← IGCSE Computer Science Paper 2

Looping through an array with FOR (7c)

Uses the loop counter Index as the array index, so Names[Index] visits every element in turn. Each one can be read, changed, input, output or totalled.

Part of: IGCSE Computer Science 0478 — 7c Pseudocode and Flowcharts — Module 40

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

What it does: INPUT Names[Index] stores each entered value in the element at the current counter value, so all 30 elements are filled. Names[1] never changes, so it would output the first name 30 times. Names[Index] moves along the array. Index is only the position number. To total the marks you must add the element Marks[Index]. The loop must stop at the last element. Going up to 30 would try to use Scores[21] and beyond, which do not exist.

Common mistakes: A FOR loop sets its counter with the assignment arrow ←, not =. It should be FOR Index ← 1 TO 30. NEXT must name the loop counter that the FOR line uses. It should be NEXT Index, not the array name.

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This module's key terms are also in a printable crossword (Modules 176–187) and word search (Modules 166–187).

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Learning

Duo

Names is an array with 30 elements, numbered 1 to 30. Which loop header visits every element once?

FOR Index ← 1 TO 30FOR Index ← 0 TO 30

Matches

FOR Index ← 1 TO 30 OUTPUT Names[30] NEXT Index

↓

The index is fixed at 30 instead of using the counter Index, so the loop outputs the last element 30 times and never visits the others.

Applying

Meanings

Term

FOR Index ← 1 TO 30

Meaning

Uses the loop counter Index as the array index, so Names[Index] visits every element in turn. Each one can be read, changed, input, output or totalled.

Practice

FOR Index ← 1 TO 30

Testing

Gaps

Names[Index] ""

Check Answer

Spots

1FOR Index = 1 TO 30

2 INPUT Names[Index]

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Quick Questions

1. Names is an array with 30 elements, numbered 1 to 30. Which loop header visits every element once?

A. FOR Index ← 1 TO 30 ✓

B. FOR Index ← 0 TO 30

The array runs from 1 to 30, so the counter must start at 1 and stop at 30. There is no element Names[0].

2. Which line inside the loop sets EVERY element of Names to an empty string?

A. Names[Index] ← "" ✓

B. Names[30] ← ""

Index changes on each pass, so Names[Index] is a different element each time. Names[30] only changes the last element, every time.

3. Which line inside the loop stores a typed name in each element of Names?

A. INPUT Names

B. INPUT Names[Index] ✓

INPUT needs one element to store the value. Names[Index] picks the next element on each pass of the loop.

4. Which loop outputs all 30 names, one after another?

A. FOR Index ← 1 TO 30 OUTPUT Names[1] NEXT Index

B. FOR Index ← 1 TO 30 OUTPUT Names[Index] NEXT Index ✓

Names[1] never changes, so it would output the first name 30 times. Names[Index] moves along the array.

5. Marks has 30 elements. Which line adds each mark to Total as the loop runs?

A. Total ← Total + Marks[Index] ✓

B. Total ← Total + Index

Index is only the position number. To total the marks you must add the element Marks[Index].

Match the Pairs

FOR Index ← 1 TO 30 Names[Index] ← "" NEXT Index → Sets every element of the Names array to an empty string, one element at a time

FOR Index ← 1 TO 30 OUTPUT Names[Index] NEXT Index → Displays every element of the Names array in turn, from element 1 to element 30

A FOR loop can store a value typed by the user in every element of an array → FOR Index ← 1 TO 30 INPUT Names[Index] NEXT Index

A FOR loop can add up every element of an array by using the counter as the index → FOR Index ← 1 TO 30 Total ← Total + Marks[Index] NEXT Index

FOR Index ← 1 TO 30 OUTPUT Names[30] NEXT Index → The index is fixed at 30 instead of using the counter Index, so the loop outputs the last element 30 times and never visits the others.

FOR Index ← 0 TO 30 INPUT Names[Index] NEXT Index → The loop bounds do not match the array. The array runs from 1 to 30, so starting at 0 refers to an element that does not exist and the loop runs 31 times.

Key Term

FOR Index ← 1 TO 30

Uses the loop counter Index as the array index, so Names[Index] visits every element in turn. Each one can be read, changed, input, output or totalled.

Fill in the Blanks

1. FOR Index ← 1 TO 30 Names[Index] ← "" NEXT Index

Hint: Type the assignment symbol that sets each element to an empty string.

2. FOR Index ← 1 TO 30 INPUT Names[Index] NEXT Index

Hint: Type the loop counter that is used as the array position.

3. FOR Index ← 1 TO 30 OUTPUT Names[Index] NEXT Index

Hint: Type the keyword that separates the start value from the end value of the loop.

4. FOR Index ← 1 TO 30 Total ← Total + Scores[Index] NEXT Index

Hint: Type the arithmetic operator that adds each element to the running total.

5. FOR Index ← 1 TO 30 OUTPUT Names[Index] NEXT Index

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

Spot the Error

Question 1

A FOR Index = 1 TO 30

B INPUT Names[Index]

C NEXT Index

Line A — A FOR loop sets its counter with the assignment arrow ←, not =. It should be FOR Index ← 1 TO 30.

Question 2

A DECLARE Names : ARRAY[1:30] OF STRING

B FOR Index ← 1 TO 30

C Names[Index] ← ""

D NEXT Names

Line D — NEXT must name the loop counter that the FOR line uses. It should be NEXT Index, not the array name.

Question 3

A DECLARE Sum : INTEGER

B Sum ← 1

C FOR Index ← 1 TO 30

D Sum ← Sum + Marks[Index]

E NEXT Index

Line B — The total must start at 0 before the loop. Starting at 1 makes the final answer one too big. It should be Sum ← 0.

Question 4

A FOR Index ← 0 TO 30

B OUTPUT Names[Index]

C NEXT Index

Line A — The array runs from index 1 to 30, so there is no element Names[0]. The loop must start at 1: FOR Index ← 1 TO 30.

Question 5

A FOR Index ← 1 TO 30

B INPUT Names

C OUTPUT Names[Index]

D NEXT Index

Line B — Without [Index] the loop does not pick out one element. To store each input in its own element, use INPUT Names[Index].

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