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

Comparing a Variable to Multiple Values Efficiently

Often in programming, it is necessary to check if a variable matches one of several options. This can be achieved through various methods, but it's essential to prioritize efficiency.

Inefficient Methods

Attempts to compare a variable to multiple values using logical operators like OR can lead to inefficient code. For example:

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

This approach evaluates each logical expression (1 || 2, 2 || 3) separately, which can result in wasted processing.

Efficient Solutions in C 11

One efficient solution in C 11 involves utilizing std::initializer_list. The following template function takes a variable and an initializer list of potential matches:

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

Now you can use it like:

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

More Efficient Solution in C 17

C 17 introduces an even more efficient solution that works exceptionally well with any type:

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

This template function uses perfect forwarding to evaluate each comparison efficiently, resulting in code that performs on par with hand-written comparisons.

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