Slice an array index wise and push into empty array

Question

I have 2 arrays. one of them(config) decides how many elements are going to be pushed in new array(finalData) from existing array(items).

I tried to use slice but at the 3rd index the value is duplicated from the 2nd index. Try to run code and you'll get the idea. Here's my sandbox

Desired output is that I get an array of arrays split exactly by using config indexes.

const config = [1,2,1, 2];
const items = ["one", "two", "three", "four", "five", "six", "seven", "eight"];

const finalData = []

for (let i = 0; i < items.length; i++) {
  if(config[i] !== undefined) { 
  finalData.push(items.slice(i, config[i] + i))
  } else {
    finalData.push([items[i]])
  }
}

console.log(finalData);

.as-console-wrapper { max-height: 100% !important; }

Could you clarify your desired output? Should it be [ ['one'], ['two', 'three'], ['four'] ... ] or [ ['one'], ['two', 'three'], ['three'] ... ] or something else? Your current code seems to output the second option — lucasvw
– lucasvw, Commented Sep 20, 2022 at 13:51
for (let i = 0; i < items.length; i += config[i] ?? 1) { ?? — 001
– 001, Commented Sep 20, 2022 at 13:55
@JohnnyMopp that works, could you please spot my mistake and explain how your solution works? — Chandler Bing
– Chandler Bing, Commented Sep 20, 2022 at 13:58
When you add more than 1 item using slice(), you need to increment i by that number of items. — 001
– 001, Commented Sep 20, 2022 at 13:59

lucasvw · Accepted Answer · 2022-09-20 14:06:03Z

2

I would suggest iterating through the config array instead of the items array

const config = [1, 2, 1, 2];
const items = ["one", "two", "three", "four", "five", "six", "seven", "eight"];

const finalData = [];
let index = 0;
for (let size of config) {
  finalData.push(items.slice(index, index + size));
  index += size;
}
// then collect any remaining
while (index < items.length) {
  finalData.push([ items[index++] ]);
}

console.log(JSON.stringify(finalData));

If the length items can be shorter than the defined config, you may need to add some checks in to prevent empty arrays from being added to final data

edited Sep 20, 2022 at 14:06

answered Sep 20, 2022 at 13:58

lucasvw

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2 Comments

Jeet Over a year ago

thanks, could you please explain why you did index += size; instead of just index++. I don't fully understand the logic behind this and it only works if we increment with size.

lucasvw Over a year ago

@JeetViramgama size is how many elements we're adding in the current iteration and index is the current location in the items array. So for each iteration, we want to move our index by the number of elements we added during that iteration

0stone0 · Accepted Answer · 2022-09-20 14:06:44Z

1

You can use map() with splice() to get the items from items array with the amount config provides.

If after the map, there are still items in the items array, use another map() to add those

const config = [1,2,1, 2];
const items = ["one", "two", "three", "four", "five", "six", "seven", "eight"];

let finalData = config.map(n => items.splice(0, n))

if (items.length > 0) {
  finalData = [ ...finalData, ...items.map(i => [ i ]) ];
}

console.log(finalData);

.as-console-wrapper { max-height: 100% !important; }

[
  [
    "one"
  ],
  [
    "two",
    "three"
  ],
  [
    "four"
  ],
  [
    "five",
    "six"
  ],
  [
    "seven"
  ],
  [
    "eight"
  ]
]

edited Sep 20, 2022 at 14:06

answered Sep 20, 2022 at 13:54

0stone0

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Comments

zer00ne · Accepted Answer · 2022-09-21 22:44:46Z

1

You could iterate through config array and use .splice(0, qty). Since .splice() mutates the array, if it called on every iteration, the first parameter is 0 index by default.

Details are commented in example

const group = [1, 2, 1, 2];
const items = ["one", "two", "three", "four", "five", "six", "seven", "eight"];

// >ext< = 8 - [1 + 2 + 1 + 2] /* returns 2 */
const ext = items.length - group.reduce((sum, add) => sum + add);

// >cfg< = [1, 2, 1, 2] merge with [1, 1]
const cfg = group.concat(Array(ext).fill(1));

// >qty< = [1, 2, 1, 2, 1, 1] returns [["one"], ["two", "three"],...]
const dat = cfg.flatMap(qty => [items.splice(0, qty)]);

console.log(dat);

.as-console-wrapper { max-height: 100% !important; }

edited Sep 21, 2022 at 22:44

answered Sep 20, 2022 at 15:31

zer00ne

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