How Can I Efficiently Compare a Variable to Multiple Values in C ?

Comparing Variables to Multiple Values Efficiently

In certain scenarios, it becomes necessary to compare a variable against several options simultaneously. Typically, developers reach for the OR operator; however, this approach often leads to complications.

The Ideal Solution

Ideally, we seek a convenient method to distinguish between multiple groups, as exemplified by the following code:

if (num = (1,2,3))

else if (num = (4,5,6))

else if (num = (7,8,9))

C 11 Solution Using std::initializer_list

In C 11, std::initializer_list can be employed to achieve the desired functionality. By defining a is_in template function, we can efficiently compare a variable to a set of options:

#include <algorithm>
#include <initializer_list>

template <typename T>
bool is_in(const T& v, std::initializer_list<T> lst)
{
    return std::find(std::begin(lst), std::end(lst), v) != std::end(lst);
}

This allows us to perform comparisons succinctly:

if (is_in(num, {1, 2, 3})) { DO STUFF }

C 17 Solution: More Efficient

C 17 introduced a highly optimized solution that works well with any type:

template<typename First, typename ... T>
bool is_in(First &&first, T && ... t)
{
    return ((first == t) || ...);
}

// ...

// s1, s2, s3, s4 are strings.
if (is_in(s1, s2, s3, s4)) // ...

This version generates efficient code even for complex types like strings, unlike the C 11 counterpart.

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