How to write an if statement for multiple conditions in a short and efficient way?

Question

C++ / C#

There is if(key == y || key == x || key ==h || key == g .....), I want to write something like this (k in {y,x,h,g} ). Or else how, in general, best practices. I thought about iterators, it's interesting to use them somehow in one line, but then the question of performance (whether gcc can) expand.

Answer 1

Something like this:

#include <set>
#include <iostream>
using namespace std;

int main() 
{
  const int alpha=301;
  
  if(set<int>{0,200,300,400}.count(alpha))
  {
    cout << "found" << endl;
  }
  else
  {
    cout << "not found" << endl;
  }
  
  return 0;
}

IDEONE
Performance - logarithmic of the number of elements. If the condition is checked many times, it makes sense to move set to a separate constant.

Answer 2

Are you paid to save lines?
No matter how you write it (even if switch - case), there will still be comparisons and branches in machine code.
So don't bother stuffing it into iterators, but try to write it down so that it's readable at a glance - and it's impossible to make a typo that wouldn't be revealed by the same first glance.
By the way, it is also easier for the compiler to apply optimizations when the code is cumbersome, but primitive.