trans_rna.cpp

#include <iostream>
#include <string>
using namespace std;
// convert DNA to RNA
int main()
{
    string s;
    // take DNA string as input from stdin
    cin >> s; 
    char n; // current nucleotide
    
    // loop through sequence, if T replace with U
    for (int i=0; i<s.length(); i++)
    {
        n = s[i];

        if (n == 'T') 
        {
            s[i] = 'U';
        }
    }
    // output to stdout
    cout << s << endl;
    return 0;
}

trans_rna2.cpp

# include <iostream>
# include <string>
# include <algorithm>
using namespace std;

int main()
{
    string s;
    cin >> s; // take input from console
    replace(s.begin(), s.end(), 'T', 'U');
    cout << s << endl; // send ouput to console
    return 0;
}