CODEWITHSUNDEEP
javascript
Ali Safapour
Ali Safapour

Reputation: 203

The splice method causes the for loop to be executed n - 2 times (n is the number of elements in an array)

I want to remove the elements of an array with n elements using a for loop and the splice method one by one. The loop is executed n - 2 times and there will be 2 elements inside the array at the end. Why is that so?

const fruits = ['Apple', 'Orange', 'Watermelon', 'Peach']

for(index in fruits){
    console.log('Execution Number:', index);
    console.log('Before:', fruits);
    fruits.splice(0, 1);
    console.log('After:', fruits);
    console.log('-'.repeat(50));
}

console.log('\nFinal Result:', fruits)

result:

Execution Number: 0
Before: [ 'Apple', 'Orange', 'Watermelon', 'Peach' ]
After: [ 'Orange', 'Watermelon', 'Peach' ]
--------------------------------------------------
Execution Number: 1
Before: [ 'Orange', 'Watermelon', 'Peach' ]
After: [ 'Watermelon', 'Peach' ]
--------------------------------------------------

Final Result: [ 'Watermelon', 'Peach' ]

I know that I can do this with a single line of code using the splice method but I wanted to test if I can do this using a for loop.

Upvotes: 2

Views: 155

Answers (4)

RenaudC5
RenaudC5

Reputation: 3829

This happen because you are reducing the length of the array in every loop. By doing this, the number of element in the array decrease but the index still increment.

Fruits Index
['Apple', 'Orange', 'Watermelon', 'Peach'] 0 0 < 4 so, Apple is deleted
['Orange', 'Watermelon', 'Peach'] 1 1 < 3 so, Orange is deleted
['Watermelon', 'Peach'] 2 2 < 2 Here 2 < 2 is false, the loop stops

A solution would be to do a for loop with a fixed length calculated before the loop 

const fruits = ['Apple', 'Orange', 'Watermelon', 'Peach']
const length = fruits.length
for (let index = 0; index < length; index++){
    console.log('Execution Number:', index);
    console.log('Before:', fruits);
    fruits.splice(0, 1);
    console.log('After:', fruits);
    console.log('-'.repeat(50));
}

console.log('\nFinal Result:', fruits)

As @reyno was mentioning, in the above snippets you can't access fruits individually. In fact if you want to access each fruit in the foor loop you can do this by accessing to the first element of the array each time : fruits[0]

Upvotes: 3

Tyler Durden
Tyler Durden

Reputation: 1070

The issue is being seen because the length of the array is decreased by one in each iteration due to fruits.splice(0,1).

  1. Iteration 1 - index = 0; fruits.length = 4
  2. Iteration 2 - index = 1; fruits.length = 3
  3. Iteration 3 - index = 2; fruits.length = 2 -> for loop breaks

Hence, the observation.

Upvotes: 2

Kanishk Anand
Kanishk Anand

Reputation: 1702

Splice mutates the original array, implying that the length of array decreases each time you do fruits.splice(0,1), hence, after 2 turns, there are not more items in the array to traverse, hence the loop breaks.

const fruits = ['Apple', 'Orange', 'Watermelon', 'Peach']

for(index in fruits){
    console.log('Execution Number:', index);
    console.log('Before:', fruits);
    fruits.splice(0, 1);
    console.log('After:', fruits, fruits.length); // Goes from 3 to 2 and then exits after 2 turns 
    console.log('-'.repeat(50));
}

console.log('\nFinal Result:', fruits)

More details on splice : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/splice

Upvotes: 1

mazei513
mazei513

Reputation: 332

When you do fruits.splice you are modifying the array used for the for loop itself. If you look at index, it is incrementing in each loop. After Execution Number: 1, it increments to 2 and looks at fruits and sees there is no more elements. Seeing that, it exits the for loop.

If you want to do it with a for loop, what you probably wanted was

const fruits = ['Apple', 'Orange', 'Watermelon', 'Peach']

const fruitsLen = fruits.length;
for(let i = 0; i < fruitsLen; i++){
    console.log('Execution Number:', i);
    console.log('Before:', fruits);
    fruits.splice(0, 1);
    console.log('After:', fruits);
    console.log('-'.repeat(50));
}

console.log('\nFinal Result:', fruits)

Upvotes: 1

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