Is there a shorter way to calculate if (a == b || a == c) in C++

Question

I am wondering if in c++11 you can calculate this:

if (a == b || a == c) {
    // do something
}

In a much shorter and more concise way such as something like this:

if (a == (b || c)) {
    // do something
}

(I know that the above code would not work [it would calculate if b or c and then check if the result is equal to a]. I am wondering if there is a similar way to implement the code before: if (a == b || a == c) {})

This sounds like an XY problem. Why do you want to rewrite this condition? Original works perfectly well. One way to rewrite is preferably a function or macro. BTW there is a short circuit while calculating the value. — Mohit Jain, Jan 02 '22 at 04:56
Are you trying to save keystrokes, or to make the code faster? — Sneftel, Jan 02 '22 at 04:57
@Sneftel I am just trying to clean up my code a little bit and make it easier to read. — Snail5008, Jan 02 '22 at 05:01
With a sufficiently modern compiler, you could write `if (std::set{b, c}.contains(a))`, but I'm not sure this is an improvement. — Igor Tandetnik, Jan 02 '22 at 05:02
@IgorTandetnik This is a good soulution for bools, but I am comparing strings, and this function seems to only allow for bools. [please correct me if I'm wrong] — Snail5008, Jan 02 '22 at 05:06
@AdrianMole Thanks, for noticing that, I meant ||, I'll edit it now. — Snail5008, Jan 02 '22 at 05:07
It would work with any type that could be put into an `std::set`, including `std::string` — Igor Tandetnik, Jan 02 '22 at 05:08
I'm sort of lost here, what types are we talking about? I can see optimizations but for bools and ints for example, perhaps for strings too. But these are different paths of solution. — , Jan 02 '22 at 05:12
For chars, you could do: if (strchr(a, “bcdefg”) != NULL) — Jeremy Friesner, Jan 02 '22 at 05:36
If you want to cleanup your code (and a,b,c aren't char's) then give them more readable names and put your or's in a function with a name what the logic is doing. Also note that 'or' and 'and' are smart enough to stop evaluating early : e.g. for 'or' if (a==b) is true then it doesn't evaluate (a==c) since it already knows the end result will be true. So actually your way of writing it down will be fast (a lot faster then using std::set) — Pepijn Kramer, Jan 02 '22 at 05:54
You could use variadic templates for this. [Here's](https://gcc.godbolt.org/z/aqj6nPK4Y) an example where you call a compare function recursively, peeling of the arguments one by one and comparing it with the first one. — jignatius, Jan 02 '22 at 07:05

Pepijn Kramer · Accepted Answer · 2022-01-02T06:39:08.530

What you are basically doing is testing if a equals a value in a set. And yes std::set can be used but its slow. This example is a bit slower then hard coding the full expression. But it shows what is being calculated and the righthand side will look like a set/collection.

#include <utility> // for std::size_t

template<typename type_t, std::size_t N>
inline constexpr bool is_any_of(const type_t& lhs, const type_t(&set)[N])
{
    for (const auto& rhs : set)
    {
        if (lhs == rhs) return true;
    }
    return false;
}

int main()
{
    constexpr int a = 1;
    constexpr int b = 3;
    constexpr int c = 2;
    constexpr int d = 1;

    static_assert(!is_any_of(a, { b, c }));
    static_assert(is_any_of(a, { b, d }));
    static_assert(is_any_of(a, { b, c, d }));
    
    return 0;
}

Is there a shorter way to calculate if (a == b || a == c) in C++

1 Answers1