how do we interpret the `||` and `&&` in an assignment statement?

Question

I have been coding from a long time though I'm still a student programmer/ I'm usually good at programming but when questions like the one below are asked I get stuck. What will be the output and why of the following program?

int main() 
    {
        int i=4,j=-1,k=0,w,x,y,z;
        w=i||j||k;
        print("%d",w);
        return 0;
    }

output: 1 why this result? what does the statement w=||j||k; means?

Another way to write this would be `w = ((i != 0) || (j != 0) || (k != 0));` where I only put the brackets for clarity. Each part such as `(i != 0)` will evaluate to `0` or `1`. Remember, in a C boolean test it is implied that `0` is `false` and all other values are `true`. — Weather Vane, Nov 18 '15 at 16:24
There are many other dups of this one on SO. You may want to read [short-circuit evaluation](https://en.wikipedia.org/wiki/Short-circuit_evaluation). — P.P, Nov 18 '15 at 16:34

score 3 · Answer 1 · edited May 23 '17 at 12:15

3

i || j || k is evaluated from left to right. It does that:

i == 4, which is true, so ORing it with any other value will yield true. That's it¹.

The rest of the statement is not evaluated because || and && are short-circuit operators, that is, if in your statement i != 0, neither j nor k will be evaluated because the result is guaranteed to be 1. && works similarly.
That's important to remember if you have something like f() || k(), where k has some side effect like an output to screen or a variable assignment; it might not be executed at all.

The bitwise OR operator | really ORs the bitwise representations of the values instead; it evaluates all its operands.

_{¹ Thanks to @SouravGosh on that!}

edited May 23 '17 at 12:15

Community

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answered Nov 18 '15 at 16:21

cadaniluk

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This answer maybe is misleading, as in case of `1 || k`, `k` is not evaluated. Please see my (downvoted) answer for a clarification. :) – Sourav Ghosh Nov 18 '15 at 16:29
@SouravGosh I do address that in the bottom of the question, if that's what you mean. – cadaniluk Nov 18 '15 at 16:30
@SouravGosh I see, will try to rectify. – cadaniluk Nov 18 '15 at 16:32
Don't get me wrong. I;m just trying to improve your answer. For some _unknown_ reason, mine got downvoted, so probably I'll remove mine. :) – Sourav Ghosh Nov 18 '15 at 16:32
@SouravGosh Don't worry, I wrote something wrong, (funnily enough got upvotes), you corrected me -> fine. – cadaniluk Nov 18 '15 at 16:38

Sourav Ghosh · Answer 2 · 2015-11-18T16:56:48.950

2

In your code,

 w=i||j||k;

is equivalent to

w= ((i||j) || k);

That means, first the (i||j) will be evaluated, and based on the result (if 0), the later part will be evaluated.

So, in your case, i being 4, (i||j) evaluates to 1 and based on the logical OR operator semantics, the later part is not evaluated and the whole expression yields 1 which is finally assigned to w.

Related quotes, from C11 standard, chapter §6.5.14, Logical OR operator

The || operator shall yield 1 if either of its operands compare unequal to 0; otherwise, it yields 0. The result has type int.

then, regarding the evaluation of arguments,

[...] If the first operand compares unequal to 0, the second operand is not evaluated.

and regarding the grouping,

[...] the || operator guarantees left-to-right evaluation;

edited Nov 18 '15 at 16:56

answered Nov 18 '15 at 16:18

Sourav Ghosh

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I did not downvote, but your answer has kind of a flaw: For `(i||j)`, `j` will not be evaluated if `i` yields _true_ already. You are basically right that this `||`-expression **will** be evaluated as such, but just not necessary both operands. – too honest for this site Nov 18 '15 at 16:50
@Olaf Sir, I understand your point, but I never mentioned that `j` will get evaluated in the evaluation of `(i || j)`. The logical OR will get evaluated, no? Can you suggest any better wordings? FWIW, to avoid any confusion, I have already added the related quotes from the standard. – Sourav Ghosh Nov 18 '15 at 17:00
To me it was ok, because I know the details. With the addition it is fine now. I too have sometimes DVs without comment. Maybe there should be an requirement to leave a comment. How else should one improve one's answer? – too honest for this site Nov 18 '15 at 17:45

pmg · Answer 3 · 2015-11-18T16:25:53.777

0

The result of boolean operators yields an int value of 0 or 1.

See 6.5.13 and 6.5.14, paragraph 3.

The [...] operator shall yield 1 [or] 0. The result has type int.

edited Nov 18 '15 at 16:25

answered Nov 18 '15 at 16:24

pmg

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how do we interpret the `||` and `&&` in an assignment statement?

3 Answers3