Breaking an if statement inside a for-loop - c

Question

How to get loop display to end after execution where gallons < 100 and hours < 24. I'm able to get the "The fish died after ... hours" but unsure how to stop the first printf() in the for loop from executing.
I've tried using a break statement indented in the if statement, but that only affects the for loop.
#include <stdio.h>
int main()
{
double add, vol = 500;
int hour;
printf("Please enter additional water added per hour: ");
scanf("%lf", &add);
for (hour = 1; hour <= 24; hour++)
{
vol = (vol * 0.90) + add;
printf("The volume is %.2f gallons after %d hours.\n", vol, hour);
if (hour <= 23 && vol < 100)
printf("The fish died after %d hours.", hour);
else if (hour == 24 && vol >= 100)
printf("Alas, The fish who lived.");
}
return 0;
}
Expected result:
Please enter additional water added per hour: 6
The volume is 456.00 gallons after 1 hours.
The volume is 416.40 gallons after 2 hours.
The volume is 380.76 gallons after 3 hours.
...
The volume is 103.33 gallons after 22 hours.
The volume is 99.00 gallons after 23 hours.
The fish died after 23 hours.
Actual result:
Please enter additional water added per hour: 6
The volume is 456.00 gallons after 1 hours.
The volume is 416.40 gallons after 2 hours.
The volume is 380.76 gallons after 3 hours.
...
The volume is 103.33 gallons after 22 hours.
The volume is 99.00 gallons after 23 hours.
The fish died after 23 hours. The volume is 95.10 gallons after 24 hours.

Answer 1

C is not Python, indentation is not significant syntactically. Compound statements must be enclosed in {..}:
if (hour <= 23 && vol < 100)
{
printf("The fish died after %d hours.", hour);
break ;
}
else if (hour == 24 && vol >= 100)
{
printf("Alas, The fish who lived.");
}
I suggest you always use {..} for conditional or loop blocks, even for single statements. It makes maintenance simpler.
However, break; is arguably a rather inelegant and poorly structured way of exiting a loop (along with continue). A better structured solution is to terminate the loop by the loop constraint alone. In this case this can be done by:
for( hour = 1;
vol > 100 && hour <= 24;
hour++)
There is the overhead of an additional test but in more complex code than this, with perhaps multiple breaks, it can become difficult to maintain, debug and comprehend.

