length property of array not working properly in for loop

Question

I don't understand what is the problem here, when i put array.length inside for loop it's giving wrong length.

let x = 'w3resource'
let y = x.split('');
let output = [];

// let len = y.length;
for(let i = 0; i < y.length; i++){
  let z = y.pop();
  output.push(z);
}

let alfa = output.join('');

console.log(alfa);

Now when i put it outside it's working correctly.

let x = 'w3resource'
let y = x.split('');
let output = [];

let len = y.length;
for(let i = 0; i < len; i++){
  let z = y.pop();
  output.push(z);
}

let alfa = output.join('');

console.log(alfa);

Please explain what's going on here?

Robby Cornelissen · Accepted Answer · 2019-09-23 03:43:54Z

6

In the first case, y.length is re-evaluated on every iteration. Since you're popping elements from the array, the array gets smaller on every iteration, and the value of y.length decrements by 1.

This is why you only have 5 characters in the output of your first snippet.

Iteration  |  i  |  y.length
-----------|-----|----------
    1      |  0  |    10
    2      |  1  |     9
    3      |  3  |     8
    4      |  4  |     7
    5      |  5  |     6

edited Sep 23, 2019 at 3:43

answered Sep 23, 2019 at 3:36

Robby Cornelissen

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Comments

Joven28 · Accepted Answer · 2019-09-23 03:44:35Z

3

Because everytime you call pop method y.pop() , the length of an array changes (y.length).

There are some methods in javascript which mutate objects internally upon calling them, such as pop and push

edited Sep 23, 2019 at 3:44

answered Sep 23, 2019 at 3:37

Joven28

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Comments

Nam Tang · Accepted Answer · 2019-09-23 03:46:19Z

1

first run:

i = 0; y = ["w","3","r","e","s","o","u","r","c","e"]; y.length = 10;
y.pop() => y =["w","3","r","e","s","o","u","r","c"]; y.length =9;

second run:

i = 1; y.length = 9;
...

continue like that the length of y array get smaller so its why you got result = 'ecruo';

answered Sep 23, 2019 at 3:46

Nam Tang

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Comments

Ranjith · Accepted Answer · 2019-09-23 04:08:07Z

1

Array.prototype.pop() is mutable method in JavaScript.

So, when you pop() out elements from array, the array y is updated.

For loop mechanism

i == 0 and y == 10, i< y.length condition true, z = y.pop() and y.length becomes 9
i == 1 and y == 9, i< y.length condition true, z = y.pop() and y.length becomes 8
i == 2 and y == 8, i< y.length condition true, z = y.pop() and y.length becomes 7
i == 3 and y == 7, < y.length condition true, z = y.pop() and y.length becomes 6
i == 4 and y == 6, i< y.length condition true, z = y.pop() and y.length becomes 5
i == 5 and y == 5, i< y.length condition false, loop exits.

edited Sep 23, 2019 at 4:08

answered Sep 23, 2019 at 3:58

Ranjith

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Comments

ankitkanojia · Accepted Answer · 2019-09-23 03:43:51Z

0

let x = 'w3resource'
let y = x.split('');
let output = [];

// let len = y.length;
for (let i = 0; i < y.length; i++) {
  let z = y[i];
  output.push(z);
}

let alfa = output.join('');
console.log(output);

y.pop() function method removes the last element from an array and returns that element, so whenever you called it the variable y will have less length than previous. so your code is perfect just need to correction at y.pop().

answered Sep 23, 2019 at 3:43

ankitkanojia

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