Iterating pairs of keys from a table

[Tim Hill <drtimhill@...>]

I recently had a need to iterate table key combinations in pairs. For example, for a table with keys (not values) of {a, b, c, d}, I wanted to iterate:

    (a,b), (a,c), (a,d), (b,c), (b,d), (c,d)

That is, I wanted unique combinations (not permutations) of non-identical keys. Sort order was not important.

Anyway, I came up with this generator function:

function keypairs(t)
    local s = next(t)
    local n, i = next(t, s), s
    return function()
        i = next(t, i)
        if i == nil then
            s, n = n, next(t, n)
            i = n
            if i == nil then s = nil end
        end
        return s, i
    end
end

Example use:

t = { a=true, b=true, c=true, d=true }
for k1, k2 in keypairs(t) do
    print(k1, k2)
end

However, I was wondering if anyone has a better (faster? smaller? cleaner?) solution they would like to share (or have I missed something very obvious in Lua?). I poked around in the archives and didn’t find anything, but it’s quite possible my limited search missed something.

(The only tweak I thought of was to swap the return order of s and i, which would make the nested “if” unnecessary. However I liked the fact that as written the generator returns the pairs in (major, minor) order that matches that returned by pairs().)

—Tim