C: Logical operators - c

Question

In this code, the user must type his/her grade (1-10) on a subject and his/her absences(0-14) on it. If he/she gets >=5 grade and <=2 absences, the student passes.
I have a problem with the possibility that both grade and absences numbers are off the limits of the programme. Which are the "right" logical operators on this:
#include <stdio.h>
#include <stdlib.h>
int main()
{
int gr, ap;
printf("GIVE YOUR SUBJECT GRADE:\n");
scanf("%d", &gr);
printf("GIVE YOUR SUBJECT ABSENCES\n");
scanf("%d", &ap);
if (ap >= 0 && ap <= 14 && gr >= 0 && gr <= 10)
{
if (gr >= 5 && ap <= 2)
printf("YOU PASSED!\n");
else if (gr < 5 && ap <=2)
printf("FAILED DUE TO YOUR GRADE\n");
else if (gr >= 5 && ap > 2)
printf("FAILED DUE TO YOUR ABSENCES\n");
else if (gr < 5 && ap > 2)
printf("FAILED DUE TO YOUR GRADE AND ABSENCES\n");
}
else
{
if (ap >= 0 && ap <= 14 && gr < 0 || gr > 10)
printf("FALSE GRADE NUMBER\n");
else if (gr >= 0 && gr <= 10 && ap < 0 || ap > 14)
printf("FALSE ABSENCES NUMBER\n");
//here is the problem
else if (gr < 0 || gr > 10 && ap < 0 || ap > 14)
printf("FALSE ABSENCES AND GRADE NUMBERS\n");
}
system("pause");
return 0;
}

Answer 1

Since && has priority over ||, you need parentheses here:
ap >= 0 && ap <= 14 && (gr < 0 || gr > 10)
// ^ ^
Otherwise, the condition is interpreted as
(ap >= 0 && ap <= 14 && gr < 0) || (gr > 10)
which causes issues down the line.
Same goes for the other two conditions - they should be as follows:
(gr >= 0 && gr <= 10) && (ap < 0 || ap > 14)
(gr < 0 || gr > 10) && (ap < 0 || ap > 14)
Note that when you keep evaluating the same conditions over and over it is better to make a variable that stores their result once, and use that variable after that:
int validGrade = gr > 0 && gr <= 10;
int validAbsences = ap >= 0 && ap < 14;
Now you can write code like this:
if (validGrade && validAbsences) {
...
} else {
if (validAbsences && !validGrade) {
...
} else if (!validAbsences && validGrade) {
...
} else {
...
}
}

Answer 2

You first check if gr is out of range but not ap, then you check if ap is out of range but not gr, and if neither of them is true then both must be out of range, and for that you only need an else statement last.
However, the two conditions above are wrong, change the || to &&. In fact, you don't really check both ap and gr, because if one is in range you know the other is out of range. So you can simplify it like this:
// If ap is in range, that means gr must be out of range
if (ap >= 0 && ap <= 14)
printf("FALSE GRADE NUMBER\n");
// else if gr is in range then ap must be out of range
else if (gr >= 0 && gr <= 10)
printf("FALSE ABSENCES NUMBER\n");
// else both ap and gr are out of range
else
printf("FALSE ABSENCES AND GRADE NUMBERS\n");
I also have another couple of suggestions, like using unsigned int instead, then the value can never be below zero. You could also use macros to ease maintainability and readability of the code.
Perhaps something like this:
#include <stdio.h>
#include <stdlib.h>
#define AP_IN_RANGE(ap) ((ap) <= 14)
#define GR_IN_RANGE(gr) ((gr) <= 10)
int main(void)
{
unsigned int gr, ap;
printf("GIVE YOUR SUBJECT GRADE:\n");
scanf("%u", &gr);
printf("GIVE YOUR SUBJECT ABSENCES\n");
scanf("%u", &ap);
if (AP_IN_RANGE(ap) && GR_IN_RANGE(gr))
{
if (gr >= 5 && ap <= 2)
printf("YOU PASSED!\n");
else if (gr < 5 && ap <=2)
printf("FAILED DUE TO YOUR GRADE\n");
else if (gr >= 5 && ap > 2)
printf("FAILED DUE TO YOUR ABSENCES\n");
else
printf("FAILED DUE TO YOUR GRADE AND ABSENCES\n");
}
else
{
if (AP_IN_RANGE(ap))
printf("FALSE GRADE NUMBER\n");
else if (GR_IN_RANGE(gr))
printf("FALSE ABSENCES NUMBER\n");
else
printf("FALSE ABSENCES AND GRADE NUMBERS\n");
}
}
Of course, you should really make sure that the user input if valid, so you need to check what scanf returns. This will protect against the user inputting some letters instead of a valid unsigned number.