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Program uses overloaded binary operators to add and subtract time. Need to add code to prevent time from becoming negative when subtracting.

This program is properly adding and subtracting time from a given time.  However, it returns a negative time if you subtract a larger number than the current hour.  I need help to add some code to prevent this.  Here is my program:

#include <iostream>
#include <iomanip>
using namespace std;

class Time
{
private:
    int hour;
    int minute;
    int second;
public:
    // Default constructor
    Time(int h=0, int m=0, int s=0) { set(h, m, s); }
    // Overload the binary += operator
    void operator+=(Time &t);
      void operator-=(Time &t);
    void display() const
    {
        cout << setw(2) << setfill('0') << hour << ":"
             << setw(2) << setfill('0') << minute << ":"
             << setw(2) << setfill('0') << second << endl;
    }
    void set(int h=0, int m=0, int s=0);
};


int main()
{
    Time t1, t2, t3;
    t1.set(23, 30, 15);
      cout<<"The original time is:\n";
      t1.display();
    t2.set(12, 01, 46);
    t1 += t2;
      cout<<"The time with 12 hours added is:\n";
    t1.display();
      t3.set(15, 00, 00);
      cout<<"The time with 15 hours subtracted is:\n";
      t1 -= t3;
      t1.display();
    return 0;
}

void Time::operator +=(Time &t)
{
    // Use the set function to take care of the addition
    set(hour + t.hour, minute + t.minute, second + t.second);
}

void Time::operator -=(Time &t)
{
      set(hour - t.hour, minute - t.minute, second - t.second);
}

void Time::set(int h, int m, int s)
{
    second = s % 60;
    m += s / 60;
    minute = m % 60;
    h += m / 60;
    hour = h % 24;
}
0
coririzzo
Asked:
coririzzo
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1 Solution
 
drichardsCommented:
Don't you also have the same problem with minutes and seconds?  Something like this maybe:
--------------------------------------------------------------
void Time::set(int h, int m, int s)
{
    if ( s < 0 )
    {
        second = (s % 60) + 60;
        m += (s/60 -1);
    }
    else
    {
        second = s % 60;
        m += s/60;
    }
    if ( m < 0 )
    {
        minute = (m % 60) + 60;
        h += (s/60 - 1);
    }
    else
    {
        minute = m % 60;
        h += m/60;
    }
    hour = h % 24;
    if ( hour < 0 ) hour += 24;
}--------------------------------------------
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drichardsCommented:
A bit more compact:

void Time::set(int h, int m, int s)
{
    int exs = (s < 0 ? 60 : 0);
    second = (s % 60) + exs;
    m += (s-exs)/60;

    exs = (m < 0 ? 60 : 0);
    minute = (m % 60) + exs;
    h += (m-exs)/60;

    hour = h % 24;
    if ( hour < 0 ) hour += 24;
}
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coririzzoAuthor Commented:
What if the set time function calls separate functions for setHour, set Minute, and set Second?  In this case the setHour works fine, but the compiler says that h is an undeclared identifier in setMinute and m is an undeclared identifier in setSecond.  How can I pass the "extra" time to these functions?  I can post another question for more points if necessary.  Thanks!

Time::Time( int hr, int min, int sec )
{
   setTime( hr, min, sec );

} // end Time constructor

Time &Time::setTime( int h, int m, int s )
{
   setSecond( s );
   setMinute( m );
   setHour( h );
   return *this;   // enables cascading
}

Time &Time::setHour( int h )
{
    hour = h % 24;
    if (hour < 0 ) hour += 24;
    return *this;   // enables cascading
}

Time &Time::setMinute( int m )
{
      if ( m < 0 )
      {
            h += (s/60 -1);
            minute = (m % 60) + 60;
      }
      else
      {
            h += m/60;
            minute = m % 60;
      }
   return *this;   // enables cascading
}

Time &Time::setSecond( int s )
{
      if (s < 0)
      {
            second = (s % 60) + 60;
            m += (s/60 -1);
      }
      else
      {
            second = s % 60;
            m+= s/60;
      }
   return *this;   // enables cascading
}
0
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drichardsCommented:
What does it mean to set one of the components but not the others?  One of these should be correct.  Depends whether you keep the current values for the parts you are not setting or not.

Zeroing out the other parts of the time:
-----------------------------------------
Time &Time::setHour( int h )
{
    SetTime(h, 0, 0);
    return *this;   // enables cascading
}

Time &Time::setMinute( int m )
{
    SetTime(0, m, 0);
    return *this;   // enables cascading
}

Time &Time::setSecond( int s )
{
    SetTime(0, 0, s);
    return *this;   // enables cascading
}
-------------------------------------------

or keeping the other parts:

-------------------------------------------
Time &Time::setHour( int h )
{
    SetTime(h, minute, second);
    return *this;   // enables cascading
}

Time &Time::setMinute( int m )
{
    SetTime(hour, m, second);
    return *this;   // enables cascading
}

Time &Time::setSecond( int s )
{
    SetTime(hour, minute, s);
    return *this;   // enables cascading
}
0
 
coririzzoAuthor Commented:
setTime calls the functions setHour, setMinute, and setSecond.  So it doesn't work to call setTime within setHour (or setMinute or set Second).  This causes a compiler exception error and stack overflow.
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drichardsCommented:
I didn't notice that you changed setTime.  You're better off doing it the opposite way - have setHour, etc. call setTime.  SetTime controls all three parts of the time.   Otherwise, setSecond will have to touch the minutes and setMinute will have to touch the hours.  May as well just defer everything to the one place that takes care of it all.

In fact, you really can't do it easily the way you've got things defined.  Look carefully:

'setTime' takes h, m ,s as parameters.  When you call 'setSecond' you may have overflow that needs to be added to the minutes.  Unfortunately, when you get back to 'setTime' to do 'setMinute', you have lost this information and you can't update m properly.  With your functions defined as they are, there's no simple way to reconcile this.  I'd put 'setTime' back the way it way and have the other three call it.

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coririzzoAuthor Commented:
Thanks for all your input on this.  Unfortunately, it is homework, and I have to use code from the textbook a a base.  It uses the setTime calling setHour, setMinute, and setSecond.  I have it working, but without your neat error checking stuff in there.  A couple of people at work are going to look at it, so maybe I'll get it figured out.
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drichardsCommented:
It sounds to me then that the setters should not accept values outside acceptable range:  minutes and seconds should be from 0 to 59 and hours from 0 to 23.  That keeps the normalozation logic in the add and subtract operations rather than in the setters.  Seems simpler.Tring to set the seconds to 75 would be illegal.  This eliminates all of the complexity.
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