I am new in programing as well as in javascript so please help me with loop

Question

let itemsArr = ["Milk", "Bread", "Potato"];
let quantityArr = [5, 2, 3];

for (i = 0; i <= itemsArr.length; i++) {
  if (i <= itemsArr.length)
    console.log(itemsArr[i] + " - " + quantityArr[i]);
}

it should
Print the following result:

Milk - 5
Bread - 2
Potato - 3

but why it is Print the following result:

Milk - 5
Bread - 2
Potato - 3
undefined - undefined

Please help!

If `i == itermsArr.length`, `itemsArr[i]` will *always* be `undefined`. Array indexes are zero-indexed. — esqew, Apr 08 '21 at 20:15
https://stackoverflow.com/questions/49723311/why-am-i-getting-typeerror-arrayi-is-undefined same question, same number of elements! — DemiPixel, Apr 08 '21 at 20:15

BenM · Answer 1 · 2021-04-08T20:20:29.257

Arrays are 0-indexed, so the last element's index is always length - 1. Therefore, you should update your loop as follows:

let itemsArr = ["Milk", "Bread", "Potato"];
let quantityArr = [5, 2, 3];

for (i = 0; i < itemsArr.length; i++) {
    console.log(itemsArr[i] + ' - ' + quantityArr[i]);
}

Notice that there is also no need to check the status inside of the loop using the if statement, since it will always evaluate to true.

For added points, you can remove the for loop entirely, and instead use forEach:

let itemsArr = ["Milk", "Bread", "Potato"];
let quantityArr = [5, 2, 3];

itemsArr.forEach((item, i) => {
  console.log(`${item} - ${quantityArr[i]}`);
});

you can even use forEach instead map because you are not returning anything :) — Sowam, Apr 08 '21 at 20:19

score 1 · Answer 2 · answered Apr 08 '21 at 20:24

Remember that itemsArr.length keeps number of all elements that are stored in an Array. For example for let itemsArr = ["Milk", "Bread", "Potato"]; it is 3. While index of array starts with 0 for Milk and ends with 2 for Potato.

This means that if you loop over the array you should use last index of an array, meaning that stop condition should look like for (i = 0; i < itemsArr.length; i++) or for (i = 0; i <= itemsArr.length - 1; i++)

This is common error that is being made even by experienced programmers (maybe in a little bit more complicated situations;-) It even has its own name off by one error - https://en.wikipedia.org/wiki/Off-by-one_error

score 1 · Answer 3 · answered Apr 08 '21 at 20:48

1

I just want to add, that you can use forEach instead of for.

answered Apr 08 '21 at 20:48

luisbar

676
5
11

score -5 · Answer 4 · answered Apr 08 '21 at 20:18

-5

Delete if(i <= itemsArr.length)

answered Apr 08 '21 at 20:18

ken666

1
1

Will not help, since the loop declaration is incorrect. – BenM Apr 08 '21 at 20:19
still need to make the <= loop into < since arrays are 0-indexed – Bmils Apr 08 '21 at 20:20

I am new in programing as well as in javascript so please help me with loop

4 Answers4