C++：为什么我的字符串没有被打印？

Question

I am trying to convert time from 12 hour format to 24 hour format. It is a hackerrank question which I am trying to solve. Here is my code:-

#include <bits/stdc++.h>

using namespace std;

string timeConversion(string s) {
    string r;
    int hh,mm,ss;
    hh = (s[0]-'0')*10+(s[1]-'0');
    mm = (s[3]-'0')*10+(s[4]-'0');
    ss = (s[6]-'0')*10+(s[7]-'0');

    if(hh<12 && s[8] == 'P') hh +=12;
    if(hh == 12 && s[8]=='A') hh = 0;
    r[0]=hh/10+'0';
    r[1]=hh%10+'0';
    r[2]=':';
    r[3]=mm/10+'0';
    r[4]=mm%10+'0';
    r[5]=':';
    r[6]=ss/10+'0';
    r[7]=ss%10+'0';
    r[8]='\0';

    return r;
}

int main() {
    string s;
    cin >> s;
    string result = timeConversion(s);
    cout << result << endl;
    return 0;
}

This code is giving blank output. Though when I cout result[0] then this gives me proper result.

Answer 1

See CppReference

operator []

No bounds checking is performed. If pos > size() , the behavior is undefined.

When you define string r , its size() is zero, so all your character assignments are undefined behaviors. You may want to resize it first:

r.resize(9);

Or alternatively, append the characters one-by-one:

r = "";
r.push_back(hh/10+'0');
r.push_back(hh%10+'0');
r.push_back(':');
r.push_back(mm/10+'0');
r.push_back(mm%10+'0');
r.push_back(':');
r.push_back(ss/10+'0');
r.push_back(ss%10+'0');
// This isn't necessary
// r.push_back('\0');

Using operator+= makes your code look more natural:

r = "";
r += hh/10+'0';
r += hh%10+'0';
r += ':';
r += mm/10+'0';
r += mm%10+'0';
r += ':';
r += ss/10+'0';
r += ss%10+'0';
// This isn't necessary
// r += '\0';

Note you don't need the terminating zero because std::basic_string isn't null-terminated (it's not C-style string).