Only last element from the array gets concatenated in every loop

Question

Issue: Only the last element of the array is getting concatenated in finalOutput in a loop

let listA = [
    [1, 2],
    [7, 8],
    [4, 5],
    [11, 12]
];
let listB = [
    ['x', 'y', 'z'],
    ['d', 'd', 'd'],
    ['f', 'y', 's']
];

let finalOutput = [];
for (let i = 1; i <= listA.length - 1; i++) {
    let dataIndx = 0;
    for (let item of listB) {
        if (item[1] !== 'd') {
            item[1] = listA[i][dataIndx];
            dataIndx++;
        }
    }
    finalOutput = finalOutput.concat(listB);
}

console.log('Undesired output:', finalOutput);

Currrent undesired output:

[
    ['x', '11', 'z'], ['d', 'd', 'd'], ['f', '12', 's'],
    ['x', '11', 'z'], ['d', 'd', 'd'], ['f', '12', 's'],
    ['x', '11', 'z'], ['d', 'd', 'd'], ['f', '12', 's'],
]

Expected finalOutput value to be

[
    ['x', '7',  'z'], ['d', 'd', 'd'], ['f', '8',  's'],
    ['x', '4',  'z'], ['d', 'd', 'd'], ['f', '5',  's'],
    ['x', '11', 'z'], ['d', 'd', 'd'], ['f', '12', 's'],
]

What're you trying to do here? Can you explain the logic a little better? — Terry
– Terry, Commented Jan 6, 2020 at 14:45

mickl · Accepted Answer · 2020-01-06 14:47:08Z

1

The problem here is that you iterate through listB several times and modify that list every time (referene to the same array). Since array is a reference the last one will be caputered in your result set. Try to clone listB instead:

let listA= [[1,2],[7,8],[4,5],[11,12]];
let listB= [['x','y','z'],['d','d','d'],['f','y','s']]; 

let finalOutput = [];
for (let i = 1; i <= listA.length - 1; i++) {
  let dataIndx = 0;
  let listBB = listB.map(x => ([...x])); // "cloning an array of arrays"
  for(let item of listBB){
  
    if (item[1] !== 'd') {
         
         item[1] = listA[i][dataIndx];
         dataIndx++;
  }
  finalOutput = finalOutput.concat(listBB);
  }
    
}
console.log(finalOutput)

answered Jan 6, 2020 at 14:47

mickl

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Comments

Nina Scholz · Accepted Answer · 2020-01-06 15:01:03Z

1

You could slice the first array and take a flatMap for the nested array.

let listA = [[1, 2], [7, 8], [4, 5], [11, 12]],
    listB = [['x', 'y', 'z'], ['d', 'd', 'd'], ['f', 'y', 's']],
    result = listA
        .slice(1)
        .flatMap(([v]) => listB.map(([a, b, c]) => [a, b === 'd' ? b : v, c]));

console.log(result);

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answered Jan 6, 2020 at 15:01

Nina Scholz

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