Array Basics (Cambridge (CIE) A Level Computer Science): Revision Note

Arrays

What is an array?

• An array is an ordered, static set of elements

• Can only store 1 data type

• The position of each element in an array is identified using the array's index

• The array's first element is the lower bound (LB)

• The array's last element is the upper bound (UB)

• The lower bound of an array is typically 0 or 1 depending on the language being used

• An array can be one-dimensional or multi-dimensional

One-dimensional (1D) arrays

• A 1D array is a linear array

• To declare a 1D array in pseudocode you must include the lower bound, upper bound and data types:

DECLARE <identifier> : ARRAY[LB:UB] OF <data type>

• In this example a 1D array of five elements each containing single character can be declared as:

DECLARE Letters : ARRAY[0:4] OF CHAR

• An example complete program could be:

// Declare the array
DECLARE Letters : ARRAY[0:4] OF CHAR

// Assign values to each index
Letters[0] ← 'B'
Letters[1] ← 'E'
Letters[2] ← 'A'
Letters[3] ← 'D'
Letters[4] ← 'S'

// Output the full array
FOR Index ← 0 TO 4
    OUTPUT Letters[Index]
NEXT Index

• The array is declared with indices from 0 to 4

• Each element stores a single character using the CHAR data type

• The loop outputs each letter in order

Two-dimensional (2D) arrays

• A 2D array can be visualised as a table

• When navigating through a 2D array you first have to go down the rows and then across the columns to find a position within the array

• In 2D arrays the following must be declared:

  • Lower bound for rows (LBR) & upper bound for rows (UBR)

  • Lower bound for columns (LBC) & upper bound for columns (UBC)

DECLARE <identifier> : ARRAY[LBR:UBR, LBC:UBC] OF <data type>

• In this example a 2D array can be declared as:

DECLARE Letters : ARRAY[0:2, 0:4] OF CHAR

• An example complete program could be:

DECLARE Letters : ARRAY[0:2, 0:4] OF CHAR

// Row 0
Grid[0,0] ← 'B'
Grid[0,1] ← 'E'
Grid[0,2] ← 'A'
Grid[0,3] ← 'D'
Grid[0,4] ← 'S'

// Row 1
Grid[1,0] ← 'S'
Grid[1,1] ← 'E'
Grid[1,2] ← 'V'
Grid[1,3] ← 'E'
Grid[1,4] ← 'N'

// Row 2
Grid[2,0] ← 'W'
Grid[2,1] ← 'H'
Grid[2,2] ← 'I'
Grid[2,3] ← 'T'
Grid[2,4] ← 'E'

• ARRAY[0:2, 0:4] creates 3 rows and 5 columns

• First index is the row, second is the column: Letters[row, column]

• All elements are of type CHAR