How to remove repeated entries from an array while preserving non-consecutive duplicates?

Question

I have an array like var arr = [5, 5, 5, 2, 2, 2, 2, 2, 9, 4, 5, 5, 5]; I really want the output to be [5,2,9,4,5]. My logic for this was:

• Go through all the element one by one.
• If the element is the same as the prev element, count the element and do something like newA = arr.slice(i, count)
• New array should be filled with just identical elements.
• For my example input, the first 3 elements are identical so newA will be like arr.slice(0, 3) and newB will be arr.slice(3,5) and so on.

I tried to turn this into the following code:

function identical(array){
    var count = 0;
    for(var i = 0; i < array.length -1; i++){
        if(array[i] == array[i + 1]){
            count++;
            // temp = array.slice(i)
        }else{
            count == 0;
        }
    }
    console.log(count);
}
identical(arr);

I am having problems figuring out how to output an element that represents a group of element that are identical in an array. If the element isn't identical it should be outputted in the order that it is in in the original array.

Answer 1

Using array.filter() you can check if each element is the same as the one before it.

Something like this:

var a = [5, 5, 5, 2, 2, 2, 2, 2, 9, 4, 5, 5, 5];

var b = a.filter(function(item, pos, arr){
  // Always keep the 0th element as there is nothing before it
  // Then check if each element is different than the one before it
  return pos === 0 || item !== arr[pos-1];
});

document.getElementById('result').innerHTML = b.join(', ');

<p id="result"></p>

Answer 2

if you are looking purely by algorithm without using any function

var arr = [5, 5, 5, 2, 2, 2, 2, 2, 9, 4, 5, 5, 5];

    function identical(array){

        var newArray = [];
        newArray.push(array[0]);
        for(var i = 0; i < array.length -1; i++) {
            if(array[i] != array[i + 1]) {
                newArray.push(array[i + 1]);
            }
        }
        console.log(newArray);
    }
    identical(arr);

Fiddle;

Answer 3

Yet another way with reduce

var arr = [5, 5, 5, 2, 2, 2, 2, 2, 9, 4, 5, 5, 5];

var result = arr.reduce(function(acc, cur) {
  if (acc.prev !== cur) {
    acc.result.push(cur);
    acc.prev = cur;
  }
  return acc;
}, {
  result: []
}).result;


document.getElementById('d').innerHTML = JSON.stringify(result);

<div id="d"></div>

Answer 4

A bit hackey, but, hell, I like it.

var arr = [5, 5, 5, 2, 2, 2, 2, 2, 9, 4, 5, 5, 5];
var arr2 = arr.join().replace(/(.),(?=\1)/g, '').split(',');

Gives you

[5,2,9,4,5]

Admittedly this will fall down if you're using sub-strings of more than one character, but as long as that's not the case, this should work fine.

Answer 5

Try this:

var a = [5, 5, 5, 2, 2, 2, 2, 2, 9, 4, 5, 5, 5];

uniqueArray = a.filter(function(item, pos) {
return a.indexOf(item) == pos;
});

See Remove Duplicates from JavaScript Array