CODEWITHSUNDEEP
javascriptarraysfor-loopindexing
Daniel Anderson
Daniel Anderson

Reputation: 3

JavaScript loops and array

I would like to know:

  1. When do you use the indexing method to assign values to a new array?
  2. and; When do you use the push method?

I know you can assign a value to an array in a for loop by using indexing.

My question is:
I have a for loop that looks like this:

for (i=array_len-2;i>0;i-=2)
{
var new_arry=array[i]
}

... that would just return the value for i. It does not store it in the new array.

but, if i do that same thing:

for (i=array_len-2;i>0;i-=2)
{
var new_arry=array.push(i)
}

it adds the new element in the array in the format of [element one , element two].

Upvotes: 0

Views: 62

Answers (3)

Kiogara
Kiogara

Reputation: 657

You'll want to use indexes for anything that should be in a specific index if it doesn't matter you can probably use push() which will return the new length and it is said on this other post that is faster than indexes.

Upvotes: 0

Chandru
Chandru

Reputation: 11184

The following example shows, how you should use the index i and the push method.

var myArray = ["hello", "world", "welcome", "javascript", "user"];
var newArray = new Array();

for(var i=myArray.length-2; i>0; i-=2) {
  newArray.push(myArray[i]);
}
console.log(newArray);

A push() simply add anything you pass as argument to an array. Index i are used to identify the "index location" on an array.

Upvotes: 1

zer00ne
zer00ne

Reputation: 43880

Either way works, just create an empty array outside the loop. Use the incrementing variable as you progress through each iteration (loop). Avoid using keyword new when making arrays, it's 99% better to use literal ex. var array = [];

Keep in mind that the loop in your examples go backward (-=2) and it's unusual in that that it stops early (i=array_len-2;i>0;), normally a loop is set at full length (i=array_len which is derived from var array_len = array.length most likely). So this unorthodox loop will only add 2 extra elements if the array is only 4 to 5 in length (I might be off...).

Demo

// Fill array by push()
var arr1 = [];

for (let i=0; i < 5; i++) {
  arr1.push(i);
}

console.log(arr1);

// Fill array by assignment
var arr2 = [];

for (let i=0; i< 5; i++) {
  arr2[i] = i;
}

console.log(arr2);

Upvotes: 1

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