基本逻辑运算符问题

Question

#include <stdio.h>
int main()
{
    int k=5;
    if(++k < 5 && k++ / 5 || ++k <= 8)
    {
         printf("%d",k);
    }
    return 0;
}

Why is the output 7 and not 8?(I am a beginner in programming so please bear with me.)

Answer 1

Operator precedence and logical expression short circuit evaluation .

The && in your logical condition binds more tightly than || , so your conditional is equivalent to:

((++k<5 && k++/5) || ++k<=8)

It's easier to read code when it is presented in a structured way, like this:

int main() {
    int k=5;
    if ((++k<5 && k++/5) || ++k<=8) {
        printf("%d",k);
    }
    return 0;
}

And now a blow-by-blow of the execution.

1. k starts at 5.
2. ++k<5 advances k to 6, which is not <5 .
3. The second half of the && expression is never evaluated, because 0 && ANYTHING == 0 .
4. Because the left hand side of the || is 0, the right hand side is not short circuited. It must be evaluated.
5. ++k<=8 advances k to 7, which is <=8 .
6. Total conditional evaluates to 1 because right hand side of || is 1.
7. The "then" clause of the if statement is executed.
8. Current value of k , which is 7, is printed.
9. The program returns 0, and terminates.

It's also worth noting that the second half of your && clause is perhaps not doing what you intended. k++/5 is integer division, and since k>5 at all times, k++/5 will always be >=1 and thus always true.