Tracing a loop in a trace table

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Quick Questions

Shade in the A or B circle for each question.

1. You are tracing a WHILE loop. When do you test the condition?

A Before each pass of the body

B After each pass of the body

2. Count is 10 before this loop: REPEAT Count ← Count + 1 UNTIL Count > 5. How many times does the body run?

A Zero times

B Once

3. Count is 7 before this loop: WHILE Count < 5 DO. How many times does the body run?

A Zero times

B Once

4. For FOR Counter ← 2 TO 5, which values does Counter take in the trace table?

A 2, 3, 4

B 2, 3, 4, 5

5. Which is the correct way to fill in a trace table for a loop?

A Add a new row on each pass showing the changed values

B Write one row at the end showing only the final values

Match the Pairs

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

Left column		Right column
1WHILE Count < 3 DO		AThe condition is tested before the body, so the loop may run zero times. Check it before every pass.
2UNTIL Total >= 10		BTotal ← Total + Count
3A FOR loop trace has one row per pass, with the counter taking each value from start to end		CFOR Count ← 1 TO 4
4Record a changed value in a new row each pass, such as a running total going up		DThe condition is tested after the body, so the loop always runs at least once. Stop when it becomes true.
5Trace of REPEAT ... UNTIL Count = 3: test Count = 3 before running the body the first time		EA REPEAT loop tests its condition after the body, not before. The body must run once first.
6Trace of FOR Count ← 1 TO 3: stop the table after the row where Count = 2		FThe final row was not checked. The loop runs for Count = 3 too, so the last row is missing.

Definitions

Tracing a loop

Fill in the Blanks

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

FOR Count ← 1 __ 4 OUTPUT Count NEXT Count

_____ Num < 3 DO Num ← Num + 1 ENDWHILE

REPEAT Num ← Num + 2 _____ Num >= 7

FOR Count ← 1 TO 3 Total ← _____ + Count NEXT Count

WHILE Num < 10 DO Num ← Num _ 2 ENDWHILE

Spot the Error

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

Question 1

A Total ← 0

B FOR Count ← 4 TO 1

C Total ← Total + Count

D NEXT Count

E OUTPUT Total

Question 2

A WHILE Num < 3 THEN

B Num ← Num + 1

C OUTPUT Num

D ENDWHILE

Question 3

A Count ← 0

B REPEAT

C Count ← Count + 0

D UNTIL Count = 3

Question 4

A Sum ← 0

B FOR Item ← 1 TO 3

C Sum ← Sum + Item

D NEXT Sum

E OUTPUT Sum

Question 5

A Count ← 3

B WHILE Count < 0 DO

C Count ← Count - 1

D ENDWHILE

Answer Key

Tracing a loop in a trace table

Section A: Quick Questions

A — Before each pass of the body

B — Once

A — Zero times

B — 2, 3, 4, 5

A — Add a new row on each pass showing the changed values

Section B: Match the Pairs

WHILE Count < 3 DO → The condition is tested before the body, so the loop may run zero times. Check it before every pass.

UNTIL Total >= 10 → The condition is tested after the body, so the loop always runs at least once. Stop when it becomes true.

A FOR loop trace has one row per pass, with the counter taking each value from start to end → FOR Count ← 1 TO 4

Record a changed value in a new row each pass, such as a running total going up → Total ← Total + Count

Trace of REPEAT ... UNTIL Count = 3: test Count = 3 before running the body the first time → A REPEAT loop tests its condition after the body, not before. The body must run once first.

Trace of FOR Count ← 1 TO 3: stop the table after the row where Count = 2 → The final row was not checked. The loop runs for Count = 3 too, so the last row is missing.

Section C: Definitions

Tracing a loop — Work through the loop body once per pass, recording changed values in a new row each time. Test the condition before the body for WHILE and after it for REPEAT. Check the final row.

Section D: Fill in the Blanks

FOR Count ← 1 TO 4
  OUTPUT Count
NEXT Count

WHILE Num < 3 DO
  Num ← Num + 1
ENDWHILE

REPEAT
  Num ← Num + 2
UNTIL Num >= 7

FOR Count ← 1 TO 3
  Total ← Total + Count
NEXT Count

WHILE Num < 10 DO
  Num ← Num * 2
ENDWHILE

Section E: Spot the Error

B — Count starts at 4 and the end value is 1, so with the default step of +1 the body never runs and the trace table has no loop rows. It should be FOR Count ← 1 TO 4, which gives Count values 1, 2, 3, 4.

A — A WHILE loop header ends with DO, not THEN (THEN belongs to IF). The condition is tested before each pass of the body, so the line should be WHILE Num < 3 DO.

C — Adding 0 means Count never changes, so every row of the trace table repeats 0 and the UNTIL test is never true. The loop never ends. It should be Count ← Count + 1.

D — NEXT must name the loop counter, which is Item. Sum is the running total, not the counter. It should be NEXT Item.

B — A WHILE loop tests its condition before the body. Count starts at 3, so 3 < 0 is false and the body never runs, leaving no loop rows in the trace table. Since the body counts down, the condition should be Count > 0.