Choosing a for loop or a while loop (JavaScript)

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Quick Questions

Shade in the A or B circle for each question.

1. You want to print the numbers 1 to 10. Which loop is the best choice?

A for loop, because you know it repeats 10 times

B while loop, because it can never be used for counting

2. A program keeps asking 'What is the password?' until the user types the right answer. Which loop fits best?

A while (answer !== password) { ... }

B for (let i = 0; i < 10; i++) { ... }

3. True or false: Any for loop can be rewritten as a while loop.

A False

B True

4. Which while loop does the same job as: for (let i = 1; i <= 5; i++) { console.log(i); }

A let i = 1; while (i <= 5) { console.log(i); i++; }

B let i = 1; while (i <= 5) { console.log(i); }

5. A game repeats a round exactly 3 times. Which loop is easier to read?

A while (true) with a counter hidden further down

B for (let round = 1; round <= 3; round++)

Match the Pairs

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

Left column		Right column
1for (let i = 1; i <= 10; i++) {		Afor (let i = 1; i <= 10; i++) { console.log(i); }
2while (answer !== "blue") {		BA for loop is a poor fit here because the number of repeats is unknown and the counter i is never used. A while loop (while (guess !== "7")) is clearer.
3Any for loop can be rewritten as a while loop by moving the setup before the loop and the update inside it.		CA while loop is a good choice here because we repeat until a condition changes, and we do not know how many tries it will take.
4Use a for loop for the readable choice when printing the numbers 1 to 10, because the counting is all in one place.		Dlet i = 1; while (i <= 10) { console.log(i); i++; }
5let guess = prompt("Number?"); for (let i = 0; guess !== "7"; ) { guess = prompt("Number?"); }		EThe loop never updates i, so it runs forever. Add i++ inside the loop, or use a for loop for a known count of 10.
6let i = 1; while (i <= 10) { console.log(i); }		FA for loop is a good choice here because we know exactly how many times to repeat: 10 times.

Definitions

for vs while

Fill in the Blanks

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

___ (let i = 1; i <= 10; i++) { console.log(i); }

let ans = ""; _____ (ans !== "yes") { ans = prompt("?"); }

let i = 1; while (i <= 10) { console.log(i); ___; }

for (let i = 1; i __ 5; i++) { console.log("Hi"); }

let total = 0; for (let n = 1; n <= _; n++) { total += n; }

Spot the Error

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

Question 1

A let n = 1;

B while (n <= 10) {

C console.log(n);

D n--;

E }

Question 2

A let guess = prompt("Guess?");

B while (guess !== "7") {

C prompt("Again?");

D }

E console.log("Correct!");

Question 3

A let count = 0;

B for (let i = 0; i < 10; i++) {

C count + 1;

D }

E console.log(count);

Question 4

A let total = 0;

B let n = 5;

C for (let i = 1, i <= n, i++) {

D total += i;

E }

Question 5

A let answer = "";

B while (answer = "yes") {

C answer = prompt("Ready?");

D }

Answer Key

Choosing a for loop or a while loop (JavaScript)

Section A: Quick Questions

A — for loop, because you know it repeats 10 times

A — while (answer !== password) { ... }

B — True

A — let i = 1; while (i <= 5) { console.log(i); i++; }

B — for (let round = 1; round <= 3; round++)

Section B: Match the Pairs

for (let i = 1; i <= 10; i++) { → A for loop is a good choice here because we know exactly how many times to repeat: 10 times.

while (answer !== "blue") { → A while loop is a good choice here because we repeat until a condition changes, and we do not know how many tries it will take.

Any for loop can be rewritten as a while loop by moving the setup before the loop and the update inside it. → let i = 1; while (i <= 10) { console.log(i); i++; }

Use a for loop for the readable choice when printing the numbers 1 to 10, because the counting is all in one place. → for (let i = 1; i <= 10; i++) { console.log(i); }

let guess = prompt("Number?"); for (let i = 0; guess !== "7"; ) { guess = prompt("Number?"); } → A for loop is a poor fit here because the number of repeats is unknown and the counter i is never used. A while loop (while (guess !== "7")) is clearer.

let i = 1; while (i <= 10) { console.log(i); } → The loop never updates i, so it runs forever. Add i++ inside the loop, or use a for loop for a known count of 10.

Section C: Definitions

for vs while — Use for when you know how many times to repeat, like 1 to 10. Use while when repeating depends on a condition, like asking until the answer is correct.

Section D: Fill in the Blanks

for (let i = 1; i <= 10; i++) {
  console.log(i);
}

let ans = "";
while (ans !== "yes") { ans = prompt("?"); }

let i = 1;
while (i <= 10) { console.log(i); i++; }

for (let i = 1; i <= 5; i++) {
  console.log("Hi");
}

let total = 0;
for (let n = 1; n <= 5; n++) { total += n; }

Section E: Spot the Error

D — n-- makes n smaller, so n <= 10 is always true and the loop never ends. A while loop must change its variable towards ending, so it should be n++;.

C — The new answer is never stored in guess, so the condition never changes and the loop keeps asking forever. It should be guess = prompt("Again?");.

C — count + 1; works out a value but never saves it, so count stays 0. It should be count = count + 1; (or count++;).

C — The three parts of a for loop header are separated by semicolons, not commas. It should be for (let i = 1; i <= n; i++) {.

B — A single = assigns a value instead of comparing. The condition should compare with !==, like while (answer !== "yes") {, so the loop keeps asking until the answer is yes.