How to replace one element of array inside loop

Question

There is an array arr = [1, 2, "a", 4, 5];. Using loop (for...of) every element of array must be pushed inside another empty array result = [];.

But, inside arr there is an element that needed to be replaced inside loop. Here, we need to replace string "a" with number 3 and than push it inside array result.

In the end we need to get an array result = [1, 2, 3, 4, 5];. As you can see string "a" was replaced and instead number 3 was pushed.

!Important initial array arr shouldn't be mutated and loop is a required condition.

Example:

const arr = [1, 2, "a", 4, 5];
const result = [];

for(let elem of arr) {
  if(typeof elem === "string") {
    // changing elem to number 3 and pushing it into result array instead of "a"
  }
// ...
}

Tried use several different array methods, but it didn't help.

why not result.push(3)? also you could use map, btw what does the 3 refer to, is it always 3 or the index of the array — cmgchess
– cmgchess, Commented Apr 15, 2023 at 14:07
use for (let i = 0; i < arr.length; i++) if u want acces to index. or both map and forEach also gives access to index — cmgchess
– cmgchess, Commented Apr 15, 2023 at 14:11

Rahul Verma · Accepted Answer · 2023-04-15 14:13:25Z

1

You can replace the string "a" with the number 3 and push every element into a new array using the for...of loop as follows:

const arr = [1, 2, "a", 4, 5];
const result = [];

for (let elem of arr) {
  if (typeof elem === "string" && elem === "a") {
    // replace "a" with number 3 and push it into the result array
    result.push(3);
  } else {
    // push the current element into the result array
    result.push(elem);
  }
}

console.log(result); // [1, 2, 3, 4, 5]

answered Apr 15, 2023 at 14:13

Rahul Verma

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1 Comment

zja Over a year ago

Also note that ‘typeof elem === "string"’ is redundant with the strict equality check on “a”

mplungjan · Accepted Answer · 2023-04-15 14:19:32Z

0

Just use an Array.map

here 3 each time

const arr = [1, 2, "a", 4, 5, "b"];
const result = arr.map(item => typeof item === "string" ? 3 : item)
console.log(result)

Here index+1

const arr = [1, 2, "a", 4, 5, "b"];
const result = arr.map((item, i) => typeof item === "string" ? i + 1 : item)
console.log(result)

answered Apr 15, 2023 at 14:19

mplungjan

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3 Comments

codingForCode Over a year ago

Allowed to only use loop, like for...of, for(let...)

mplungjan Over a year ago

.map is also an iterator...

codingForCode Over a year ago

yes, but a requirement mentioned allowing using only loop iterators

KooiInc · Accepted Answer · 2023-04-15 16:29:10Z

You may want to use Array.reduce here. Something like:

const reducer = (acc, v, i) => 
  [...acc, isNaN(+v) ? (acc[i-1] || 0) + 1 : +v];

const arr = [1, 2, "a", 4, 5];
const result = arr.reduce(reducer, []);
console.log(`in: ${JSON.stringify(arr)}`);
console.log(`out: ${JSON.stringify(result)}`);

const arr2 = ["b", 2, "a", 4, 5];
const result2 = arr2.reduce(reducer, []);
console.log(`in: ${JSON.stringify(arr2)}`);
console.log(`out: ${JSON.stringify(result2)}`);

const arr3 = ["b", 2, "a", 4, "5"];
const result3 = arr3.reduce(reducer, []);
console.log(`in: ${JSON.stringify(arr3)}`);
console.log(`out: ${JSON.stringify(result3)}`);

// or in increments by choice
const reducerFactory = step => (acc, v, i) => 
  [...acc, isNaN(+v) ? (acc[i-1] || 0) + step : +v];

const arr4 = ["b", 4, "a", 8, 10, "12"];
const result4 = arr4.reduce(reducerFactory(2), []);
console.log(`in: ${JSON.stringify(arr4)}`);
console.log(`out: ${JSON.stringify(result4)}`);

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

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