Nested loops: a loop inside a loop
Name:
Date:
Class:
A
Quick Questions
Shade in the A or B circle for each question.
1. A for loop runs 3 times on the outside and a loop inside it runs 4 times. How many times does the inner code run in total?
2. Which describes what a nested loop does?
3. Which is a correct nested loop using the counter names i and j?
4. An outer loop runs 2 times and an inner loop runs 5 times. The inner loop contains console.log('*'). How many stars are printed?
5. You want a multiplication grid with 3 rows and 4 columns. Which loop layout is right?
B
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 <= 3; i++) { for (let j = 1; j <= 4; j++) { | AInside a nested loop, this prints one multiplication result on every pass, using both counters to build a multiplication grid. | |
| 2console.log(i * j); | BThe inner loop reuses the counter name i from the outer loop, which makes the code confusing and error-prone. The inner loop should use a different counter name such as j. | |
| 3Building a row of stars: the inner loop adds a star to a string for each column, then the outer loop moves on to the next row. | CA nested loop: the inner j loop runs 4 times for each of the 3 runs of the outer i loop, giving 12 passes in total. | |
| 4An outer loop of 3 repeats with an inner loop of 4 repeats makes the inner loop body run 12 times in total. | Dlet count = 0; for (let i = 0; i < 3; i++) { for (let j = 0; j < 4; j++) { count++; } } console.log(count); | |
| 5for (let i = 1; i <= 3; i++) { for (let i = 1; i <= 4; i++) { console.log(i); } } | Efor (let i = 0; i < 3; i++) { let row = ""; for (let j = 0; j < 4; j++) { row += "*"; } console.log(row); } | |
| 6for (let i = 1; i <= 3; i++) { for (let j = 1; j <= 4; j++) { console.log(i * j); } console.log("-"); } } | FThere is an extra closing brace at the end. Each loop needs exactly one matching closing brace, so a nested loop with two opening braces needs only two closing braces. |
C
Definitions
nested loop
D
Fill in the Blanks
Complete each line of code by filling in the missing parts.
1.
for (let i = 1; i <= 3; i++) {
for (let _ = 1; j <= 4; j++) count++;
}
2.
for (let i = 1; i <= 3; i++) {
for (let j = 1; j <= _; j++) count++;
}
3.
for (let i = 1; i <= 3; i++) {
for (let j = 1; j <= 3; j++) t += i _ j;
}
4.
for (let i = 1; i <= 3; i++) {
for (let j = 1; j <= 5; j++) row __ "*";
}
5.
for (let i = 1; i <= 3; i++) {
___ (let j = 1; j <= 4; j++) count++;
}
E
Spot the Error
Shade in the letter of the line that contains an error.
Question 1
A
let count = 0;
B
for (let i=0; i<3; i++) {
C
for (let i=0; i<4; i++) {
D
count++;
E
}
F
}
Question 2
A
for (let i=0; i<3; i--) {
B
let row = '';
C
for (let j=0; j<4; j++)
D
row += '*';
E
console.log(row);
F
}
Question 3
A
for (let i=1, i<=3, i++) {
B
for (let j=1; j<=3; j++) {
C
console.log(i * j);
D
}
E
}
Question 4
A
let passes = 0;
B
for (let i=0; i<3; i++)
C
for (let j=0; j<4; j++)
D
passes + 1;
E
console.log(passes);
Question 5
A
let total = 0;
B
For (let i=0; i<3; i++) {
C
for (let j=0; j<4; j++) {
D
total++;
E
}
F
}
Answer Key
Nested loops: a loop inside a loop
Section A: Quick Questions
Q1
B — 12 times
Q2
A — The inner loop finishes all its repeats for every one repeat of the outer loop
Q3
A — for (let i = 0; i < 3; i++) { for (let j = 0; j < 4; j++) { } }
Q4
A — 10 stars
Q5
B — Outer loop for the rows, inner loop for the columns
Section B: Match the Pairs
Q1
for (let i = 1; i <= 3; i++) {
for (let j = 1; j <= 4; j++) { → A nested loop: the inner j loop runs 4 times for each of the 3 runs of the outer i loop, giving 12 passes in total.
Q2
console.log(i * j); → Inside a nested loop, this prints one multiplication result on every pass, using both counters to build a multiplication grid.
Q3
Building a row of stars: the inner loop adds a star to a string for each column, then the outer loop moves on to the next row. → for (let i = 0; i < 3; i++) {
let row = "";
for (let j = 0; j < 4; j++) { row += "*"; }
console.log(row);
}
Q4
An outer loop of 3 repeats with an inner loop of 4 repeats makes the inner loop body run 12 times in total. → let count = 0;
for (let i = 0; i < 3; i++) {
for (let j = 0; j < 4; j++) { count++; }
}
console.log(count);
Q5
for (let i = 1; i <= 3; i++) {
for (let i = 1; i <= 4; i++) {
console.log(i);
}
} → The inner loop reuses the counter name i from the outer loop, which makes the code confusing and error-prone. The inner loop should use a different counter name such as j.
Q6
for (let i = 1; i <= 3; i++) {
for (let j = 1; j <= 4; j++) {
console.log(i * j);
}
console.log("-");
}
} → There is an extra closing brace at the end. Each loop needs exactly one matching closing brace, so a nested loop with two opening braces needs only two closing braces.
Section C: Definitions
Q1
nested loop — A loop inside another loop. The inner loop runs all its repeats for every single repeat of the outer loop, so an outer 3 and inner 4 gives 12 passes.Section D: Fill in the Blanks
Q1
for (let i = 1; i <= 3; i++) {
for (let j = 1; j <= 4; j++) count++;
}Q2
for (let i = 1; i <= 3; i++) {
for (let j = 1; j <= 4; j++) count++;
}Q3
for (let i = 1; i <= 3; i++) {
for (let j = 1; j <= 3; j++) t += i * j;
}Q4
for (let i = 1; i <= 3; i++) {
for (let j = 1; j <= 5; j++) row += "*";
}Q5
for (let i = 1; i <= 3; i++) {
for (let j = 1; j <= 4; j++) count++;
}Section E: Spot the Error
Q1
C — The inner loop reuses the counter name i, which clashes with the outer loop's counter. Give it its own name: for (let j=0; j<4; j++).
Q2
A — The outer loop counts down with i-- but the condition i<3 stays true, so it never ends. It should be i++ so the outer loop repeats 3 times.
Q3
A — The parts of a for loop header are separated by semicolons, not commas. It should be for (let i=1; i<=3; i++) {.
Q4
D — passes + 1; works out a value but never saves it, so passes stays 0. Use passes++; so each of the 12 passes adds 1.
Q5
B — JavaScript is case sensitive, so For is not the keyword. It must be lowercase: for (let i=0; i<3; i++) {.