assert and enforce Solutions - D Programming Language
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# assert and enforce Solutions

#### `assert` and `enforce` Dersi Problem Çözümleri

1. You will notice that the program terminates normally when you enter `06:09` and `1:2`. However, you may notice that the start time is not what has been entered by the user:

       1 hours and 2 minutes after 09:06 is 10:08.

   As you can see, although the time that has been entered as `06:09`, the output contains `09:06`. This error will be caught by the help of an `assert` in the next problem.

2. The `assert` failure after entering `06:09` and `15:2` takes us to the following line:

       string timeToString(int hour, int minute) {
           assert((hour >= 0) && (hour <= 23));
           // ...
       }

   For this `assert` check to fail, this function must have been called with invalid `hour` value.

   The only two calls to `timeToString()` in the program do not appear to have any problems:

           writefln("%s hours and %s minutes after %s is %s.",
                    durationHour, durationMinute,
                    timeToString(startHour, startMinute),
                    timeToString(endHour, endMinute));

   A little more investigation should reveal the actual cause of the bug: The hour and minute variables are swapped when reading the start time:

           readTime("Start time", startMinute, startHour);

   That programming error causes the time to be interpreted as `09:06` and incrementing it by duration `15:2` causes an invalid hour value.

   An obvious correction is to pass the hour and minute variables in the right order:

           readTime("Start time", startHour, startMinute);

   The output:

       Start time? (HH:MM) 06:09
       Duration? (HH:MM) 15:2
       15 hours and 2 minutes after 06:09 is 21:11.

3. It is again the same `assert` check:

           assert((hour >= 0) && (hour <= 23));

   The reason is that `addDuration()` can produce hour values that are greater than 23. Adding a remainder operation at the end would ensure one of the output guarantees of the function:

       void addDuration(int startHour, int startMinute,
                        int durationHour, int durationMinute,
                        out int resultHour, out int resultMinute) {
           resultHour = startHour + durationHour;
           resultMinute = startMinute + durationMinute;

           if (resultMinute > 59) {
               ++resultHour;
           }

           resultHour %= 24;
       }

   Observe that the function has other problems. For example, `resultMinute` may end up being greater than 59. The following function calculates the minute value correctly and makes sure that the function's output guarantees are enforced:

       void addDuration(int startHour, int startMinute,
                        int durationHour, int durationMinute,
                        out int resultHour, out int resultMinute) {
           resultHour = startHour + durationHour;
           resultMinute = startMinute + durationMinute;

           resultHour += resultMinute / 60;
           resultHour %= 24;
           resultMinute %= 60;

           assert((resultHour >= 0) && (resultHour <= 23));
           assert((resultMinute >= 0) && (resultMinute <= 59));
       }

4. Good luck.

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