Modify repeated values in javaScript

Question

There is an array of JavaScript:

let arr=[
[220,A,12],
[560,B,5],
[300,A,0],
[300,H,3], 
[300,K,T],
[450,P,9],
[500,E,6],
[500,M,8]
];

The first element of sub-arrays have some repeated values,
I want to find them and modify the repeated values, as follow:

[300,A,0],  ===>  remain unchanged
[300,H,3],  ===>  [320,H,3],  //First element add 20
[300,K,T],  ===>  [340,K,T],  //First element add 20 again

[500,E,6],  ===>  remain unchanged
[500,M,8]   ===>  [520,M,8] //First element add 20

That is, the first element adds 20 in proper order if repeated.
Other elements don't need to be considered.
How to do it in javaScript?

Answer 1

One option is to use reduce . I pass an object as the initial value to reduce :

{
    last: null,  // The previous number encountered
    offset: 20,  // The amount to add on repeat
    data: []     // The new array
}

Then, use this to build the new array:

 let arr=[ [220,'A',12], [560,'B',5], [300,'A',0], [300,'H',3], [300,'K','T'], [450,'P',9], [500,'E',6], [500,'M',8] ]; arr = arr.reduce((prev, cur) => { // Is this a repeat? if (prev.last == cur[0]) { cur[0] += prev.offset; prev.offset += 20; } // Not a repeat. Reset. else { prev.last = cur[0]; prev.offset = 20; } prev.data.push(cur); return prev; }, {last: null, offset: 20, data: []}).data; console.log(arr);

Answer 2

using map key:value, where key is number and value is its occurrence so far

 let arr = [ [220, 12], [560, 5], [300, 0], [300, 3], [300, 0], [450, 9], [500, 6], [500, 8] ]; m = {} for (let i = 0; i < arr.length; ++i) { const x = arr[i][0] if (m[x] > 0){ arr[i][0] += (m[x]) *20 m[x]++ } else m[x] = 1 } console.log(arr)