How to count Matching values in Array of Javascript

Question

please tell me any good algorithm/code to get list of unique values from array and count of its occurrence in array. (i am using javascript).

Answer 1

a straightforward way is to loop over the array once and count values in a hash

a = [11, 22, 33, 22, 11];
count = {}
for(var i = 0; i < a.length; i++)
   count[a[i]] = (count[a[i]] || 0) + 1

the "count" would be like this { 11: 2, 22: 2, 33: 1 }

for sorted array the following would be faster

a = [11, 11, 11, 22, 33, 33, 33, 44];
a.sort()
uniq = [];
len = a.length
for(var i = 0; i < len;) {
   for(var k = i; k < len && a[k] == a[i]; k++);
   if(k == i + 1) uniq.push(a[i])
   i = k
}   
// here uniq contains elements that occur only once in a