Error: loop that never ends

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Quick Questions

Shade in the A or B circle for each question.

1. Count ← 1 WHILE Count <= 5 DO OUTPUT Count ENDWHILE Why does this loop never end?

A OUTPUT cannot be used inside a WHILE loop

B Nothing in the body changes Count, so Count <= 5 is always TRUE

2. Which change stops this loop from running forever? Num ← 10 WHILE Num > 0 DO OUTPUT Num ENDWHILE

A Add Num ← Num - 1 inside the loop

B Add OUTPUT Num after ENDWHILE

3. Total ← 0 REPEAT INPUT Number Total ← Total + Number UNTIL Total < 0 The user only enters positive numbers. What is the problem?

A The UNTIL condition can never become TRUE, so the loop never ends

B REPEAT loops must always run exactly once

4. Which loop has the test in the wrong direction and so never ends?

A Count ← 1 WHILE Count <= 10 DO Count ← Count + 1 ENDWHILE

B Count ← 1 WHILE Count >= 1 DO Count ← Count + 1 ENDWHILE

5. A WHILE loop asks for a password: INPUT Password WHILE Password <> "abc123" DO OUTPUT "Try again" ENDWHILE What must be added to the body so it can end?

A INPUT Password

B OUTPUT Password

Match the Pairs

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

Left column		Right column
1WHILE Count <= 5 DO OUTPUT Count ENDWHILE		AWrong direction: Count goes up away from 0, so Count > 0 is always TRUE. Fix by using Count ← Count - 1.
2REPEAT INPUT Num UNTIL Num = 0 AND Num = 1		BInfinite loop: Count is never changed in the body, so Count <= 5 stays TRUE forever
3Adding an update to the loop body makes the tested variable move towards the exit value, so the loop ends		CThe UNTIL condition can never be TRUE because no number is both above 100 and below 0. Fix by using OR instead of AND.
4Putting INPUT inside the loop body lets the user change the tested variable so the exit value can be reached		DInfinite loop: the UNTIL condition can never be TRUE because Num cannot be 0 and 1 at once
5Count ← 10 WHILE Count > 0 DO Count ← Count + 1 ENDWHILE		EWHILE Answer <> "Q" DO OUTPUT "Enter Q to quit" INPUT Answer ENDWHILE
6REPEAT INPUT Score UNTIL Score > 100 AND Score < 0		FWHILE Count <= 5 DO OUTPUT Count Count ← Count + 1 ENDWHILE

Definitions

infinite loop

Fill in the Blanks

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

WHILE Count < 5 DO Count ← _____ + 1 ENDWHILE

WHILE Num _ 10 DO Num ← Num + 1 ENDWHILE

REPEAT Num ← Num - 1 UNTIL Num __ 0

WHILE Answer <> "yes" DO _____ Answer ENDWHILE

REPEAT INPUT Guess _____ Guess = 7

Spot the Error

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

Question 1

A Count ← 1

B WHILE Count <= 5 DO

C OUTPUT Count

D Count ← Count - 1

E ENDWHILE

Question 2

A Num ← 2

B WHILE Num > 0 DO

C OUTPUT Num

D Num ← Num + 2

E ENDWHILE

Question 3

A WHILE TRUE DO

B OUTPUT "Driving"

C Fuel ← Fuel - 5

D ENDWHILE

Question 4

A Count ← 0

B REPEAT

C Count ← 1

D OUTPUT Count

E UNTIL Count = 10

Question 5

A Score ← 0

B REPEAT

C Score ← Score + 5

D UNTIL Score = 102

Answer Key

Error: loop that never ends

Section A: Quick Questions

B — Nothing in the body changes Count, so Count <= 5 is always TRUE

A — Add Num ← Num - 1 inside the loop

A — The UNTIL condition can never become TRUE, so the loop never ends

B — Count ← 1 WHILE Count >= 1 DO Count ← Count + 1 ENDWHILE

A — INPUT Password

Section B: Match the Pairs

WHILE Count <= 5 DO OUTPUT Count ENDWHILE → Infinite loop: Count is never changed in the body, so Count <= 5 stays TRUE forever

REPEAT INPUT Num UNTIL Num = 0 AND Num = 1 → Infinite loop: the UNTIL condition can never be TRUE because Num cannot be 0 and 1 at once

Adding an update to the loop body makes the tested variable move towards the exit value, so the loop ends → WHILE Count <= 5 DO OUTPUT Count Count ← Count + 1 ENDWHILE

Putting INPUT inside the loop body lets the user change the tested variable so the exit value can be reached → WHILE Answer <> "Q" DO OUTPUT "Enter Q to quit" INPUT Answer ENDWHILE

Count ← 10 WHILE Count > 0 DO Count ← Count + 1 ENDWHILE → Wrong direction: Count goes up away from 0, so Count > 0 is always TRUE. Fix by using Count ← Count - 1.

REPEAT INPUT Score UNTIL Score > 100 AND Score < 0 → The UNTIL condition can never be TRUE because no number is both above 100 and below 0. Fix by using OR instead of AND.

Section C: Definitions

infinite loop — A loop that never ends because nothing in the body changes the tested variable, or the condition can never reach its exit value.

Section D: Fill in the Blanks

WHILE Count < 5 DO
    Count ← Count + 1
ENDWHILE

WHILE Num < 10 DO
    Num ← Num + 1
ENDWHILE

REPEAT
    Num ← Num - 1
UNTIL Num <= 0

WHILE Answer <> "yes" DO
    INPUT Answer
ENDWHILE

REPEAT
    INPUT Guess
UNTIL Guess = 7

Section E: Spot the Error

D — Count goes down, so it moves away from the exit value and Count <= 5 stays TRUE forever. The update must be Count ← Count + 1.

B — Num only increases, so Num > 0 is always TRUE and the loop never stops. The test points the wrong way. It should be WHILE Num < 20 DO.

A — The condition TRUE can never become FALSE, so the loop never ends whatever the body does. It should test the variable that changes: WHILE Fuel > 0 DO.

C — Count is reset to 1 on every pass, so it never reaches 10 and UNTIL Count = 10 is never TRUE. The body must add to Count: Count ← Count + 1.

D — Score goes up in steps of 5 (..., 100, 105), so it never equals 102. The UNTIL condition never becomes TRUE. It should be UNTIL Score = 100.