Tracing WHILE and REPEAT loops

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Quick Questions

Shade in the A or B circle for each question.

1. A trace of WHILE Count < 3 DO starts with Count = 5. How many times does the loop body run?

A 0 times, because the condition is tested first and is FALSE

B 1 time, because the body always runs once

2. A REPEAT loop ends with UNTIL Num > 0. The first test input is 7. How many times does the body run?

A Once, then the loop stops

B 0 times, because 7 already passes the test

3. When tracing a REPEAT ... UNTIL loop, where do you check the condition on each pass?

A After the body has run

B Before the body runs

4. REPEAT INPUT Num UNTIL Num >= 0 is traced with test inputs -3, -1, 6. How many values are read?

A 1, because the loop stops after the first input

B 3, because the loop stops only when 6 is entered

5. Trace WHILE Total < 10 DO Total ← Total + 4 ENDWHILE with Total starting at 0. What is Total when the loop ends?

A 10

B 12

Match the Pairs

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

Left column		Right column
1WHILE Total < 10 DO Total ← Total + 4 ENDWHILE		AThe student ran the body without testing first. X < 5 is false at the start (8 is not less than 5), so the body never runs and X stays 8.
2REPEAT INPUT Guess UNTIL Guess = 7		BREPEAT tests after the body, so the body always runs once. N becomes 11, and then N > 5 is true, so the loop stops after one pass.
3When tracing a loop that reads input, use the next test input on each pass and stop when the sentinel value is read.		CCount ← 1 WHILE Count < 5 DO OUTPUT Count ENDWHILE
4A loop whose condition never changes to end it will never stop, so the trace table keeps adding rows forever.		DThe body runs first and the condition Guess = 7 is tested AFTER it, so the loop runs at least once and stops when the condition is true.
5Trace of: X ← 8 WHILE X < 5 DO X ← X + 1 ENDWHILE Trace shows X = 9 after the loop		EINPUT Num WHILE Num <> -1 DO Total ← Total + Num INPUT Num ENDWHILE
6Trace of: N ← 10 REPEAT N ← N + 1 UNTIL N > 5 Trace shows the loop does not run		FThe condition Total < 10 is tested BEFORE each pass of the body, so the body may run zero times.

Definitions

WHILE and REPEAT loop trace

Fill in the Blanks

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

WHILE Count < 3 DO Count ← Count + 1 ________

REPEAT Count ← Count + 1 _____ Count = 3

Count ← 5 _____ Count < 3 DO OUTPUT Count

REPEAT INPUT _____ UNTIL Value = 0

WHILE Count < 3 DO Count ← _____ + 1 ENDWHILE

Spot the Error

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

Question 1

A Count ← 1

B WHILE Count <= 3

C OUTPUT Count

D Count ← Count + 1

E ENDWHILE

Question 2

A Total ← 0

B WHILE Total < 10 DO

C Total ← Total - 4

D ENDWHILE

Question 3

A Num ← 0

B WHILE Num < 20 DO

C Num ← Num * 2

D ENDWHILE

E OUTPUT Num

Question 4

A Tries ← 1

B REPEAT

C Tries ← Tries + 1

D INPUT Guess

E UNTIL Guess = 7

Question 5

A INPUT Mark

B WHILE Mark < 0 AND Mark > 100 DO

C OUTPUT "Try again"

D INPUT Mark

E ENDWHILE

Answer Key

Tracing WHILE and REPEAT loops

Section A: Quick Questions

A — 0 times, because the condition is tested first and is FALSE

A — Once, then the loop stops

A — After the body has run

B — 3, because the loop stops only when 6 is entered

B — 12

Section B: Match the Pairs

WHILE Total < 10 DO Total ← Total + 4 ENDWHILE → The condition Total < 10 is tested BEFORE each pass of the body, so the body may run zero times.

REPEAT INPUT Guess UNTIL Guess = 7 → The body runs first and the condition Guess = 7 is tested AFTER it, so the loop runs at least once and stops when the condition is true.

When tracing a loop that reads input, use the next test input on each pass and stop when the sentinel value is read. → INPUT Num WHILE Num <> -1 DO Total ← Total + Num INPUT Num ENDWHILE

A loop whose condition never changes to end it will never stop, so the trace table keeps adding rows forever. → Count ← 1 WHILE Count < 5 DO OUTPUT Count ENDWHILE

Trace of: X ← 8 WHILE X < 5 DO X ← X + 1 ENDWHILE Trace shows X = 9 after the loop → The student ran the body without testing first. X < 5 is false at the start (8 is not less than 5), so the body never runs and X stays 8.

Trace of: N ← 10 REPEAT N ← N + 1 UNTIL N > 5 Trace shows the loop does not run → REPEAT tests after the body, so the body always runs once. N becomes 11, and then N > 5 is true, so the loop stops after one pass.

Section C: Definitions

WHILE and REPEAT loop trace — Trace a loop by testing its condition each pass: WHILE tests before the body and may never run; REPEAT tests after the body so runs once at least. Stop when the loop ends.

Section D: Fill in the Blanks

WHILE Count < 3 DO
    Count ← Count + 1
ENDWHILE

REPEAT
    Count ← Count + 1
UNTIL Count = 3

Count ← 5
WHILE Count < 3 DO
    OUTPUT Count

REPEAT
    INPUT Value
UNTIL Value = 0

WHILE Count < 3 DO
    Count ← Count + 1
ENDWHILE

Section E: Spot the Error

B — A WHILE loop header needs the keyword DO after the condition. It should be WHILE Count <= 3 DO.

C — Total goes 0, -4, -8... and never reaches 10, so the condition Total < 10 is always true and the loop never ends. It should be Total ← Total + 4.

A — If Num starts at 0, then 0 * 2 is always 0. Num never reaches 20, so the loop never ends. It should start with Num ← 1.

A — The counter must start at 0. REPEAT runs the body once before testing, so Tries becomes 1 after the first pass. Starting at 1 would count one attempt too many.

B — A mark cannot be below 0 and above 100 at the same time, so the condition is always false. The WHILE loop never runs, even for invalid marks. It should use OR.