Operators : Short circuit behavior of logical operators

Question

Short circuiting is a functionality that skips evaluating parts of a (if/while/...) condition when able. 

Answer 1

Short circuiting is a functionality that skips evaluating parts of a (if/while/...) condition when able. In case of a logical operation on two operands, the first operand is evaluated (to true or false) and if there is a verdict (i.e first operand is false when using &&, first operand is true when using ||) the second operand is not evaluated.

Example:

#include <stdio.h> 

int main(void) {  int a = 20;  int b = -5; 

  /* here 'b == -5' is not evaluated,     since a 'a != 20' is false. */  

if (a != 20 && b == -5) 

{    

printf("I won't be printed!\n");  

}     

return 0;

}

Check it out yourself:

#include <stdio.h> 

int print(int i) 

{  

printf("print function %d\n", i);  

return i; 

} 

int main(void) 

{  

int a = 20; 

  /* here 'print(a)' is not called,     since a 'a != 20' is false. */  

if (a != 20 && print(a)) 

{    

printf("I won't be printed!\n");  

}

  /* here 'print(a)' is called,     since a 'a == 20' is true. */  

if (a == 20 && print(a)) 

{    

printf("I will be printed!\n");  

}

  return 0; 

}

Output:

$ ./a.out 

print function 20 

I will be printed!

Short circuiting is important, when you want to avoid evaluating terms that are (computationally) costly. Moreover, it can heavily affect the flow of your program.