CODEWITHSUNDEEP
javascript
Ankit
Ankit

Reputation: 350

length property of array not working properly in for loop

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?

Upvotes: 0

Views: 1064

Answers (5)

Ranjith
Ranjith

Reputation: 157

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

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

For loop mechanism

  1. i == 0 and y == 10, i< y.length condition true, z = y.pop() and y.length becomes 9

  2. i == 1 and y == 9, i< y.length condition true, z = y.pop() and y.length becomes 8

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

Upvotes: 1

Nam Tang
Nam Tang

Reputation: 100

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';

Upvotes: 1

Joven28
Joven28

Reputation: 769

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

Upvotes: 3

Robby Cornelissen
Robby Cornelissen

Reputation: 97381

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

Upvotes: 6

ankitkanojia
ankitkanojia

Reputation: 3122

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().

Upvotes: 0

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