Very basic "sleep calculator" algorithm in C using cs50 library

Question

I've been following along with CS50 online and tried my first shot at making a program from scratch and without guidelines. I hope you may be able to rip it to shreds for me.

//
// this program determines how many hours you slept last night. 
// it assumes that: 
// you're using a 12 hour clock
// you sleep for a maximum of 24 hours
//
#include <stdio.h>
#include <cs50.h>
int main(void)
{ // get user bedtime
 printf("When did you go to bed last night?\n");
 int bedtime = GetInt();
 // determine AM or PM
 printf("AM [1]\nor\nPM [2]\n");
 int b = GetInt();
 // get user wake up time
 printf("When did you wake up this morning?\n");
 int wakeup = GetInt();
 // determine AM or PM
 printf("AM [1]\nor\nPM [2]\n");
 int w = GetInt();
 // interpret inputs and determine how to calculate hours slept
 {if ((b == 1) && (w == 1))
 {
 int slept = wakeup-bedtime;
 printf("You slept for %i hours last night\n",slept);
 }
 else if (((b == 1) && (w == 2)) || ((b == 2) && (w == 1)))
 {
 int slept = 12-bedtime+wakeup;
 printf("You slept for %i hours last night\n",slept);
 }
 else
 {
 int slept = (12-bedtime)+12+wakeup;
 printf("You slept for %i hours last night\n",slept);
 }
 }
}

Answer 1

A few things:

• Naming: b and w aren't particularly descriptive. Even bampm and wampm are better.

• You've got extra {} around your if statement for some reason.

• Your algorithm does a lot of tests. What if you convert to 24-hour time?

  Doing so will let you get rid of b and w by changing bedtime and wakeup after you read them.

  printf("When did you go to bed last night?\n");
  int bedtime = GetInt();
  // determine AM or PM
  printf("AM [1]\nor\nPM [2]\n");
  int ampm = GetInt();
  // convert to 24-hour time
  if (ampm == 2) bedtime = bedtime + 12;
  // get user wake up time
  printf("When did you wake up this morning?\n");
  int wakeup = GetInt();
  // determine AM or PM
  printf("AM [1]\nor\nPM [2]\n");
  ampm = GetInt();
  // convert to 24-hour time
  if (ampm == 2) wakeup = wakeup + 12;
  

  Also, the conditionals get easier:

  int slept = wakeup - bedtime;
  if (slept < 0) { slept = slept + 24 } // math is your friend
  printf("You slept for %i hours last night\n",slept);
  

• Once principle of programming is "Don't Repeat Yourself" aka "DRY". Do you really need 3 copies of the output printf?

  Of course not. You can move it after the end of the if statement.

• Finally, I don't know if this was covered in class, but you're not doing any input checking. What if the user puts in 3 instead of 1 or 2 ? What if they put in times like -5 or 27 ?