#include <iostream>
#include <string>
using namespace std;
int main()
{
    string A;
    cin >>A;
    char vocale[10]={'a','e','i','o','u','A','E','I','O','U'};
    for(int i=0; i<(A.length()) ;i++){
        for(int k=0; k<10; k++){
            if(A[i]==vocale[k]){
                A[i]='_';}
        }
    }
    cout <<A;



    return 0;
}

cc1plus: warnings being treated as errors
In function 'int main()':
Line 9: warning: comparison between signed and unsigned integer expressions