count occurence of elements in an array

Question

Hello I want to ask how to find same number or string on array The output must be a number or string

Example var arr = [1,2,4,4,5,1,1,1,8,9]

Do we need to use loop , function?

do you want to know the number of duplicate items in an array? — Saif Azmi, Sep 15 '18 at 00:30
can you precise if you need to use pure vanilla js, or if you can use external libraries for that ? — JR Utily, Sep 15 '18 at 00:40
I want to just need pure js , I am still learning and I get a little confused when to use loop and when to use function, — Lion Z, Sep 15 '18 at 00:42
If you agree this is duplicate, you can delete this question. And if not, please edit it to precise what do you need that is not explained there. — JR Utily, Sep 15 '18 at 01:15

score 0 · Answer 1 · edited Sep 15 '18 at 01:39

You can create a frequency array. Admittedly this might not be the most efficient way. You can use dictionaries for this. I hope you know what dictionaries are.

function fun(arr) {
     var sval;
     var dict = new Object();
     for (var val in arr) {
            sval = string(val) 
            if(dict[sval] === undefined) {
                  dict[sval] = 1;
            }
            else {
                  dict[sval]++;
            }
      }
      var max_count = -5;
      var max_count_num;
      for (var key in dict) {
             if (max_count < dict[key]) {
                    max_count = dict[key];
                    max_count_num = Number(key);
                  }
      }
}

First you create an Object, which is for our purposes serves as a dictionary, then you iterate through the array. If the key is not found, hence undefined, then we create such entry, else we increment the value which is the count by 1.

We then loop through the dictionary trying to find the number with the maximum value, hence the maximum count.

I hope this is what you are looking for. Please fix any errors in my code, I'm a bit rusty on JavaScript.

Romildo da Silva · Answer 2 · 2018-09-15T02:11:22.973

Iterate for each item on the array (i index), and iterate for each item for the rest of the array (j index, j = i+1), and create a count to any repetition of current item.

function maxRepeated(arr) {

  // count global to keep the max value count
  const maxRepeat = {
    value: 0,
    times: 0
  };
  for (var i = 0; i < arr.length - 1; i++) {
    var maxLocal = 1; // to count ar[i] item repetitions
    for (var j = i + 1; j < arr.length; j++) {
      if (arr[j] == arr[i]) maxLocal++;
      console.log( maxLocal )
      console.log( "I " + i + " arr[i] " + arr[i] )
      
    }

    // check if maxLocal is great than max global
    if (maxLocal > maxRepeat.times) {
      maxRepeat.value = arr[i];
      maxRepeat.times = maxLocal;
    }
  }
  return maxRepeat;
}
const ar = [1, 2, 3, 1, 3, 5, 1, 1, 1, 5, 4, 3, 3, 2, 2, 3];
const b = ["LUNA", "LUNA", "JS", "JS", "JS"];

//console.log(maxRepeated(ar))
console.log(maxRepeated(b))

count occurence of elements in an array

2 Answers2