How many times a FOR loop runs

Name:

Date:

Class:

Quick Questions

Shade in the A or B circle for each question.

1. How many times does FOR i ← 1 TO 5 run its loop body?

A 5 times

B 4 times

2. Which formula gives the number of times a FOR loop runs?

A end - start

B end - start + 1

3. FOR i ← 3 TO 3 ... NEXT i. What happens?

A The loop body never runs

B The loop body runs exactly once

4. FOR i ← 5 TO 1 ... NEXT i. What happens?

A The loop body runs zero times

B The loop body runs 5 times, counting down

5. A student wants a loop to run 10 times. Which loop header is correct?

A FOR i ← 1 TO 10

B FOR i ← 1 TO 9

Match the Pairs

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

Left column		Right column
1FOR i ← 1 TO 5		AFOR Count ← 4 TO 11
2FOR i ← 5 TO 1		BThe loop body runs zero times, because the start value is bigger than the end value
3When the start and end values are equal, the loop runs exactly once		COff-by-one error: 1 TO 10 runs 10 times, not 9. The count was worked out as end - start and the + 1 was forgotten.
4The number of repetitions is end - start + 1, so this loop runs 8 times		DThe loop body runs 5 times, because 5 - 1 + 1 = 5
5FOR i ← 1 TO 10 // meant to run 9 times		EFOR i ← 3 TO 3
6FOR i ← 10 TO 1 // meant to run 10 times		FThe start value is bigger than the end value, so the loop runs zero times instead of 10.

Definitions

end - start + 1

Fill in the Blanks

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

FOR i ← 1 TO _ OUTPUT i NEXT i

FOR i ← 3 TO _ OUTPUT i NEXT i

FOR i ← 5 TO 1 // runs _ times OUTPUT i NEXT i

FOR i ← 3 TO 8 // runs 8 - 3 + _ times OUTPUT i NEXT i

FOR i ← 0 TO _ // runs 10 times OUTPUT i NEXT i

Spot the Error

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

Question 1

A Count ← 0

B FOR i ← 1 TO 5

C Count = Count + 1

D NEXT i

E OUTPUT Count

Question 2

A // Runs once

B FOR i ← 3 TO 2

C OUTPUT i

D NEXT i

Question 3

A FOR i ← 1 TO 4

B OUTPUT "Hello"

C NEXT i

D // Hello is shown 5 times

Question 4

A FOR i ← 3 TO 1

B OUTPUT i * 2

C NEXT i

D // Output: 2 4 6

Question 5

A Runs ← 0

B FOR i ← 2 TO 6

C Runs ← Runs + 1

D NEXT Runs

E OUTPUT Runs

Answer Key

How many times a FOR loop runs

Section A: Quick Questions

A — 5 times

B — end - start + 1

B — The loop body runs exactly once

A — The loop body runs zero times

A — FOR i ← 1 TO 10

Section B: Match the Pairs

FOR i ← 1 TO 5 → The loop body runs 5 times, because 5 - 1 + 1 = 5

FOR i ← 5 TO 1 → The loop body runs zero times, because the start value is bigger than the end value

When the start and end values are equal, the loop runs exactly once → FOR i ← 3 TO 3

The number of repetitions is end - start + 1, so this loop runs 8 times → FOR Count ← 4 TO 11

FOR i ← 1 TO 10 // meant to run 9 times → Off-by-one error: 1 TO 10 runs 10 times, not 9. The count was worked out as end - start and the + 1 was forgotten.

FOR i ← 10 TO 1 // meant to run 10 times → The start value is bigger than the end value, so the loop runs zero times instead of 10.

Section C: Definitions

end - start + 1 — The number of times a FOR loop runs. FOR i ← 1 TO 5 runs 5 times, 3 TO 3 runs once, and 5 TO 1 runs zero times because the start value is bigger than the end value.

Section D: Fill in the Blanks

FOR i ← 1 TO 5
    OUTPUT i
NEXT i

FOR i ← 3 TO 3
    OUTPUT i
NEXT i

FOR i ← 5 TO 1 // runs 0 times
    OUTPUT i
NEXT i

FOR i ← 3 TO 8 // runs 8 - 3 + 1 times
    OUTPUT i
NEXT i

FOR i ← 0 TO 9 // runs 10 times
    OUTPUT i
NEXT i

Section E: Spot the Error

C — Assignment uses the arrow ←, not =. The line should be Count ← Count + 1, so Count goes up by 1 on each of the 5 runs of the loop.

B — The loop should run exactly once, which happens when the start and end values are equal. It should be FOR i ← 3 TO 3. With 3 TO 2 the start is bigger than the end, so the loop runs zero times.

A — This is an off-by-one mistake. 1 TO 4 runs 4 - 1 + 1 = 4 times. To run 5 times the line must be FOR i ← 1 TO 5.

A — The start value 3 is bigger than the end value 1, so the loop would run zero times and print nothing. It should be FOR i ← 1 TO 3, which runs 3 times.

D — NEXT must name the loop variable used in the FOR line. It should be NEXT i. The loop runs 6 - 2 + 1 = 5 times, so Runs ends as 5.