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Arrays

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Notes de leçon

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).

1D Array Example1210528[1][2][3][4][5]Indices (pseudocode) start at 1

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

2D Array as a TableNameScoreAlice25Bob30Charlie22Daisy28Rows: players, Columns: name & score

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  1. 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. 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. 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. 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. 5.A 2D array is declared as DECLARE table: ARRAY[1:4, 1:3] OF INTEGER. How many elements does it contain?

    Easy
    • A7
    • B12
    • C4
    • D3

  6. 6.In CIE pseudocode, what is the correct way to assign the value 'Red' to the fourth element of a 1D array called colours?

    Easy
    • Acolours[4] ← 'Red'
    • Bcolours[3] ← 'Red'
    • Ccolours[4] = 'Red'
    • Dcolours[0] ← 'Red'

  7. 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]?

    Easy
    • A1
    • B2
    • C3
    • D4

  8. 8.In CIE pseudocode, which loop structure is typically used to iterate through all elements of a 1D array?

    Easy
    • AFOR...NEXT loop
    • BWHILE loop
    • CREPEAT...UNTIL loop
    • DIF...THEN statement

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