Why do most programming languages only have binary equality comparison operators?

Question

In natural languages, we would say \"some color is a primary color if the color is red, blue, or yellow.\"

In every programming language I\'ve seen, that translates

Answer 1

My guess would be that languages are designed by force of habit. Early languages only would have had binary comparison operators because they are simpler to implement. Everyone got used to saying (x > 0 and x < y) until language designers didn't ever bother to support the common form in mathematics, (0 < x < y).

In most languages a comparison operator returns a boolean type. In the case of 0 < x < y, if this is interpreted as (0 < x) < y it would be meaningless, since < does not make sense for comparing booleans. Therefore, a new compiler could interpret 0 < x < y as tmp:=x, 0 < tmp && tmp < y without breaking backward compatibility. In the case of x == y == z, however, if the variables are already booleans, it is ambiguous whether this means x == y && y == z or (x == y) == z.

In C# I use the following extension method so that you can write someColor.IsOneOf("Red", "Blue", "Yellow"). It is less efficient than direct comparison (what with the array, loop, Equals() calls and boxing if T is a value type), but it sure is convenient.

public static bool IsOneOf(this T value, params T[] set) 
{
    object value2 = value;
    for (int i = 0; i < set.Length; i++)
        if (set[i].Equals(value2))
            return true;
    return false;
}