Trouble with Javascript loop

September 21, 2023

I have an array with numbers starting from 1 to 20. I want to write a loop that takes every n-th element in the current array, for example (1, 6, 11, 16). After the first loop, it should take every 5-th element too, but start from 2 (2, 7, 12, 17)

I tried this:

const row = 5;

const cellArray = [
  1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20,
];

const newArray = [];

for (let i = 0; i <= row; i++) {
  cellArray.forEach((item, k) => {
    if (k === i) {
      newArray.push(item);
    }

    console.log(i);

    if (k % (row + i) == 0 && k !== 0) {
      newArray.push(item);
    }
  });
}

Output:

[1, 6, 11, 16, 2, 7, 13, 19, 3, 8, 15, 4, 9, 17, 5, 10, 19, 6, 11]

What I expected to:

[1, 6, 11, 16, 2, 7, 12, 17, 3, 8, 13, 18, 4, 9, 14, 19, 5, 10, 15, 20]

>Solution :

Way too complicated approach, with those two separate checks you implemented there.

If the item index k modulo 5 (so row, effectively) matches i, then you want to put the item into your result array.

const row = 5;

const cellArray = [
  1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20,
];

const newArray = [];

for (let i = 0; i <= row; i++) {
  cellArray.forEach((item, k) => {
    if (k % row === i) {
      newArray.push(item);
    }
  });
}

console.log(JSON.stringify(newArray))