What does the ! operator do in this situation?

Question

I am learning C from a textbook, and I stumbled onto the code of a function, where the following part had little explanation to what it does. It looked something like this:

int func(char *a, char *b) {
    if(!a || !b)
        return 0;
    return 1;
}

My understanding is it checks that a and b are not null? Is this correct? Any help is welcome for this beginner :)

I don't get the downvote, the question is absolutely fine for a beginner. — dtell, Aug 11 '20 at 18:56
If *either* of them is `NULL`. But it would be better to be explicit: `if(a == NULL || b == NULL)`. The function returns `1` if both pointers are valid. — Weather Vane, Aug 11 '20 at 18:56
@CherryDT NULL evaluates to the address zero, not the number zero. — nicomp, Aug 11 '20 at 19:03

Acorn · Answer 1 · 2020-08-11T19:04:02.713

4

! is the logical not operator.

The if condition !a || !b is true if either a or b are zero, i.e. NULL pointers. Then the func returns 0 in that case, 1 otherwise:

a        b        condition  func
null     null     true       0
null     non-null true       0
non-null null     true       0
non-null non-null false      1

It is easier to understand if you negate the condition and swap the return values:

int func(char *a, char *b) {
    if(a && b)
        return 1;
    return 0;
}

Since in C the relational operators give their result as an int, i.e. 0 for false and 1 for true, you can simplify further to:

int func(char *a, char *b) {
    return a && b;
}

edited Aug 11 '20 at 19:04

answered Aug 11 '20 at 18:57

Acorn

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This is a very clear answer and the code "simplifications" are great! Thank you! – anInputName Aug 11 '20 at 19:08
@anInputName You're welcome! :) – Acorn Aug 11 '20 at 19:14

score 0 · Answer 2 · answered Aug 11 '20 at 19:24

0

Not exactly. When a is null or b is null then the logical NOT operator will cause the if conditional to evaluate as true and the return value is zero, otherwise the code returns 1 indicating that either a or b is not null.

answered Aug 11 '20 at 19:24

slevy1

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score -1 · Answer 3 · answered Aug 11 '20 at 18:59

The ! operator in most programming languages reverses the logic(or the not operator). i.e.(From True to False or vice versa)

For example:

a=True;
if(!a) {
    return 1;
}else {
    return 2;
}

Output is 2. So in this case, you are saying if not A OR not B, return 0.

What does the ! operator do in this situation?

3 Answers3