Variable changing value in loop "IF" without correctly condition-CodePudding

I want know why me variable test change to 1, considering that I don't have a sequence like: 0 or 255 > 0 or 255 > 0 or 255 > 0 or 255

Why my if condition doesn't working?

I have this code:

#include <stdio.h>

int main()
{

  int item[4]={1,2,3,4};
  int test =0;

  printf("test: %d\n", test);
int i =0;
            if( (item[i] == 0 || 255) && (item[i 1] == 0 || 255) && (item[i 2] == 0 || 255)  &&
             (item[i 3] == 0 || 255) && (item[i 4]== 0 || 255))
              {
                test =1;
              }
                   

  printf("test after if:%d\n\n\n",test);
}

result:

test: 0 test after if:1

I want result test after if = 0 because my condition is: sequence 0 or 255 4 times

CodePudding user response：

Because of operator precedence the expression

item[i] == 0 || 255

is equivalent to

(item[i] == 0) || 255

That means the condition is true if item[i] == 0 is true, or if 255 is true. And because 255 is not 0 (0 is the only integer value that is false) it's true, making your whole condition true.

Since all the sub-conditions in your if are the same, you have (basically) true && true && true && true. Which is, of course, true.

To check if an element is 0 or 255 you need to check the element twice:

item[i] == 0 || item[i] == 255

Modify all your sub-conditions similarly.