Infinite loops and updating the loop variable
Name:
Date:
Class:
A
Quick Questions
Shade in the A or B circle for each question.
1. let count = 0;
while (count < 5) {
console.log(count);
}
What happens when this loop runs?
2. Which loop body lets the condition count < 5 eventually become false?
3. True or false: A while loop stops by itself after a few turns, even if its variable never changes.
4. Which loop is an infinite loop?
5. let n = 10;
while (n > 0) {
n++;
}
Why does this loop never stop?
B
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) { count++; } | Alet lives = 3; while (lives > 0) { lives--; } | |
| 2while (true) { console.log("Hi"); } | Blet n = 1; while (n > 0) { n++; } | |
| 3To make a loop stop, update the variable that the condition tests inside the loop body. | CThe loop stops because count++ changes count each time, so count < 5 eventually becomes false. | |
| 4A condition that can never become false makes the loop run forever, even if the variable is updated. | Dcount goes up, moving away from 0, so count > 0 is always true. Use count-- so it counts down to 0. | |
| 5let count = 0; while (count < 5) { console.log(count); } | EThe loop never changes count, so count < 5 stays true forever. Add count++; inside the loop body. | |
| 6let count = 10; while (count > 0) { count++; } | FAn infinite loop: the condition is always true and nothing in the body can make it false. |
C
Definitions
while (count < 5) { count++; }
D
Fill in the Blanks
Complete each line of code by filling in the missing parts.
1.
while (count < 5) { _______; }
2.
let lives = 3;
while (lives > 0) { lives__; }
3.
while (____) { console.log("Hi"); }
4.
let n = 0;
while (_ < 3) { n++; }
5.
while (true) { if (n > 4) _____; n++; }
E
Spot the Error
Shade in the letter of the line that contains an error.
Question 1
A
for (let i = 0; i < 5; i--) {
B
console.log(i);
C
}
Question 2
A
let count = 0;
B
while (count >= 0) {
C
count++;
D
}
Question 3
A
let n = 10;
B
while (n < 20) {
C
n--;
D
}
Question 4
A
let count = 0;
B
while (count < 3) {
C
console.log("Hi");
D
count--;
E
}
Question 5
A
for (let i = 5; i > 0; ) {
B
console.log(i);
C
}
Answer Key
Infinite loops and updating the loop variable
Section A: Quick Questions
Q1
B — It never stops, because count is never changed
Q2
A — console.log(count); count++;
Q3
A — False
Q4
B — while (true) { console.log("Hi"); }
Q5
A — n++ makes n bigger, so n > 0 stays true
Section B: Match the Pairs
Q1
while (count < 5) { count++; } → The loop stops because count++ changes count each time, so count < 5 eventually becomes false.
Q2
while (true) { console.log("Hi"); } → An infinite loop: the condition is always true and nothing in the body can make it false.
Q3
To make a loop stop, update the variable that the condition tests inside the loop body. → let lives = 3;
while (lives > 0) {
lives--;
}
Q4
A condition that can never become false makes the loop run forever, even if the variable is updated. → let n = 1;
while (n > 0) {
n++;
}
Q5
let count = 0;
while (count < 5) {
console.log(count);
} → The loop never changes count, so count < 5 stays true forever. Add count++; inside the loop body.
Q6
let count = 10;
while (count > 0) {
count++;
} → count goes up, moving away from 0, so count > 0 is always true. Use count-- so it counts down to 0.
Section C: Definitions
Q1
while (count < 5) { count++; } — A loop runs forever if nothing in its body changes the variable its condition tests. Updating count each time lets the condition become false so the loop stops.Section D: Fill in the Blanks
Q1
while (count < 5) { count++; }Q2
let lives = 3;
while (lives > 0) { lives--; }Q3
while (true) { console.log("Hi"); }Q4
let n = 0;
while (n < 3) { n++; }Q5
while (true) { if (n > 4) break; n++; }Section E: Spot the Error
Q1
A — i-- makes i smaller each time, so it never reaches 5 and the loop never stops. It should be i++ so i counts up to 5.
Q2
B — count only goes up, so count >= 0 is always true and the condition can never become false. It should be count < 5.
Q3
B — n goes down from 10, so it is always less than 20 and the condition never becomes false. It should be n > 0 so the loop ends when n reaches 0.
Q4
D — count-- moves count away from 3, so count < 3 stays true forever. The loop variable must move towards the stopping value, so use count++.
Q5
A — The update part has been left out, so i stays at 5 and i > 0 is always true. It should end with i-- to count down.