What do these operators do in C

Question

I'm very new to C and I'm going through some example code and I'm not sure what these operators in the if statements are "asking", so to say.

Here is the code:

int main(void){

    int a = 99;

    int b = 0;

    int c = 74;

    if( a || b )
      printf("first\n");

    else
      printf("second\n");

    if( a && c )
      printf("third\n");

    else
      printf("fourth\n");

    if( !a )
      printf("fifth\n");

    else
      printf("sixth\n");

    if( (a && b) || c )
      printf("seventh\n");

    else
      printf("eighth\n");

    if( !c || !b )
      printf("nineth\n");

    else
      printf("tenth\n");
}

I know what the operators mean, I just don't understand what is going on when they're going through the "if" statements. Could someone please explain this to me?

If you know what the operators mean, then you know they have a truth value, right? And an `if` statement will check the truth value and, if it's true, execute the following statement or block. (NOTE: A number in C has a truth value of "true" if it's non-zero, and "false" if it's zero.) — lurker, Sep 27 '13 at 02:13
These are boolean operators . I suggest writing a small application to play with the different expressions and different input values — TGH, Sep 27 '13 at 02:14
This question appears to be off-topic because the OP has done no basic research — , Sep 27 '13 at 02:14
You don't know what they mean if you don't know what they are doing... — Ed S., Sep 27 '13 at 02:15
@mbratch Right, but if the statement is if(a || b) what is it asking? All I'm seeing is "if a or b" I don't understand the full extent of it. — Preston May, Sep 27 '13 at 02:16
possible duplicate of [Logical Operators in C](http://stackoverflow.com/questions/12332316/logical-operators-in-c) — Shafik Yaghmour, Sep 27 '13 at 02:17
You said you know what the operators mean. What does `||` mean? Then apply that logic to the values of `a` and `b` based upon what I said previously. — lurker, Sep 27 '13 at 02:18

score 1 · Accepted Answer · answered Sep 27 '13 at 02:15

1

The key to answering this question is realization of how C treats integers that participate in logical operations:

Zero is treated as FALSE
All values other than zero are treated as TRUE

Here are the truth tables for the three operators from your code snippet:

!FALSE -> TRUE
!TRUE  -> FALSE

FALSE || FALSE -> FALSE
FALSE || TRUE  -> TRUE
TRUE  || FALSE -> TRUE
TRUE  || TRUE  -> TRUE

FALSE && FALSE -> FALSE
FALSE && TRUE  -> FALSE
TRUE  && FALSE -> FALSE
TRUE  && TRUE  -> TRUE

When multiple operators are used in an expression without parentheses, unary ! is applied ahead of the binary && or ||.

Now you have enough information to figure out the output yourself.

answered Sep 27 '13 at 02:15

Sergey Kalinichenko

714,442
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How do I know whether they're True or False? – Preston May Sep 27 '13 at 02:17
See the two bullet points at the top of the answer :-) – Sergey Kalinichenko Sep 27 '13 at 02:18
I was about to edit my comment after I reread your answer. Thank you! That is what I was asking! :D – Preston May Sep 27 '13 at 02:20

score 0 · Answer 2 · answered Sep 27 '13 at 02:16

0

They are just normal operators:

|| = or
&& = and
! = not

So...

if( a || b )
  printf("first\n");

Is saying...

if a or b then print "first"

And...

if ( !a )

Is...

if not a

Equivalent to:

if a is equal to 0 or NULL

Really basic stuff!

answered Sep 27 '13 at 02:16

Craig

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score 0 · Answer 3 · answered Sep 27 '13 at 02:24

It might be easier to learn about boolean operators if you are working with variables of type bool. I find boolean checks to be less useful when dealing with ints since it's less common to think of a number as being true or false. With ints you typically consider their numerical values which makes less sense in boolean logic

What do these operators do in C

3 Answers3