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

I think that most people consider something like

isPrimaryColor = ["Red", "Blue", "Yellow"].contains(someColor)

to be sufficiently clear that they don't need extra syntax for this.

Answer 2

In Python you can say ...

isPrimaryColor = someColor in ('Red', 'Blue', 'Yellow')

... which I find more readable than your (== "Red" or == "Blue") syntax. There's a few reasons to add syntax support for a language feature:

• Efficiency: Not a reason here, since there's no speed improvement.
• Functionality: Also not a concern; there's nothing you can do in the new syntax that you can't do in the old.
• Legibility: Most languages handle the case where you're checking the equality of multiple values just fine. In other cases (e.g., someNumber (> 1 and < 10)) it might be more useful, but even then it doesn't buy you much (and Python allows you to say 1 < someNumber < 10, which is even clearer).

So it's not clear the proposed change is particularly helpful.

Answer 3

Icon has the facility you describe.

if y < (x | 5) then write("y=", y)

I rather like that aspect of Icon.

Answer 4

I'm reminded of when I first started to learn programming, in Basic, and at one point I wrote

if X=3 OR 4

I intended this like you are describing, if X is either 3 or 4. The compiler interpreted it as:

if (X=3) OR (4)

That is, if X=3 is true, or if 4 is true. As it defined anything non-zero as true, 4 is true, anything OR TRUE is true, and so the expression was always true. I spent a long time figuring that one out.

I don't claim this adds anything to the discussion. I just thought it might be a mildly amusing anecdote.

Answer 5

In python you can do something like this:

color = "green"

if color in ["red", "green", "blue"]:
    print 'Yay'

It is called in operator, which tests for set membership.

Answer 6

This can be replicated in Lua with some metatable magic :D

local function operator(func)
    return setmetatable({},
        {__sub = function(a, _)
            return setmetatable({a},
                {__sub = function(self, b)
                    return f(self[1], b)
                end}
            )
        end}
    )
end


local smartOr = operator(function(a, b)
    for i = 1, #b do
        if a == b[i] then
            return true
        end
    end
    return false
end)


local isPrimaryColor = someColor -smartOr- {"Red", "Blue", "Either"}

Note: You can change the name of -smartOr- to something like -isEither- to make it even MORE readable.