Subtract time from another time

There are a lot of ways to solve this problem.  Instinctively, our brains want to subtract two time values just as we do two integers.  But the "borrow" can get a little bit messy because the time format of Hh:Mm.  When subtracting column m, if a borrow is necessary we decrement M and add 10 the smaller value.  When subtracting column M, we decrement h and add 6 to M.

It becomes an excercise in programming that we learn to avoid.

The easiest way is to convert each time value to a common integer value.  In this case, the number of minutes per day (24*60=1440) works well, so does the number of seconds per day (24*60*60=86400).  Then subtract the two integers and convert back to HH:mm

Here are the formulas.  Since this sounds like homework, you'll have to plug them in yourself:

MinuteValue = Hours * 60 + Minutes;  // Convert HH:MM to minutes

Hh = MinuteValue / 60;
Mm = MinuteValue % 60;


Good Luck,
Kent

Hi Pete420,

Your program doesn't deal with input values very well.  Here's a style that is a bit more useful:

#define BUFFL 1024

#include <stdlib.h>

void main (void){
  char Buffer[BUFFL];  
  int hours1, hours2, mins1, mins2;
  int time1, time2;

      printf("Please enter the first time (hh:mm): ");
     fgets (Buffer, BUFFL, stdin);
     hours1 = mins1 = -1;
     scanf (Buffer, "%d:%d", &hours1, &mins1);
     if (hours1 < 0 || hours1 > 23 || mins1 < 0 || mins 1 > 59)
     {
       printf ("You did not enter a valid date\n");
       exit (0);
     }

     printf("Please enter the second time (hh:mm): ");
     fgets (Buffer, BUFFL, stdin);
     hours1 = mins1 = -1;
     scanf (Buffer, "%d:%d", &hours2, &mins2);
     if (hours2 < 0 || hours2 > 23 || mins2 < 0 || mins2 > 59)
     {
       printf ("You did not enter a valid date\n");
       exit (0);
     }

    time1 = hours1 * 60 + mins1;
    time2 = hours2 * 60 + mins2;

/*
    Just in case this is homework, I really can't write the entire program for you.  :)
    Subtracting time2 from time1 will give you the number of minutes that have elapsed.

    Convert the difference into HH:MM format with the equations that I gave you in my first post and display them.
*/
}


Kent

@pete420:

> this program is to subtract time from another time in 24 hour format. eg. 1hr
> 0mins - 1hr 30mins should read 23.30. however in my program when it works out
> it reads 1-1 = 0 and not 23.  
> any ideas? thanks

I would use the C standard library to accomplish this.  The functions you need are basically "gmtime" and "mktime".  This page has good info:  http://www.acm.uiuc.edu/webmonkeys/book/c_guide/2.15.html

Here is your program re-written to use those functions:
      #include <stdio.h>
      #include <time.h>

      void main (void)
      {
            struct tm theTime;
            int nHours, nMinutes;
            time_t tTemp;

            /* Start with the current time */
            time( &tTemp );
            memcpy( &theTime, gmtime(&tTemp), sizeof(theTime) );
            theTime.tm_isdst = -1; /* Set DST */
            theTime.tm_sec = 0;

            /* Read the start time */
            printf("Please enter what hour it is: ");
            scanf("%d", &theTime.tm_hour);
            fflush(stdin);

            printf("Please enter how many minutes have passed: ");
            scanf("%d", &theTime.tm_min);
            
            /* Read the elapsed time */
            printf("Please enter hour 2: ");
            scanf("%d", &nHours);
            fflush(stdin);

            printf("Please enter how many minutes have passed: ");
            scanf("%d", &nMinutes);

            /* Adjust the time */
            theTime.tm_hour -= nHours;
            theTime.tm_min -= nMinutes;
            mktime(&theTime);

            /* Display the result */
            printf("Answer time is : %d:%d\n", theTime.tm_hour, theTime.tm_min);
      }

Hope That Helps,
Dex*

my updated code now reads. however im still gettin the wrong answer. ken thanks for the advice on the better input way however im  not sure of every feature so i think it best not 2 use it.  dexstar, altho ur way seems to be a gd way to solve this problem again i am not sure of the functions.

#include <stdio.h>

void main (void){
      
int hours1, hours2, mins1, mins2, Hh, Mm, time1, time2, minval;

      printf("Please enter what hour it is: ");
      scanf("%d", &hours1);
      fflush(stdin);

      printf("Please enter how many minutes have passed: ");
      scanf("%d", &mins1);

      printf("Please enter hour 2: ");
      scanf("%d", &hours2);
      fflush(stdin);

      printf("Please enter how many minutes have passed: ");
      scanf("%d", &mins2);

      time1 = hours1 * 60 +mins1;
      printf("time1 %d", time1);
      time2= hours2 * 60+ mins2;
      printf("time2 %d", time2);
      minval = time1 - time2;

      Hh = minval / 60;
      Mm = minval % 60;

       printf("Answer time is : %d:%d\n", Hh, Mm);

}

Just try the code that I posted, and then ask me about the parts that you do not understand...

The key of it is the mktime function (More Info: http://www.unidata.ucar.edu/cgi-bin/man-cgi?mktime+3).  It will correct a struct that contains the time values to be correct.  Here's how the program works:

1) It fills in the time structure with the current time and date information.
2) I change the hour and minute parts of the time to be the first hour and minute entered by the user.
So, with your sample data, then:
     theTime.tm_hour = 1;
     theTime.tm_min = 0;
(The other date information will have today's date)

Then, I subtract off the other hour and minute from those values, so with your sample data:
     theTime.tm_hour = 0;
     theTime.tm_min = -30;

Clearly that looks wrong because you can't have -30 minutes!  But, after you do that, if you call "mktime", it will "correct" your struct to have the correct time.  It will look like this:
     theTime.tm_hour = 23;
     theTime.tm_min = 30;
And the date information will be from YESTERDAY!

Just try the code I gave you... I tested it and everything, it should work just fine.

Dex*

yes ur solution works first class however i cannot use it for this assignment due to none of the features being mentioned before. thanks loads for the help but i think ken's solution is more along the lines of what is expected.. any ideas of how to do it that way?

Cheers man! works a treat!