← IGCSE Computer Science Paper 2

Parallel arrays

Two or more arrays of the same size where the same index holds related data, e.g. StudentNames[3] and Marks[3] both describe the same student.

Part of: IGCSE Computer Science 0478 — 8.2 Arrays — Module 24

Understanding Parallel arrays in IGCSE Computer Science Paper 2

What it does: The single loop variable Index fills the name and the mark for the same student on each pass. Parallel arrays are separate 1D arrays linked by index. A grid of rows and columns is a 2D array. A parallel array to a 30-element StudentNames array must also have bounds 1 to 30, but it can hold a different data type.

Common mistakes: Parallel arrays must use the same index for the same student. Marks[I+1] shows the next student's mark next to this name. It should be Marks[I]. Both arrays hold 5 items, so the loop must stop at 5. Going to 6 uses an index that does not exist in either array. It should be FOR I ← 1 TO 5.

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This module's key terms are also in a printable crossword (Modules 459–468) and word search (Modules 459–478).

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Learning

Duo

StudentNames[4] holds "Mia". Which element of Marks holds Mia's mark if the arrays are parallel?

Marks[4]Marks[5]

Matches

DECLARE StudentNames : ARRAY[1:30] OF STRING DECLARE Marks : ARRAY[1:25] OF INTEGER

↓

The arrays have different lengths (30 and 25), so some students would have no matching mark and the indexes would not line up.

Applying

Meanings

Term

parallel arrays

Meaning

Two or more arrays of the same size where the same index holds related data, e.g. StudentNames[3] and Marks[3] both describe the same student.

Practice

DECLARE StudentNames : ARRAY[1:3] OF STRING

Testing

Gaps

DECLARE StudentNames : ARRAY[1:5] OF

Check Answer

Spots

3 INPUT Marks[I]

4 OUTPUT Names[I], Marks[I+1]

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Quick Questions

1. StudentNames[4] holds "Mia". Which element of Marks holds Mia's mark if the arrays are parallel?

A. Marks[5]

B. Marks[4] ✓

In parallel arrays the same index refers to the same item, so Mia's name and mark are both at index 4.

2. Which statement describes parallel arrays?

A. Two arrays of the same size where the same index holds related data ✓

B. One array whose elements are stored in two rows and two columns

Parallel arrays are separate 1D arrays linked by index. A grid of rows and columns is a 2D array.

3. Which loop reads a name and a mark for each student into parallel arrays (indexes 1 to 30)?

A. FOR Index ← 1 TO 30 INPUT StudentNames[Index] INPUT Marks[Index + 1] NEXT Index

B. FOR Index ← 1 TO 30 INPUT StudentNames[Index] INPUT Marks[Index] NEXT Index ✓

One loop can fill both arrays, but it must use the same index for both so the related values stay together.

4. A school stores names in StudentNames and marks in Marks. Which pair of declarations suits parallel arrays?

A. ARRAY[1:30] OF STRING and ARRAY[1:30] OF INTEGER ✓

B. ARRAY[1:30] OF STRING and ARRAY[1:20] OF INTEGER

Parallel arrays must have the same length so every index in one array has a matching element in the other. They can have different data types.

5. Can parallel arrays store different data types, such as STRING names and INTEGER marks?

A. Yes, each array has one type but the arrays can differ ✓

B. No, all parallel arrays must use the same data type

Each array holds one data type, but separate arrays can use different types, which lets them store one record's details.

Match the Pairs

OUTPUT StudentNames[3], Marks[3] → Displays the name and the mark of the same student, because index 3 is used in both parallel arrays.

DECLARE Marks : ARRAY[1:30] OF INTEGER → Declares an INTEGER array with 30 elements that can sit beside a 30-element names array, so each index matches one student.

One loop can read both parallel arrays, because the same index refers to the same student in each array. → FOR Index ← 1 TO 30 INPUT StudentNames[Index] INPUT Marks[Index] NEXT Index

Parallel arrays can hold different data types for one record, such as STRING names and INTEGER marks. → DECLARE StudentNames : ARRAY[1:30] OF STRING DECLARE Marks : ARRAY[1:30] OF INTEGER

DECLARE StudentNames : ARRAY[1:30] OF STRING DECLARE Marks : ARRAY[1:25] OF INTEGER → The arrays have different lengths (30 and 25), so some students would have no matching mark and the indexes would not line up.

OUTPUT StudentNames[3], Marks[4] → Different indexes are used, so the name of student 3 is shown with the mark of student 4, which are not related.

Key Term

parallel arrays

Two or more arrays of the same size where the same index holds related data, e.g. StudentNames[3] and Marks[3] both describe the same student.

Fill in the Blanks

1. DECLARE StudentNames : ARRAY[1:5] OF STRING

Hint: Type the data type that suits an array of student names.

2. DECLARE Marks : ARRAY[1:5] OF INTEGER

Hint: Type the data type that suits whole-number marks.

3. OUTPUT StudentNames[Index], Marks[Index]

Hint: Type the variable that picks the same position as in the first array.

4. DECLARE StudentNames : ARRAY[1:30] OF STRING DECLARE Marks : ARRAY[1:30] OF INTEGER

Hint: Type the upper bound that keeps both arrays the same length.

5. FOR Index ← 1 TO 30 OUTPUT StudentNames[Index], Marks[Index] NEXT Index

Hint: Type the loop variable that closes the FOR loop.

Spot the Error

Question 1

A FOR I ← 1 TO 3

B INPUT Names[I]

C INPUT Marks[I]

D OUTPUT Names[I], Marks[I+1]

E NEXT I

Line D — Parallel arrays must use the same index for the same student. Marks[I+1] shows the next student's mark next to this name. It should be Marks[I].

Question 2

A // 5 students in each array

B FOR I ← 1 TO 6

C OUTPUT Names[I]

D OUTPUT Marks[I]

E NEXT I

Line B — Both arrays hold 5 items, so the loop must stop at 5. Going to 6 uses an index that does not exist in either array. It should be FOR I ← 1 TO 5.

Question 3

A Names[1] ← "Ava"

B Marks[1] ← 88

C Passed[1] ← "TRUE"

Line C — Passed is a BOOLEAN array, so it can only store TRUE or FALSE, not a STRING. The quotes must be removed: Passed[1] ← TRUE.

Question 4

A FOR I ← 1 TO 3

B OUTPUT Names[I]

C OUTPUT Marks[I]

D NEXT J

Line D — The loop counter is I, so the loop must end with NEXT I. NEXT J names a variable that is not the loop counter, so the same index would not be used for both arrays.

Question 5

A FOR I ← 1 TO 5

B IF Names[I] = "Zoe" THEN

C OUTPUT Names[I]

D ENDIF

E NEXT I

Line C — Once Zoe is found at index I, her mark is in the parallel array at the same index. It should output Marks[I], not Names[I], which just repeats her name.

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