24HR to 12HR convert program, new values not recognized in output? - c++

Question

I am trying to learn C++ and have an issue with a small conversion program about time formats.
#include <iostream>
using namespace std;
void conversion(int hours, int minutes) {
if (hours > 12) {
hours -= 12;
minutes = minutes;
}
else
hours = hours;
minutes = minutes;
cout << hours << ":" << minutes << endl;
}
void output(int hours, int minutes) {
cout << hours << ":" << minutes;
}
int main() {
int hours, minutes;
cout << "Enter the hours: ";
cin >> hours;
cout << "Enter minutes: ";
cin >> minutes;
conversion(hours, minutes);
output(hours, minutes);
}
In the main function, the output call is not receiving the updated values for hours and minutes.

Answer 1

This has nothing to do with time formats. A much smaller program that shows the same problem is:
#include <iostream>
using namespace std;
void f(int i) {
i = 20;
}
int main() {
int i = 10;
f(i);
cout << "i is still " << i << endl;
}
The problem is that int i in the parameter list means "pass by value". The function gets a copy of the value. Any changes you make to that copy are thrown away when the function exits. Either pass by reference (void f(int& i)) or by pointer (void f(int *pi)). Look up reference and pointers in your text book.
There are also a number of issues in your function:
void conversion(int hours, int minutes) {
if (hours > 12) {
hours -= 12;
minutes = minutes; // This line does nothing.
}
else
hours = hours; // This line does nothing.
minutes = minutes; // This line will get executed even if hours was originally
// >12. (Only the hours=hours line is covered by the else).
cout << hours << ":" << minutes << endl;
}
You probably wanted:
void conversion(int& hours, int& minutes) {
if (hours > 12) {
hours -= 12;
}
cout << hours << ":" << minutes << endl;
}