[SOLVED] A Little explanation on logical OR.

alikamu · 07-25-2011, 12:09 PM

Hi friends,

i have a code :

int x,y,z; x=1;y=z=0;x=x||y&&z;
printf("%d",x,y,z);

here value of x will depend on the result of x OR y AND z.
so the x OR y will be 1, therefore it will not check Logical AND condition.

If i am wrong please do let me know (thanks)

but in second example:
(c=='\0' || c=='\n' || c=='\t')

here will it follow as before and only left OR will be executed or the complete expression will be executed.

Thanks for reading and answering.

Heraton · 07-25-2011, 01:15 PM

Hi!

Would have been nice to let us know, which language you are referring to. I will simply assume C. A second hint i would like to give is to use code tags. This little bastards help reading your code.

I am not sure, but i guess you mixed up the comparison operator "==" and the assignment operator "=". Your second example will change the value of c to '\0', '\n' or '\t', depending in which order you write your "logical expression". Each of the three expressions first changes the value of c, and after that, it will take the new value and will interpret that as a logical value, which will almost always be true.

If you did use the "=" for a reason, i don't have a clue about what your question is exactly...

Edit:
There is another little flaw in your post about operator precedence, if we are talking about C:
Logical AND "&&" has a precedence of 5 while logical OR "||" has only 4. That for your upper expression should be evaluated as:

X OR ( Y AND Z )

This example demonstrates the behavior:

Code:

#include <stdio.h>

int main(int c, char * argv[])
{
	int x = 1;
	int y = 1;
	int z = 0;

	if (x||y&&z)
		printf("true\n");
	else
		printf("false\n");
}

If your assumption was right, x and y would evaluate to true, but true and false would result in a false...

Anyway, tests on my machine result in true...

Regards, Heraton

devnull10 · 07-25-2011, 01:16 PM

Different languages have different implementations (which are you using??) however most will execute just the left hand side in a logical or expression if that part of the expression evaluates to true. An example in c:

Code:

     1  #include<stdio.h>
       
     2  int main() {
     3    int x=1;
     4    int y=0;
     5    x==y||printf("Executed\n");
     6  }

Code:

 tmp $ ./a.out 
Executed

Code:

     1  #include<stdio.h>
       
     2  int main() {
     3    int x=0;
     4    int y=0;
     5    x==y||printf("Executed\n");
     6  }

Code:

 tmp $ ./a.out 
 tmp $

We often used to do things like this in PHP:

Code:

do_something(etc) or die("Couldn't do something");

MTK358 · 07-25-2011, 02:41 PM

Quote:

Originally Posted by alikamu

Last edited by alikamu; Today at 14:36. Reason: Sorry, as pointed out by Heraton.

Don't edit your first post in response to replies. It makes the thread very difficult to understand.