javascript: problem assigning indexes of array within array, with if statement in a loop

Question

Given str="GC" I need an output of:
[ [ 'G', 'C' ], [ 'C', 'G' ] ]
My code below seems to run the first if statement, create the first entry for the "G":
[["G" "C"]]
But then when the second iteration ("C"), which runs the else if, it converts both to
[ [ 'C', 'G' ], [ 'C', 'G' ] ]

I don't understand, the values I assign in the first if statement are overwritten, and I don't understand why.

function pairElement(str) {
  let a = []
  let b = []

  for (let i=0; i<str.length;i++) {
    a[i]=b
    if (str[i]=="G") {
      console.log("case 1")
      a[i][0]="G"
      a[i][1]="C"
      console.log(a)
    }
    else if (str[i]=="C") {
      console.log("case 2")
      a[i][0]="C"
      a[i][1]="G"
      console.log(a)
    }
  }

  return a;
}

console.log(pairElement("GC")); //desired output: [ [ 'G', 'C' ], [ 'C', 'G' ] ]

Jay Nyxed · Accepted Answer · 2020-06-12 21:03:52Z

1

You create your b value before the loop and that what is cosing the problem: Just create it inside the loop and you should get your desired result.

function pairElement(str) {
  let a = []
  

  for (let i=0; i<str.length;i++) {
	let b = [];
    a[i] = b;
    if (str[i]=="G") {
      console.log("case 1")
      a[i][0]="G"
      a[i][1]="C"
      console.log(a)
    }
    else if (str[i]=="C") {
      console.log("case 2")
      a[i][0]="C"
      a[i][1]="G"
      console.log(a)
    }
  }

  return a;
}

console.log(pairElement("GC"));

answered Jun 12, 2020 at 21:03

Jay Nyxed

4975 silver badges13 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

kite Over a year ago

Why is that? I'm looking at it and I don't see 'b' being assigned to a new value within the for loop....so why is it putting it inside fixing things...? I'm missing something still.

kite Over a year ago

Okay I see it is updating b within the for loop..where is it doing this tho...?

Jay Nyxed Over a year ago

When you are creating b outside the loop, you are basicly reassing its values inside the loop. In the first iteration i=0 -> b = ['G','C'], but on the second iteration i=1 yoy are reassigning it to b=['C','G']

kite Over a year ago

Oh nvm I see now....cause its an array the a[i]=b is updating the original b as well cause it's referencing the same data.

Jay Nyxed Over a year ago

Yep, and when b is created inside the loop you are creating new object.

Collectives™ on Stack Overflow

javascript: problem assigning indexes of array within array, with if statement in a loop

1 Answer 1

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related