Arrays: index, for...of and length together

Let's build a step counter. Steps = distance in metres ÷ step length (0.7 m), rounded with Math.round because a step count is a whole number. We store a week of distances in an array, starting on Monday:

Day	Mon	Tue	Wed	Thu	Fri	Sat	Sun
Distance	4500	5600	2400	3800	4200	6700	3600
Index	0	1	2	3	4	5	6

For one day use the index: Monday is distanceList[0].

JavaScript

const distanceList = [4500, 5600, 2400, 3800, 4200, 6700, 3600];
const stepCount = Math.round(distanceList[0] / 0.7);
console.log(stepCount);

Console

For every day use for...of and write distance instead of distanceList[0]. Then, for the average, add the distances up and divide by length:

JavaScript

const distanceList = [4500, 5600, 2400, 3800, 4200, 6700, 3600];
let sum = 0;
for (const distance of distanceList) {
  sum = sum + distance;
}
console.log(sum);
console.log(sum / distanceList.length);

Console

30800
4400

sum uses let because its value changes. If you type 7 instead of distanceList.length, the average becomes wrong as soon as you add more days.

Largest and smallest value

How do we find the longest distance? Assume the first value is the largest, then go through every value: whenever one is bigger than what we are holding, keep it instead. At the end we are holding the largest.

JavaScript

const distanceList = [4500, 5600, 2400, 3800, 4200, 6700, 3600];
let max = distanceList[0];
let min = distanceList[0];
for (const distance of distanceList) {
  if (distance > max) {
    max = distance;
  }
  if (distance < min) {
    min = distance;
  }
}
console.log("Longest: " + max);
console.log("Shortest: " + min);

Console

Longest: 6700
Shortest: 2400

Start max and min with the first value, not 0. If min started at 0, no value would be smaller, and the answer would be 0, which is not even in the array.

To know which day had the longest distance we need the index, so we go back to a counter loop and keep i:

JavaScript

const days = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"];
const distanceList = [4500, 5600, 2400, 3800, 4200, 6700, 3600];
let best = 0;
for (let i = 1; i < distanceList.length; i++) {
  if (distanceList[i] > distanceList[best]) {
    best = i;
  }
}
console.log(days[best] + ": " + distanceList[best]);

Console

Sat: 6700

Practice

1Write the programCurriculum practice

From line 2: define the constant stepCount as Monday's distance ÷ 0.7, rounded with Math.round, then print it.

JavaScript Ctrl/⌘ + Enter to run · Esc then Tab to leave the editor

const distanceList = [4500, 5600, 2400, 3800, 4200, 6700, 3600];

Console

Press Run to see the result.

2Fill in the blankCurriculum practice

Fill the blank to work out the steps for Saturday (distance 6700 ÷ 0.7, rounded with Math.round) without typing 6700 directly.

JavaScript Ctrl/⌘ + Enter to run · Esc then Tab to leave the editor

const distanceList = [4500, 5600, 2400, 3800, 4200, 6700, 3600];
const stepCount = ___;
console.log(stepCount);

Console

Press Run to see the result.

3Fill in the blankCurriculum practice

Fill both blanks: use for...of to take the values of distanceList into the constant distance, so the program prints the steps for each day.

JavaScript Ctrl/⌘ + Enter to run · Esc then Tab to leave the editor

const distanceList = [4500, 5600, 2400, 3800, 4200, 6700, 3600];

___ (___) {
  const stepCount = Math.round(distance / 0.7);
  console.log(stepCount);
}

Console

Press Run to see the result.

4Fill in the blankCurriculum practice

Fill the blank with a for...of loop that takes the values of scoreList into the constant score, so the program prints the total score.

JavaScript Ctrl/⌘ + Enter to run · Esc then Tab to leave the editor

const scoreList = [40, 55, 67, 52, 120];
let sum = 0;

for (___) {
  sum = sum + score;
}
console.log(sum);

Console

Press Run to see the result.

5Fill in the blankCurriculum practice

Fill the blank to print the average: the total ÷ the number of values in scoreList. Write it so it stays correct if the number of values changes.

JavaScript Ctrl/⌘ + Enter to run · Esc then Tab to leave the editor

const scoreList = [40, 55, 67, 52, 120];
let sum = 0;
for (const score of scoreList) {
  sum = sum + score;
}
console.log(___);

Console

Press Run to see the result.

6Write the programExtra practice

Temperatures in St Catherine over a winter week. Find the lowest one with a loop (without Math.min) and print Coldest: -4.

Hint

Start min at temps[0].

JavaScript Ctrl/⌘ + Enter to run · Esc then Tab to leave the editor

const temps = [3, -1, 2, -4, 0, 1, 5];

Console

Press Run to see the result.

7Write the programExtra practice

In the two arrays, student number i is names[i] with mark marks[i]. Find the top student and print Top: Salma with 97.

Hint

Keep the index best, not the mark itself, then use it with both arrays.

JavaScript Ctrl/⌘ + Enter to run · Esc then Tab to leave the editor

const names = ["Omar", "Nour", "Salma", "Karim", "Hana"];
const marks = [88, 91, 97, 79, 94];

Console

Press Run to see the result.

Key points

One value: list[i]. Every value: for (const x of list). Average: sum / list.length.
For the largest value, start with list[0] and replace it with any bigger value inside the loop; the smallest works the same with <.
If you need where a value is, use a counter loop and keep i.