Counting sort problem in understanding code

#1

void countSort(int *array, int size) {
int output[size+1];
int max = getMax(array, size);
int count[max+1]; //create count array (max+1 number of elements)
for(int i = 0; i<=max; i++)
count[i] = 0; //initialize count array to all zero
for(int i = 1; i <=size; i++)
count[array[i]]++; //increase number count in count array.
for(int i = 1; i<=max; i++)
count[i] += count[i-1]; //find cumulative frequency
for(int i = size; i>=1; i–) {
output[count[array[i]]] = array[i];
count[array[i]] -= 1; //decrease count for same numbers
}
for(int i = 1; i<=size; i++) {
array[i] = output[i]; //store output array to main array
}
}

the doubt i have is
for(int i = size; i>=1; i–) {
output[count[array[i]]] = array[i];
count[array[i]] -= 1; //decrease count for same numbers
}
i am not getting this for loop kindly help me to understand it

#2

hey @angivanshikaangi, they are actually buliding output in reverse order which is correct also.
Consider this one, here output array is building from 0 index. It is easy to understand.
for (i = 0; arr[i]; ++i)
{
output[count[arr[i]]-1] = arr[i];
–count[arr[i]];
}

Dry run this code once, than try that for loop.