How does the increment operator work in an if statement?

问题

    #include <stdio.h>

    int main()
    {
        int x = 0;

        if (x++)
            printf("true\n");
        else if (x == 1)
            printf("false\n");
        return 0;
    }

Output:

false

Why is the output false?

x++ is post increment; this means that the value of x is used then it is incremented. If it is so, then x=0 should be used and the answer should be true.

回答1:

In C, 0 is treated as false. In x++, the value of x, i.e, 0 is used in the expression and it becomes

if(0)  // It is false
    printf("true\n");

The body of if doesn't get executed. After that x is now 1. Now the condition in else if, i.e, x == 1 is checked. since x is 1 , this condition evaluates to true and hence its body gets executed and prints "false".

回答2:

Post increment means that it returns the current value (in this case for the purpose of the if) and increments it afterwards. It is equivalent to

if(x) {
  x++;
  // ...
} else {
  x++;
  // ...
}

回答3:

0 is equivalent to false in C. As you are using post-increment operator, condition is evaluated before increment so x is false and printf("true\n"); is never executed. Then goes to else and succeeds evaluating x == 1, then prints false.

As a good practice, try to avoid assignations in condition sentences.

回答4:

0 is false in C. You're using the post-increment operator.

回答5:

You yourself wrote: "x++ is post increment, this means that the value of x is used then it is incremented"

Consider what that means:

x is 0
The expression is evaluated, 0 is false, so the expression is false.
The post increment happens, changing x from 0 to 1.
(After the expression was evaluated)

回答6:

I believe this simply could fix the error


#include <stdio.h>

    int main()
    {
        int x = 0;

        if (++x)
            printf("true\n");
        else if (x == 1)
            printf("false\n");
        return 0;
    }

来源：https://stackoverflow.com/questions/21435072/how-does-the-increment-operator-work-in-an-if-statement

标签

if-statement

post-increment