Using a variable as an array index

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Quick Questions

Shade in the A or B circle for each question.

1. Index ← 3 then OUTPUT Marks[Index]. Which element is displayed?

A Marks[Index], because the name Index is used as the position

B Marks[3], because Index holds 3

2. The same statement OUTPUT Marks[Index] runs twice. Index is 2 the first time and 5 the second time. What happens?

A It displays Marks[2] and then Marks[5]

B It displays Marks[2] both times

3. Which declaration suits a variable used as an array index?

A DECLARE Index : INTEGER

B DECLARE Index : STRING

4. Which line stores the value 72 in the element at the position held by Index?

A Index ← Marks[72]

B Marks[Index] ← 72

5. Index ← 4. Which line copies the element at that position into Score?

A Score ← Marks[Index]

B Marks[Index] ← Score

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1Index ← 3 OUTPUT Marks[Index]		AStores 50 in whichever element Index currently points to
2Marks[Index] ← 50		BIndex ← 2 OUTPUT Marks[Index] Index ← 5 OUTPUT Marks[Index]
3Changing the value of the index variable makes the same statement reach a different element		CIndex is in quotation marks, so it is treated as text, not as the variable holding 4. It should be Marks[Index].
4The variable used as an index must be an INTEGER		DDisplays the value stored in element 3 of Marks, because Index currently holds 3
5DECLARE Index : STRING Index ← 3 OUTPUT Marks[Index]		EDECLARE Index : INTEGER
6Index ← 4 OUTPUT Marks["Index"]		FIndex is declared as STRING, but a variable used as an array index must be an INTEGER whole number.

Definitions

Marks[Index]

Fill in the Blanks

Complete each line of code by filling in the missing parts.

Index ← 3 OUTPUT Marks[_____]

DECLARE Index : _______ Index ← 3 Marks[Index] ← 85

Index ← 3 Index ← Index _ 1 OUTPUT Marks[Index]

Index ← 3 Marks[Index] _ 85

Index ← 5 ______ Marks[Index]

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A DECLARE Index : STRING

B Index ← 3

C Marks[Index] ← 75

D OUTPUT Marks[Index]

Question 2

A Index ← 2

B Marks[Index] ← 60

C Marks(Index + 1) ← 70

D OUTPUT Marks[Index + 1]

Question 3

A Index ← 4

B Total ← Marks[Idx]

C OUTPUT Total

Question 4

A Index ← 2

B Marks[Index] ← 77

C Index ← 3

D Marks["Index"] ← 64

Question 5

A DECLARE Pos : INTEGER

B Pos ← 2.5

C Marks[Pos] ← 90

D OUTPUT Marks[Pos]

Answer Key

Using a variable as an array index

Section A: Quick Questions

B — Marks[3], because Index holds 3

A — It displays Marks[2] and then Marks[5]

A — DECLARE Index : INTEGER

B — Marks[Index] ← 72

A — Score ← Marks[Index]

Section B: Match the Pairs

Index ← 3 OUTPUT Marks[Index] → Displays the value stored in element 3 of Marks, because Index currently holds 3

Marks[Index] ← 50 → Stores 50 in whichever element Index currently points to

Changing the value of the index variable makes the same statement reach a different element → Index ← 2 OUTPUT Marks[Index] Index ← 5 OUTPUT Marks[Index]

The variable used as an index must be an INTEGER → DECLARE Index : INTEGER

DECLARE Index : STRING Index ← 3 OUTPUT Marks[Index] → Index is declared as STRING, but a variable used as an array index must be an INTEGER whole number.

Index ← 4 OUTPUT Marks["Index"] → Index is in quotation marks, so it is treated as text, not as the variable holding 4. It should be Marks[Index].

Section C: Definitions

Marks[Index] — A variable inside the brackets picks the element whose index matches its value. Change Index (an INTEGER) and the same statement reaches a different element of Marks.

Section D: Fill in the Blanks

Index ← 3
OUTPUT Marks[Index]

DECLARE Index : INTEGER
Index ← 3
Marks[Index] ← 85

Index ← 3
Index ← Index + 1
OUTPUT Marks[Index]

Index ← 3
Marks[Index] ← 85

Index ← 5
OUTPUT Marks[Index]

Section E: Spot the Error

A — A variable used as an array index must be an INTEGER. Index is declared as STRING here. It should be DECLARE Index : INTEGER.

C — Array indexes go inside square brackets, not round brackets. It should be Marks[Index + 1] ← 70.

B — The index variable is called Index, but Idx has never been given a value. The brackets must use the same variable name: Total ← Marks[Index].

D — Quotes turn Index into a piece of text, not the variable. The variable name must be written without quotes: Marks[Index] ← 64.

B — Pos is an INTEGER and is used as an index, so it must be a whole number. An index such as 2.5 does not exist. It should be Pos ← 2.