make an array of arrays from for loop

Question

I want to put each iteration into an array which is in a main array. Like this: array = [[a,b], [c,d], [e,f]]

push method and slice, splice

var descendants = [A,C,C,D,C,B], 
chunk = 2, 
descendantPairs = [];
for (var i = 0; i < descendants.length; i += chunk) {
descendantPairs = descendantPairs.push(descendants.slice(i, i + chunk));
console.log("descendantPairs: " + descendantPairs});
}

Right now I'm able to get the pairs like this but I need to do more logic on ALL the pairs, instead of just the last pair, therefore I need an array which holds all the pairs.

I get this now in console.log:

descendantPairs: A,C descendantPairs: C,D descendantPairs: C,B

• Pranav C Balan: descendantPairs = descendantPairs.push(descendants.slice(i, i + chunk)); ???
• artworkjpm: @MarkMeyer it needs to keep the order of descendants, to be in pairs, ie array = [ [A,C], [C,D], [D,C]]
• charlietfl: Don't overwrite the whole variable with result of push()
• BOZ: Why you doesn't using if (value%2 == 0) (even)

Answer 1

The push method returns the length of the added element, not the array after pushing the element.

replace descendantPairs = descendantPairs.push(descendants.slice(i, i + chunk)); with

descendantPairs.push(descendants.slice(i, i + chunk));

Answer 2

Something like this will do the trick:

let descendants = ['A', 'C', 'C', 'D', 'C', 'B'];
let chunk = 2;
let result = [];
for (let i = 0; i < descendants.length; i += chunk) {
 result.push(descendants.slice(i, i+chunk));
}
console.log(result);

Answer 3

Your code contains one issue, you are reassigning descendantPairs with returned value of Array#push method which will be the length of the array and it leads to throwing an error on the next iteration(since there is no push method for type Number).

To make it work just remove the reassigning part and log the value after the for loop.

var descendants = ['A', 'C', 'C', 'D', 'C', 'B'],
 chunk = 2,
 descendantPairs = [];
for (var i = 0; i < descendants.length; i += chunk) {
 descendantPairs.push(descendants.slice(i, i + chunk));
}
console.log("descendantPairs: ", descendantPairs);

Answer 4

You could push each slice to the result array, without assining the new length of the array. The next loop throws an error, because your descendantPairs is now a number and not anymore an array, which is needed for using the push method.

var descendants = ['A', 'C', 'C', 'D', 'C', 'B'],
 chunk = 2,
 result = [],
 i = 0;
while (i < descendants.length) {
 result.push(descendants.slice(i, i += chunk));
}
console.log(result);

• Scott Hunter: Can you explain how this is different from what OP did, since she was pushing each slice to the result array?
• Nina Scholz: it does not assign the result of the pushing (the new length of the array) to the result set.

Answer 5

You can use a simple for loop and a temp variable and push values on every second index to final output

var arr = [`A`,`C`,`C`,`D`,`C`,`B`]
let op = []
let temp = []
for(let i = 0; i < arr.length; i++){
 temp.push(arr[i])
 if( i % 2 == 1 ){
 op.push(temp)
 temp = []
 }
}
if( temp.length > 0 ){
 op.push(temp)
}
console.log(op)

Answer 6

var arr = ['A','C','C','D','C','B'];
let chunk = 2;
let pairs = [];
for(let i = 0; i < arr.length; i+= chunk) {
 pairs.push(arr.slice(i, i + chunk));
}
console.log(pairs);

• charlietfl: Should really test this. Has serious flaws
• charlietfl: There's a built in code editor that lets you run your code if you click on <> when editing answer