js: create an array that holds the indexof duplicates in array

Question

I'm looking for a js/jquery solution for the following

I want to check an array for duplicates, lets say:

var destRes = ['BH17 7EP', 'TA2 6LL', 'OX15 4LJ', 'OX15 4LJ', 'OX15 4LJ', 'BH17 7EP', 'TA2 6LL']

and return an array that holds the first index of that value; for example for the above ex this should be the return:

var newArr = ['0', '1', '2', '2', '2', '0', '1' ]

BH17 7EP is at index 0 and 5, so in the new array the 0 and the 5 indexes should be 0, the first instance of BH17 7EP

TA2 6LL is at index 1 and 6 so in the new array the 1 and the 6 indexes should be 1, the first instance of TA2 6LL

Dij · Accepted Answer · 2017-08-16 12:31:12Z

6

you can iterate over the array and use indexOf() it always returns the first index of a value in the array. something like this:

var destRes = ['BH17 7EP', 'TA2 6LL', 'OX15 4LJ', 'OX15 4LJ', 'OX15 4LJ', 'BH17 7EP', 'TA2 6LL'];

var indexes = destRes.map(x => destRes.indexOf(x));

console.log(indexes);

answered Aug 16, 2017 at 12:31

Dij

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1 Comment

florinescu Over a year ago

Love it; Works like a charm; exactly what I was looking . Cheers

Nina Scholz · Accepted Answer · 2017-08-16 12:33:00Z

0

You could map the indices with a hash table, without using another loop for Array#indexOf.

var array = ['BH17 7EP', 'TA2 6LL', 'OX15 4LJ', 'OX15 4LJ', 'OX15 4LJ', 'BH17 7EP', 'TA2 6LL'],
    temp = Object.create(null),
    result = array.map(function (a, i) {
        return a in temp ? temp[a] : temp[a] = i;
    });
    
console.log(result);
console.log(temp);

.as-console-wrapper { max-height: 100% !important; top: 0; }

answered Aug 16, 2017 at 12:33

Nina Scholz

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2 Comments

Serge K. Over a year ago

Doesn't in operator perform a loop in background ?

Nina Scholz Over a year ago

yes, but not implicit. hash table perform better than searches over the elements.

Serge K. · Accepted Answer · 2017-08-16 12:45:10Z

0

try this one

var destRes = ['BH17 7EP', 'TA2 6LL', 'OX15 4LJ', 'OX15 4LJ', 'OX15 4LJ', 'BH17 7EP', 'TA2 6LL'];
var newArr = [];
for (var i = (destRes.length - 1); i >= 0; i--) {
  var temp = 0;
  for (var j = 0; j < i; j++) {
    if (destRes[j] == destRes[i]) {
      temp = 1;
      break;
    }
  }
  if (temp == 1) {
    newArr[i] = j;
  } else {
    newArr[i] = i;
  }
}
console.log(newArr);

edited Aug 16, 2017 at 12:45

Serge K.

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answered Aug 16, 2017 at 12:43

Ravi Kharinta

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1 Comment

Serge K. Over a year ago

You could replace you second for-loop with a call to indexOf

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