Looping through an array by index

Name:

Date:

Class:

Quick Questions

Shade in the A or B circle for each question.

1. Which loop header visits every item in the array scores?

A for (let i = 0; i < scores.length; i++)

B for (let i = 0; i <= scores.length; i++)

2. What should the counter i start at to begin with the first item of an array?

A 1

B 0

3. scores has 4 items. Which is the index of the last item?

A 3

B 4

4. Which line inside the loop reads the current item?

A console.log(scores.i);

B console.log(scores[i]);

5. Which line adds 5 bonus points to every score, changing the items in place?

A scores[i] = scores[i] + 5;

B scores = scores[i] + 5;

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 = 0; i < scores.length; i++)		AStarting i at 1 skips the first item at index 0. The counter should start at 0.
2scores[i] = scores[i] + 5;		Bfor (let i = 0; i < scores.length; i++) { console.log(scores[i]); }
3Use the loop counter as the index to read each item and print it		CCounts i from 0 up to the last index so every item in the scores array can be visited
4Stop the loop before i reaches the array length, so no index goes past the end		Di < scores.length
5for (let i = 0; i <= scores.length; i++)		EUsing <= lets i reach scores.length, which is one step past the end, so scores[i] is undefined on the last pass. Use < instead.
6for (let i = 1; i < scores.length; i++)		FChanges the item at index i in place by adding 5 to it

Definitions

for (let i = 0; i < scores.length; i++)

Fill in the Blanks

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

for (let i = 0; i < scores.______; i++) { console.log(scores[i]); }

for (let i = 0; i _ scores.length; i++) { console.log(scores[i]); }

for (let i = _; i < scores.length; i++) { console.log(scores[i]); }

for (let i = 0; i < scores.length; i++) { console.log(scores[_]); }

for (let i = 0; i < scores.length; i++) { scores[i] __ 5; }

Spot the Error

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

Question 1

A const pts = [4, 8, 6];

B let sum = 0;

C for (let i=0;i<pts.length;i++)

D sum += i;

E console.log(sum);

Question 2

A const pts = [4, 8, 6];

B for (let i=1;i<pts.length;i++)

C pts[i] = pts[i] + 1;

D console.log(pts);

Question 3

A const a = [3, 6, 9];

B let n = 0;

C for (let i=0;i<=a.length;i++)

D n += a[i];

E console.log(n);

Question 4

A const pts = [2, 4, 6];

B for (let i=0;i<pts.length;i++)

C console.log(pts(i));

Question 5

A const pts = {4, 8, 6};

B for (let i=0;i<pts.length;i++)

C console.log(pts[i] * 2);

Answer Key

Looping through an array by index

Section A: Quick Questions

A — for (let i = 0; i < scores.length; i++)

B — 0

A — 3

B — console.log(scores[i]);

A — scores[i] = scores[i] + 5;

Section B: Match the Pairs

for (let i = 0; i < scores.length; i++) → Counts i from 0 up to the last index so every item in the scores array can be visited

scores[i] = scores[i] + 5; → Changes the item at index i in place by adding 5 to it

Use the loop counter as the index to read each item and print it → for (let i = 0; i < scores.length; i++) { console.log(scores[i]); }

Stop the loop before i reaches the array length, so no index goes past the end → i < scores.length

for (let i = 0; i <= scores.length; i++) → Using <= lets i reach scores.length, which is one step past the end, so scores[i] is undefined on the last pass. Use < instead.

for (let i = 1; i < scores.length; i++) → Starting i at 1 skips the first item at index 0. The counter should start at 0.

Section C: Definitions

for (let i = 0; i < scores.length; i++) — A loop that counts i from 0 up to the last index, so scores[i] visits every item in the array. Use < not <=, or i goes one step past the end and reads undefined.

Section D: Fill in the Blanks

for (let i = 0; i < scores.length; i++) {
  console.log(scores[i]);
}

for (let i = 0; i < scores.length; i++) {
  console.log(scores[i]);
}

for (let i = 0; i < scores.length; i++) {
  console.log(scores[i]);
}

for (let i = 0; i < scores.length; i++) {
  console.log(scores[i]);
}

for (let i = 0; i < scores.length; i++) {
  scores[i] += 5;
}

Section E: Spot the Error

D — This adds the counter i instead of the item at that index, so the total is 0 + 1 + 2 = 3. It should be sum += pts[i]; to add each score.

B — The counter starts at 1, so index 0 is skipped and the first score is never changed. A loop over every item must start at i=0.

C — Using <= makes i reach 3, which is one step past the last index (2). a[3] is undefined, so n becomes NaN. It should be i<a.length.

C — Round brackets try to call pts as a function, which causes a TypeError. Items are read with square brackets: pts[i].

A — Curly braces are for objects and blocks, so this is a syntax error. An array uses square brackets: [4, 8, 6].