← IGCSE Computer Science Paper 2

Looping through an array with FOR (7d)

A loop whose counter is the array index. Inside it, Marks[Index] picks each element in turn, so the loop visits every item from position 1 to 10 once.

Part of: IGCSE Computer Science 0478 — 7d Standard Methods of Solution — Module 17

Understanding Looping through an array with FOR (7d) in IGCSE Computer Science Paper 2

What it does: INPUT stores each typed value into Marks[Index], so the loop fills positions 1 to 10 one at a time. The counter goes through every position from 1 to 10, so each element of Marks is output exactly once. Cambridge arrays usually start at index 1, so a 10-item array has indexes 1 to 10. Starting at 0 would refer to an element that does not exist. Both the start value 1 and the end value 10 are included, so Index takes 10 values. Cambridge pseudocode closes a FOR loop with NEXT followed by the counter name. ENDFOR is not used.

Common mistakes: A FOR loop sets its counter with the assignment arrow ←, not =. It should be FOR Index ← 1 TO 10. The end value of a FOR loop follows the keyword TO, not UNTIL. UNTIL belongs to REPEAT loops. It should be FOR Index ← 1 TO 10.

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Learning

Duo

Which loop header visits every element of a 10-item array Marks, starting at index 1?

FOR Index ← 1 TO 10FOR Index ← 0 TO 10

Matches

FOR Index ← 1 TO 10 OUTPUT Marks[10] NEXT Index

↓

The index is fixed at 10, so the same element is output ten times instead of using Index to visit each element.

Applying

Meanings

Term

FOR Index ← 1 TO 10

Meaning

A loop whose counter is the array index. Inside it, Marks[Index] picks each element in turn, so the loop visits every item from position 1 to 10 once.

Practice

FOR Index ← 1 TO 10

Testing

Gaps

FOR Index ← 1 10

Check Answer

Spots

1FOR Index = 1 TO 10

2 INPUT Marks[Index]

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Quick Questions

1. Which loop header visits every element of a 10-item array Marks, starting at index 1?

A. FOR Index ← 1 TO 10 ✓

B. FOR Index ← 0 TO 10

Cambridge arrays usually start at index 1, so a 10-item array has indexes 1 to 10. Starting at 0 would refer to an element that does not exist.

2. Inside FOR Index ← 1 TO 10, which line outputs the current element of Marks?

A. OUTPUT Marks[10]

B. OUTPUT Marks[Index] ✓

Marks[Index] changes each time round the loop, so every element is output. Marks[10] always outputs the 10th element.

3. Which statement about the loop FOR Index ← 1 TO 10 is true?

A. Index is the value stored in the array

B. Index is the counter and also the position in the array ✓

Index counts 1, 2, 3 ... 10 and is used inside the square brackets to choose a position. The value stored at that position is Marks[Index].

4. Which loop fills an array with 10 marks typed by the user?

A. FOR Index ← 1 TO 10 INPUT Marks[Index] NEXT Index ✓

B. FOR Index ← 1 TO 10 INPUT Marks NEXT Index

Each INPUT must store the value in a particular element, so you write Marks[Index]. Without the index, you do not say which element receives the value.

5. How many times does the body of FOR Index ← 1 TO 10 run?

A. 10 times ✓

B. 9 times

Both the start value 1 and the end value 10 are included, so Index takes 10 values.

Match the Pairs

FOR Index ← 1 TO 10 → Repeats the loop body once for each array position from 1 to 10, with Index counting up each time

OUTPUT Marks[Index] → Displays the element at the current position, so each pass of the loop shows the next mark

Fill an array by asking the user for a value at each position as the counter moves along → FOR Index ← 1 TO 10 INPUT Marks[Index] NEXT Index

Output every element of the array in order using the loop counter as the index → FOR Index ← 1 TO 10 OUTPUT Marks[Index] NEXT Index

FOR Index ← 1 TO 10 OUTPUT Marks[10] NEXT Index → The index is fixed at 10, so the same element is output ten times instead of using Index to visit each element.

FOR Index ← 0 TO 10 INPUT Marks[Index] NEXT Index → The loop starts at 0 and runs 11 times, but the array has positions 1 to 10, so Marks[0] does not exist.

Key Term

FOR Index ← 1 TO 10

A loop whose counter is the array index. Inside it, Marks[Index] picks each element in turn, so the loop visits every item from position 1 to 10 once.

Fill in the Blanks

1. FOR Index ← 1 TO 10 INPUT Marks[Index] NEXT Index

Hint: Type the keyword that joins the start value and the end value of the loop.

2. FOR Index ← 1 TO 10 OUTPUT Marks[Index] NEXT Index

Hint: Type the keyword that displays each array element on the screen.

3. FOR Index ← 1 TO 10 INPUT Marks[Index] NEXT Index

Hint: Type the loop counter name that picks the current element.

4. DECLARE Marks : ARRAY[1:10] OF INTEGER FOR Index ← 1 TO 10 INPUT Marks[Index]

Hint: Type the last index so the loop reaches every element of the array.

5. FOR Index ← 1 TO 10 OUTPUT Marks[Index] NEXT Index

Hint: Type the keyword that ends the loop body and moves the counter on.

Spot the Error

Question 1

A FOR Index = 1 TO 10

B INPUT Marks[Index]

C NEXT Index

Line A — A FOR loop sets its counter with the assignment arrow ←, not =. It should be FOR Index ← 1 TO 10.

Question 2

A FOR Index ← 1 UNTIL 10

B OUTPUT Marks[Index]

C NEXT Index

Line A — The end value of a FOR loop follows the keyword TO, not UNTIL. UNTIL belongs to REPEAT loops. It should be FOR Index ← 1 TO 10.

Question 3

A Sum ← 0

B FOR Index ← 1 TO 10

C Sum ← Sum + Marks[1]

D NEXT Index

Line C — Marks[1] always picks the first element, so every pass adds the same mark. Use the loop counter: Marks[Index] so each element is visited in turn.

Question 4

A FOR Index ← 1 TO 10

B OUTPUT "Enter mark"

C INPUT Marks[Index]

D ENDFOR

Line D — In Cambridge pseudocode a FOR loop ends with NEXT and the counter name, not ENDFOR. It should be NEXT Index.

Question 5

A FOR Index ← 1 TO 10

B OUTPUT Marks(Index)

C NEXT Index

Line B — Array elements are picked with square brackets, not round brackets. It should be OUTPUT Marks[Index].

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