Javascript - How to number an array?

nbs335
August 13, 2024
222 views
3 votes
3 Answers

I have an array like this : [‘a’, ‘a’, ‘b’, ‘c’, ‘a’ ]

I want to obtain this : [0, 0, 1, 2, 0]

The idea is to give the first value of the arr1 array 0 value, the second 1, etc…

I tried this :

function onlyUnique(value, index, array) {
  return array.indexOf(value) === index;
}
let arr1 = ['a', 'a', 'b', 'c', 'a' ]
var arr1_uniq= arr1.filter(onlyUnique);
let k=0
var list = []
for (let i in arr1_uniq){
    list.push(k)
    k++
}

So I have the arr1 without duplicate : [‘a’, ‘b’, ‘c’] And the corresponding values : [0, 1, 2]

And finally I tried to use the findIndex function to reallocate arr1 values but I have issues.

Tags: javascript

Answers

It should be possible by mapping the values to its unique index like this:

function indexMapper(arr) {
    const uniqueValues = [...new Set(arr)];
    const inverseMap = Object.fromEntries(uniqueValues.map((val, index) => [val, index]));
    return arr.map(value => inverseMap[value]);
}

// Example usage
let arr1 = ['a', 'a', 'b', 'c', 'a'];
let result = indexMapper(arr1);
console.log(result);  // [0, 0, 1, 2, 0]

You can use a Map<Object, Number> and a counter to keep track of the indices.

const indexArray = (arr) => {
  const lookup = new Map();
  let currIndex = 0;
  return arr.map((item) => {
    if (lookup.has(item)) return lookup.get(item);
    lookup.set(item, currIndex);
    return currIndex++;
  });
};

const arr = ['a', 'a', 'b', 'c', 'a'];
const res = indexArray(arr);

console.log(...res); // [0, 0, 1, 2, 0]

You can remove the need for the currIndex if you utilize the size property of the map.

const indexArray = (arr) => {
  const lookup = new Map();
  return arr.map((item) => {
    if (!lookup.has(item)) {
      lookup.set(item, lookup.size);
    }
    return lookup.get(item);
  });
};

const arr = ['a', 'a', 'b', 'c', 'a'];
const res = indexArray(arr);

console.log(...res); // [0, 0, 1, 2, 0]

Here it is in one line:

const indexArray = (arr) =>
  (lookup =>
    arr.map((item) =>
      (lookup.has(item)
        ? lookup
        : lookup.set(item, lookup.size)
      ).get(item)))
  (new Map());

const arr = ['a', 'a', 'b', 'c', 'a'];
const res = indexArray(arr);

console.log(...res); // [0, 0, 1, 2, 0]

And now, the code golf:

// 64 bytes
f=a=>(m=>a.map(e=>(m.has(e)?m:m.set(e,m.size)).get(e)))(new Map)

console.log(...f(['a', 'a', 'b', 'c', 'a'])) // [0, 0, 1, 2, 0]

Here is the desired code sample using Map:

function mapToIndexes(arr) {
    const map = new Map();
    return arr.map((item, index) => {
        if (!map.has(item)) {
            map.set(item, map.size);
        }
        return map.get(item);
    });
}

const arr = ['a', 'a', 'b', 'c', 'a'];
const result = mapToIndexes(arr);
console.log(result); // Output: [0, 0, 1, 2, 0]

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Javascript – How to number an array?

Answers