how to get unique elements and how many times they appear in an array of ints (for C, but compiled with C++)

unique_array.cpp

// Example program
#include <iostream>
#include <string>

int main(void) {
    
    int arr[13] = {1, 2, 2, 123121, 123121, 3, 5, 6 , 7, 7, 14, 2, 16};
    int len = 13;
    
    int unique[len];
    int duplicates[len];
    int j, n, count, flag;

    unique[0] = arr[0];

    count = 1;/*one element is counted*/

    for(j=0; j <= len-1; ++j) {
        duplicates[j] = 1;
        flag = 1;;
        /*the unique array will always have 'count' elements*/
        for(n=0; n < count; ++n) {
            if(arr[j] == unique[n]) {
                if(j != n){
                    duplicates[n] += 1;
                }
                flag = 0;
            }
        }
        if(flag == 1) {
            ++count;
            unique[count-1] = arr[j];
        }
    }
    
    
    printf("count: %d \n", count);
    for(int k=0;k<count;k++){
        std::cout << "unique elem: " << unique[k] << " appearing " << duplicates[k] << " times" <<"\n";
    }
    return 0;
}