Answer 2

You are still having a bit of problem figuring out your loop control. You are also inviting an Endless Loop if a matching failure occurs during input because you fail to check the return of scanf (probably one of the biggest pitfalls that new C programmers fall into). Try it. Type "one gallon" at your prompt for input and see what happens. Further, after checking the return, you have to handle 3-cases:
scanf can be used, if used correctly. This means you are responsible for checking the return of scanf every time.
(return == EOF) the user canceled input by generating a manual EOF by pressing Ctrl+d (or on windows Ctrl+z, but see CTRL+Z does not generate EOF in Windows 10 (early versions));
(return < expected No. of conversions) a matching or input failure occurred. For a matching failure you must account for every character left in your input buffer. (scan forward in the input buffer reading and discarding characters until a '\n' or EOF is found); and finally
(return == expected No. of conversions) indicating a successful read -- it is then up to you to check whether the input meets any additional criteria (e.g. positive integer, positive floating-point, within a needed range, etc..).
Onto your loop logic. Given your example and your question "Breaking an if statement inside a for-loop", I think I know where you want to go -- and you can get there with one of the simplest, least used and frankly, mandatory, expressions for nested loop control. The good ole goto statement. It still has a place in programming, and while it should not be overly used, there are a few circumstances, such as those above, and in fact you circumstance, where it is fine to use the goto to jump out of a loop and pass control a few lines down. (the limitation is a longjmp -- you don't want to use it to jump out of a function)
For example, if I understand what you are going for in fish-survival, you can clean up your logic and incorporate the scanf validations by putting the pieces together similar to the following:
#include <stdio.h>
int main (void) {
double add, vol = 500;
int hour;
for (;;) { /* loop continually until valid input is received */
int rtn; /* varible for scanf return - EVERY TIME */
/* there is no conversion taking place, fputs will do */
fputs ("Please enter additional water added per hour: ", stdout);
if ((rtn = scanf ("%lf", &add)) == EOF) { /* handle EOF case */
fputs ("(user canceled input)\n", stderr);
return 1;
}
else if (rtn == 0) { /* handle matching failure */
fputs ("error: invalid double input.\n", stderr);
/* empty all characters from stdin before next input */
for (int c = getchar(); c != '\n' && c != EOF; c = getchar()) {}
}
else /* handle good input */
break;
}
for (hour = 0; hour < 24; hour++) { /* loop 0 < 24 */
vol = (vol * 0.90) + add; /* adjust output hour + 1 below */
printf ("The volume is %.2f gallons after %d hours.\n", vol, hour+1);
if (vol < 100) { /* don't forget the '\n' */
printf ("The fish died after %d hours.\n", hour + 1);
goto deadfish; /* jump to deadfish label */
}
}
puts ("Alas, The fish who lived."); /* again, no conversion */
deadfish:; /* good ole goto label */
return 0;
}
(note: generally loop in C will run 0 < n and you adjust the output as required above. While this is not mandatory, it is very much by convention, and accommodates arrays being zero-indexed in C, as well as the hours on a clock that run 0 -> 23, not 1 - > 24. The title of the movie "Zero Dark Thirty" is a good analogy (e.g. zero hour thirty minutes or 12:30 am.) You can't get there looping 1 <= 24, so those limits fail to model the concept of time correctly)
Notice above the use of the goto. It is used in conjunction with the vol < 100 check. If vol < 100 tests TRUE, then control is passed to the label associated with the goto statement. (the label being deadfish:; here). This provides several benefits here. Your for loop can simply track time -- the hours the tank is leaking up to 24. Note how the volume falling below 100 causes control to jump over the living fish output which would execute on normal loop exit.
While you may be able to get your desired dead fish output by incorporating the if (vol < 100) /* print dead fish */; break; else if ... it becomes very difficult to get your /* print live fish */ method on normal loop exit that way without checking the loop counter again as part of the else if clause. The goto provides an elegant solution to just that problem by allowing you to pass control over the /* print live fish */ case when the volume falls below the limit, while at the same time preserving the normal loop exit without having to test your loop counter variable a second time within the loop itself.
Example Use/Output
Make sure your program can handle a cat stepping on the keyboard for every user input. You must Validate every time, e.g.
$ ./bin/deadfish
Please enter additional water added per hour: one gallon
error: invalid double input.
Please enter additional water added per hour: 1
The volume is 451.00 gallons after 1 hours.
The volume is 406.90 gallons after 2 hours.
The volume is 367.21 gallons after 3 hours.
The volume is 331.49 gallons after 4 hours.
The volume is 299.34 gallons after 5 hours.
The volume is 270.41 gallons after 6 hours.
The volume is 244.37 gallons after 7 hours.
The volume is 220.93 gallons after 8 hours.
The volume is 199.84 gallons after 9 hours.
The volume is 180.85 gallons after 10 hours.
The volume is 163.77 gallons after 11 hours.
The volume is 148.39 gallons after 12 hours.
The volume is 134.55 gallons after 13 hours.
The volume is 122.10 gallons after 14 hours.
The volume is 110.89 gallons after 15 hours.
The volume is 100.80 gallons after 16 hours.
The volume is 91.72 gallons after 17 hours.
The fish died after 17 hours.
(cat on keyboard test passed...)
The closest dead fish to 100 gallons:
$ ./bin/deadfish
Please enter additional water added per hour: 6.53
The volume is 456.53 gallons after 1 hours.
The volume is 417.41 gallons after 2 hours.
The volume is 382.20 gallons after 3 hours.
The volume is 350.51 gallons after 4 hours.
The volume is 321.99 gallons after 5 hours.
The volume is 296.32 gallons after 6 hours.
The volume is 273.22 gallons after 7 hours.
The volume is 252.42 gallons after 8 hours.
The volume is 233.71 gallons after 9 hours.
The volume is 216.87 gallons after 10 hours.
The volume is 201.71 gallons after 11 hours.
The volume is 188.07 gallons after 12 hours.
The volume is 175.79 gallons after 13 hours.
The volume is 164.75 gallons after 14 hours.
The volume is 154.80 gallons after 15 hours.
The volume is 145.85 gallons after 16 hours.
The volume is 137.80 gallons after 17 hours.
The volume is 130.55 gallons after 18 hours.
The volume is 124.02 gallons after 19 hours.
The volume is 118.15 gallons after 20 hours.
The volume is 112.86 gallons after 21 hours.
The volume is 108.11 gallons after 22 hours.
The volume is 103.83 gallons after 23 hours.
The volume is 99.97 gallons after 24 hours.
The fish died after 24 hours.
(3 - 100th of a gallon dead)
The first live fish:
$ ./bin/deadfish
Please enter additional water added per hour: 6.54
The volume is 456.54 gallons after 1 hours.
The volume is 417.43 gallons after 2 hours.
The volume is 382.22 gallons after 3 hours.
The volume is 350.54 gallons after 4 hours.
The volume is 322.03 gallons after 5 hours.
The volume is 296.36 gallons after 6 hours.
The volume is 273.27 gallons after 7 hours.
The volume is 252.48 gallons after 8 hours.
The volume is 233.77 gallons after 9 hours.
The volume is 216.94 gallons after 10 hours.
The volume is 201.78 gallons after 11 hours.
The volume is 188.14 gallons after 12 hours.
The volume is 175.87 gallons after 13 hours.
The volume is 164.82 gallons after 14 hours.
The volume is 154.88 gallons after 15 hours.
The volume is 145.93 gallons after 16 hours.
The volume is 137.88 gallons after 17 hours.
The volume is 130.63 gallons after 18 hours.
The volume is 124.11 gallons after 19 hours.
The volume is 118.24 gallons after 20 hours.
The volume is 112.95 gallons after 21 hours.
The volume is 108.20 gallons after 22 hours.
The volume is 103.92 gallons after 23 hours.
The volume is 100.07 gallons after 24 hours.
Alas, The fish who lived.
(saved by 7 - 100th of a gallon -- I'll leave it to you to bisect further into the 1000th of a gallon if you need to)
Look things over and let me know if you have further questions. Put the goto into you C toolbox knowing it should not be over-used, but that it has a few uses where it can provide the optimal solution, and in the case of breaking nested loop, is the only tool in your toolbox that can do it.