JavaScript updating 2d array value

Question

I'm trying to update every element of a 2d array only once. But unexpectedly array item gets update multiple times.

For example:

const s = "ab";
const m = [...Array(s.length).fill([...Array(s.length).fill("")])]

for(let row = 0; row < s.length; row++) {
  for (let col = 0; col < s.length; col++) {
    console.log(row, col)
    m[row][col] += `<${row}${col}>`
  }
}

console.log(m)

it should return m = [ [ '<00>', '<01>' ], [ '<10>', '<11>' ] ]

but it returns m = [ [ '<00><10>', '<01><11>' ], [ '<00><10>', '<01><11>' ] ] instead.

Can anyone explain it, please?

Update:

Here I'm looping through each item once, so there should be no chance of updating the item twice ( two value should not concat here )

Dhana D. · Accepted Answer · 2021-09-02 05:34:28Z

1

Instead of initializing array with specific length. You may just initialize empty array then push the elements. Because if you do + of 2 strings, it may just concatenate it, for example:

console.log("I "+"am");

Full working code:

const s = "ab";
let m = [];

for(let row = 0; row < s.length; row++) {
  m.push([]);
  for (let col = 0; col < s.length; col++) {
    m[row].push(`<${row}${col}>`);
  }
}

console.log(m);

edited Sep 2, 2021 at 5:34

answered Sep 2, 2021 at 5:31

Dhana D.

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7 Comments

Moni Over a year ago

could you explain what's wrong with my code?

Moni Over a year ago

for some reason, I need to initialize the array before starting the loop

mplungjan Over a year ago

Your double fill likely does not create what you think it does

Moni Over a year ago

I know plus sign is the problem, but it means somehow array item updating twice, right? but why? @DhanaD.

Dhana D. Over a year ago

I think it's because of the fill. Have you tried logging the value of the initialized array before the loop?

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Muhammad Mujtaba Shafique · Accepted Answer · 2021-09-02 05:36:40Z

The issue is that you are using <${row}${col}>, You should simple use let size=0 and use Array[i][j] = size++; in place of <${row}${col}>

you should use the following way for 2D Arrays

//Create a new 1D array

let myArray  = new Array(2);

// Loop to create 2D array using 1D array

for (let i = 0; i < myArray.length; i++) {
    myArray[i] = new Array(2);
}

//declare size

let size = 0;

// Loop to initialize 2D array elements.

for (let i = 0; i < 2; i++) {
    for (let j = 0; j < 2; j++) {
        myArray[i][j] = size++;
    }
}

// Loop to display the elements of 2D array.

for (let i = 0; i < 2; i++) {
    for (let j = 0; j < 2; j++)    {
        document.write(myArray[i][j] + " ");
    }
    document.write("<br>");
}

lukasz · Accepted Answer · 2021-09-02 05:59:33Z

0

You don't even need a loop, you can use map instead.

const a = Array(2).fill(null).map(
  (_, row) => Array(2).fill(null).map(
    (_, col) => `<${row}${col}>`
  )
);

answered Sep 2, 2021 at 5:59

lukasz

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