Swapping two values with a temporary variable

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Quick Questions

Shade in the A or B circle for each question.

1. Which set of lines correctly swaps Marks[Index] and Marks[Index + 1]?

A Temp ← Marks[Index] Marks[Index] ← Marks[Index + 1] Marks[Index + 1] ← Temp

B Marks[Index] ← Marks[Index + 1] Marks[Index + 1] ← Marks[Index]

2. Why do we store a value in Temp before swapping two array elements?

A So the first value is not lost when it is overwritten

B So the array becomes sorted automatically

3. Marks[3] is 40 and Marks[4] is 70. You run Marks[3] ← Marks[4] and then Marks[4] ← Marks[3]. What are the values now?

A Both are 70

B Marks[3] is 70 and Marks[4] is 40

4. Which line should come first in a swap of Marks[Index] and Marks[Index + 1]?

A Marks[Index + 1] ← Temp

B Temp ← Marks[Index]

5. What does the line Marks[Index + 1] ← Temp do at the end of a swap?

A Saves the value of Marks[Index + 1] into Temp

B Puts the saved original value of Marks[Index] into Marks[Index + 1]

Match the Pairs

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

Left column		Right column
1Temp ← Marks[Index]		ACopies the saved value from Temp into the next element, finishing the swap
2Marks[Index + 1] ← Temp		BTemp is assigned after Marks[Index] has already been overwritten, so Temp saves the wrong value and the original is lost.
3The middle step of a swap copies the next element into the current one, which is safe because the old value is already in Temp		CMarks[Index] ← Marks[Index + 1]
4A complete swap of two neighbouring array elements uses three assignments in the correct order		DSaves the value in Marks[Index] in Temp so it is not lost when that element is overwritten
5Marks[Index] ← Marks[Index + 1] Marks[Index + 1] ← Marks[Index]		ETemp ← Marks[Index] Marks[Index] ← Marks[Index + 1] Marks[Index + 1] ← Temp
6Marks[Index] ← Marks[Index + 1] Temp ← Marks[Index] Marks[Index + 1] ← Temp		FThere is no temporary variable. After the first line both elements hold the same value, because the original Marks[Index] was overwritten and lost.

Definitions

Temp ← Marks[Index]

Fill in the Blanks

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

____ ← Marks[Index] Marks[Index] ← Marks[Index + 1] Marks[Index + 1] ← Temp

Temp ← Marks[_____] Marks[Index] ← Marks[Index + 1] Marks[Index + 1] ← Temp

Temp ← Marks[Index] Marks[Index] ← Marks[Index + 1] Marks[Index + 1] ← ____

Temp _ Marks[Index] Marks[Index] ← Marks[Index + 1] Marks[Index + 1] ← Temp

Temp ← Marks[Index] Marks[Index] ← Marks[_________] Marks[Index + 1] ← Temp

Spot the Error

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

Question 1

A Temp ← Marks[Index+1]

B Marks[Index] ← Marks[Index+1]

C Marks[Index+1] ← Temp

Question 2

A Marks[1] ← 40

B Marks[2] ← 75

C Temp ← Marks[1]

D Marks[2] ← Marks[1]

E Marks[2] ← Temp

Question 3

A Temp ← Marks[Index]

B Marks[Index] ← Marks[Index+1]

C Marks[Index+1] ← Marks[Index]

Question 4

A DECLARE Temp : INTEGER

B Temp ← Marks[3]

C Marks[3] = Marks[4]

D Marks[4] ← Temp

Question 5

A IF Marks[1] > Marks[2] THEN

B Marks[1] ← Temp

C Marks[1] ← Marks[2]

D Marks[2] ← Temp

E ENDIF

Answer Key

Swapping two values with a temporary variable

Section A: Quick Questions

A — Temp ← Marks[Index] Marks[Index] ← Marks[Index + 1] Marks[Index + 1] ← Temp

A — So the first value is not lost when it is overwritten

A — Both are 70

B — Temp ← Marks[Index]

B — Puts the saved original value of Marks[Index] into Marks[Index + 1]

Section B: Match the Pairs

Temp ← Marks[Index] → Saves the value in Marks[Index] in Temp so it is not lost when that element is overwritten

Marks[Index + 1] ← Temp → Copies the saved value from Temp into the next element, finishing the swap

The middle step of a swap copies the next element into the current one, which is safe because the old value is already in Temp → Marks[Index] ← Marks[Index + 1]

A complete swap of two neighbouring array elements uses three assignments in the correct order → Temp ← Marks[Index] Marks[Index] ← Marks[Index + 1] Marks[Index + 1] ← Temp

Marks[Index] ← Marks[Index + 1] Marks[Index + 1] ← Marks[Index] → There is no temporary variable. After the first line both elements hold the same value, because the original Marks[Index] was overwritten and lost.

Marks[Index] ← Marks[Index + 1] Temp ← Marks[Index] Marks[Index + 1] ← Temp → Temp is assigned after Marks[Index] has already been overwritten, so Temp saves the wrong value and the original is lost.

Section C: Definitions

Temp ← Marks[Index] — Saves the value of Marks[Index] in Temp so it is not lost. Marks[Index] can then be overwritten, and Temp is copied into Marks[Index + 1] to finish the swap.

Section D: Fill in the Blanks

Temp ← Marks[Index]
Marks[Index] ← Marks[Index + 1]
Marks[Index + 1] ← Temp

Temp ← Marks[Index]
Marks[Index] ← Marks[Index + 1]
Marks[Index + 1] ← Temp

Temp ← Marks[Index]
Marks[Index] ← Marks[Index + 1]
Marks[Index + 1] ← Temp

Temp ← Marks[Index]
Marks[Index] ← Marks[Index + 1]
Marks[Index + 1] ← Temp

Temp ← Marks[Index]
Marks[Index] ← Marks[Index + 1]
Marks[Index + 1] ← Temp

Section E: Spot the Error

A — Temp must save Marks[Index], the value that is about to be overwritten. Saving Marks[Index+1] means the original Marks[Index] is lost. It should be Temp ← Marks[Index].

D — The second element is overwritten with Marks[1] when it should be Marks[1] that is overwritten with Marks[2]. Both elements would end up as 40. It should be Marks[1] ← Marks[2].

C — Marks[Index] has already been overwritten on line 2, so copying it again gives the same value twice. The original value is in Temp. It should be Marks[Index+1] ← Temp.

C — The = sign is used for comparison, not for assignment. Cambridge pseudocode uses ← to store a value. It should be Marks[3] ← Marks[4].

B — Temp has not been given a value yet, so this overwrites Marks[1] and loses it. The first step must be to save Marks[1] in Temp. It should be Temp ← Marks[1].