Arrays and files; the scenario question

An array is a data structure that holds many values of the same data type under one name.

Each value is an element. Each element has a position number called its index.

Without an array, 30 marks need 30 variables. With an array they need one name: `Marks`.

`ARRAY[1:5]` means the first index is 1 and the last is 5. The first index can be 0 or 1, and the exam says which.

DECLARE Marks : ARRAY[1:5] OF INTEGER
Marks[1] ← 56
Marks[2] ← 71
Marks[3] ← 48
Marks[4] ← 90
Marks[5] ← 65
OUTPUT Marks[4]

marks = [56, 71, 48, 90, 65]
print(marks[3])

The index can be a variable. `Marks[Count]` means the element whose index is the value in Count.

Put that inside a FOR loop, and the loop visits every element in turn.

DECLARE Marks : ARRAY[1:5] OF INTEGER
DECLARE Count : INTEGER
DECLARE Total : INTEGER
DECLARE Highest : INTEGER
FOR Count ← 1 TO 5
  OUTPUT "Enter mark ", Count
  INPUT Marks[Count]
NEXT Count
Total ← 0
Highest ← Marks[1]

FOR Count ← 1 TO 5
  Total ← Total + Marks[Count]
  IF Marks[Count] > Highest
    THEN
      Highest ← Marks[Count]
  ENDIF
NEXT Count
OUTPUT "Total: ", Total
OUTPUT "Highest: ", Highest

marks = [0] * 5
for count in range(5):
    marks[count] = int(input("Enter mark " + str(count + 1) + ": "))
total = 0
highest = marks[0]
for count in range(5):
    total = total + marks[count]
    if marks[count] > highest:
        highest = marks[count]
print("Total:", total)
print("Highest:", highest)

A 2D array is a table of values with rows and columns. Every element has the same data type.

`ARRAY[1:3, 1:4]` has 3 rows and 4 columns, so 3 × 4 = 12 elements.

You give two indexes: row first, then column. `Scores[2, 3]` is row 2, column 3.

Try the toy: tap a cell to read its address and value, then type a new value in Change the value. Then press Step to run the nested loop total. The toy uses different marks from the table below, so its Scores[2, 3] holds 16.

DECLARE Scores : ARRAY[1:3, 1:4] OF INTEGER
// (Scores holds the values in the table)
OUTPUT Scores[2, 3]
Scores[2, 3] ← 10
OUTPUT Scores[2, 3]

scores = [[7, 5, 9, 6],
          [8, 6, 4, 9],
          [5, 9, 7, 8]]
print(scores[1][2])
scores[1][2] = 10
print(scores[1][2])

There is something to try for this step in the IGCSE library.

The lesson page has the rest of the explanation, step by step:

• Nested loops to visit every element
• Initialising a 2D array
• Why programs store data in files
• Open, read or write, close
• Writing and reading a single item
• Writing and reading lines of text with a loop
• Be precise, and amend with care
• The 15-mark scenario question: plan first
• Model answer in pseudocode and Python
• How the 15 marks are awarded

Open the lesson page