Two nested for of for and map?

Question

When I need to generate a sequence which needs "two loops", is it better to do something like this:

(for [x (range 1 4)] (map #(* x %) (range 6 9)))

or something like this:

(for [x (range 1 4)] (for [y (range 6 9)] (* x y)))

Both give the same result:

((6 7 8) (12 14 16) (18 21 24))

Is one more idiomatic than the other and what are the differences between these two?

Also, is it possible to get the same result by nesting two map?

Answer 1

Either is fine, but the second version scales better to arbitrary nesting levels.

Nesting map calls works, too:

user=> (map (fn [x] (map (fn [y] (* x y)) (range 6 9))) (range 1 4))
((6 7 8) (12 14 16) (18 21 24))

You cannot nest the shortcut function syntax, though.

By the way, just in case you actually need more set-comprehension-like semantics, for also has nesting built-in. The result is somewhat different:

user=> (for [x (range 1 4), y (range 6 9)] (* x y))
(6 7 8 12 14 16 18 21 24)