: arrayName[index] ← newValue (e.g., scores[2] ← 10).
In Python, create with: arrayName = [value1, value2, ...] and access with arrayName[index].

To declare an array called 'ages' that can hold 10 integers in CIE pseudocode, you write: DECLARE ages : ARRAY[1:____] OF ____

Iterating Through a 1D Array

Use a FOR loop to iterate through all elements.
In pseudocode: FOR index ← 1 TO length ... NEXT index
In Python: for element in arrayName: or for i in range(len(arrayName)):
Example: FOR i ← 1 TO 5 : OUTPUT myArray[i] : NEXT i outputs each element.

Given the pseudocode: DECLARE nums : ARRAY[1:4] OF INTEGER; nums[1]←5; nums[2]←10; nums[3]←15; nums[4]←20; What is the output of: OUTPUT nums[3]

What is a 2D Array?

A 2D array extends a 1D array by adding another dimension, like a table with rows and columns.
Declare in pseudocode: DECLARE arrayName : ARRAY[rowLower:rowUpper, colLower:colUpper] OF dataType
Example: DECLARE table : ARRAY[1:3, 1:4] OF INTEGER creates a 3×4 array.
Access an element: arrayName[row, col] (e.g., table[2,3] is row 2, column 3).
In Python: arrayName = [[r1c1, r1c2], [r2c1, r2c2]]; access with arrayName[row][col].

In pseudocode, to access the element in the 2nd row and 3rd column of a 2D array called 'grid', you write: grid[____, ____]

Iterating Through a 2D Array

Use nested FOR loops: outer loop for rows, inner loop for columns.
Pseudocode: FOR row ← 1 TO numRows : FOR col ← 1 TO numCols : OUTPUT array[row, col] : NEXT col : NEXT row
Python: for row in array2D: for item in row: print(item)
This visits every element in row-major order.

Given the pseudocode: DECLARE mat : ARRAY[1:2,1:3] OF INTEGER; mat[1,1]←1; mat[1,2]←2; mat[1,3]←3; mat[2,1]←4; mat[2,2]←5; mat[2,3]←6; What is the value of mat[2,1]?

Examiner Tips for 2D Arrays

In exam questions, always check the example given to see the order of indices (row first or column first).
Some questions use (x,y) and others (y,x); refer to the example before answering.
Be careful with zero-indexed vs one-indexed notation between Python and pseudocode.

Which of these are true about arrays?

An array can store multiple data types at the same time.
In CIE pseudocode, array indices start at 1.
A 2D array can be thought of as a table with rows and columns.
In Python, the first element of an array is at index 1.

Worked Example: 2D Array Access

A 2D array 'minsWatched' stores TV minutes for 4 children (columns 0-3) over 5 days (rows 0-4).
Monday is row 0, Quinn is column 0, Elias is column 3.
To output Lyla's minutes on Tuesday (row 1, column 1): print(minsWatched[1][1]) or print(minsWatched[1,1])
To output Harry's minutes on Friday (row 4, column 2): print(minsWatched[4][2])
To output Quinn's minutes on Wednesday (row 2, column 0): print(minsWatched[2][0])

Using the 'minsWatched' array from the worked example, which line correctly outputs Elias's minutes on Thursday? (Assume zero-indexed)

print(minsWatched[3][3])
print(minsWatched[3,3])
print(minsWatched[4][3])
print(minsWatched[3][4])

Summary

Arrays store multiple values of the same data type in a fixed-size structure.
1D arrays are linear; 2D arrays are like tables with rows and columns.
CIE pseudocode uses 1-based indexing; Python uses 0-based indexing.
Use loops to iterate through arrays: FOR loops for pseudocode, for loops in Python.
Always check the index order in exam questions.

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