Unknown behaviour of OR operator in C++

Question

I have this code running by chance and when i put anything as the answer it is showing me correct. I know we have to put ans before YES and yay, but this code was compiled too, as i mentioned if i put any word as the input the output is correct:

string ans;
cin >> ans;

if(ans == "yes" || "YES" || "yay") {
    cout << "Correct";
}else {
    cout << "Incorrect";
}

Boolean operators don't work like that. All they do is take two bool values (true or false) and return a bool value. That's the only thing they do. — eesiraed, Apr 27 '18 at 04:33
Anything nonzero is true. `"YES"` is always true. `"yay"` is always true. You are forgetting to compare those against something. — Drew Dormann, Apr 27 '18 at 04:33
Forget everything you know about the word "OR" in English. Like I said, the only thing `||` does is return true if at least one of its inputs is true. — eesiraed, Apr 27 '18 at 04:36
@FeiXiang i know what is a OR in logic. But if i input ANYTHING even if i put NO, i will get Correct as an output. I know the working form is to write ans=="yes" || ans=="YES" || ans=="yay", but why the form in the question is working like that. — Shahryar, Apr 27 '18 at 04:38
You are trying to make three comparisons, but you have only written one. — Drew Dormann, Apr 27 '18 at 04:39
When you give `"YES"` to the `||` operator which takes a boolean value, it get's converted to a boolean value. When you convert something to a boolean, you get false if it is 0, and true for anything else. — eesiraed, Apr 27 '18 at 04:40
@codekaizer there will be, but for naught. This isn't the sort of thing you can easily Google. — user4581301, Apr 27 '18 at 04:41
I just asked a question (which is stackoverflow is made for), but you are giving negative rates. Nice. — Shahryar, Apr 27 '18 at 04:44
`if(ans == "yes" || "YES" || "yay")` is equivalent to `if(ans == "yes" || true || true )`. That is always true. Maybe or true or true is never false. — Drew Dormann, Apr 27 '18 at 04:45
@Shahryar downvotes are most likely because this question is asked every other day and is easily explained by working through statement and taking [operator precedence](http://en.cppreference.com/w/cpp/language/operator_precedence) into account. I don't downvote over this one because, like I said, above, it's really hard to websearch. — user4581301, Apr 27 '18 at 04:47
I didn't downvote, but others probably did because they considered this to be too basic, and can be solved by carefully reading a textbook or asking your professor. — eesiraed, Apr 27 '18 at 04:48
I'm not saying that you shouldn't try stuff that you haven't been taught, but just don't assume that they will work and get confused when they don't. — eesiraed, Apr 27 '18 at 04:49
Thanks. A question may seem to be basic. But look at the answers some people provided below. I have questions with 10000 views... I just didn't know this answer. — Shahryar, Apr 27 '18 at 04:51
Someone should make a canonical "Why does my comparison to multiple values always return true" question. — AShelly, Apr 27 '18 at 04:51
@AShelly I thought of doing that, but I was afraid it would be downvoted as too basic. — eesiraed, Apr 27 '18 at 04:52

Joseph D. · Accepted Answer · 2018-04-27T06:06:17.953

5

Ok here's the precedence (L->R) and associativity of logic wise operators:

(((ans == "yes") || "YES") || "yay")

Since C/C++ has no chaining unlike Python.

1st: ans == "yes" -> str to str comparison

2nd: bool result of 1st || "YES" -> bool and str comparison = always true for "YES" is not null

3rd: true || "YES" = always true

Thus, the condition will always be true for "YES" and "yay" are not null.

edited Apr 27 '18 at 06:06

answered Apr 27 '18 at 04:44

Joseph D.

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Thanks i understand it now. – Shahryar Apr 27 '18 at 04:48
"always true for "YES" is not empty" isn't quite correct. It's always true as "YES" is a non-null pointer. `if ("")` would also be true – Alan Birtles Apr 27 '18 at 05:44
@AlanBirtles, modified. thanks! – Joseph D. Apr 27 '18 at 06:06

score 3 · Answer 2 · answered Apr 27 '18 at 04:39

3

You have the precedence wrong.

if (var == A || B || C) means "if (var is equal to A) OR (B is not zero) OR (C is not zero)"

You want to do if (var == A || var == B || var == C). That means "if (var is equal to A) OR (var is equal to B) or (var is equal to C)"

answered Apr 27 '18 at 04:39

AShelly

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i know what is a OR in logic. But if i input ANYTHING even if i put NO, i will get Correct as an output. I know the working form is to write ans=="yes" || ans=="YES" || ans=="yay", but why the form in the question is working like that. – Shahryar Apr 27 '18 at 04:42
Because "YES" is a non-zero value, so inside the if statement, it is always true. – AShelly Apr 27 '18 at 04:46

score 1 · Answer 3 · answered Apr 27 '18 at 04:37

You should compare ans with each value. Every comparison with value is not "0" or "fail" value is assumed as "true". So if you input the if a condition like if("yes") it always returns a true.

int main()
{
    std::string ans;
    std::cin >> ans;

    if("yes" == ans || "YES" == ans || "yay" == ans)
    {
        std::cout << "Correct\n";
    }
    else
    {
        std::cout << "Incorrect";
    }
}

Unknown behaviour of OR operator in C++

3 Answers3