Reproducing nested loops using Map?

Question

I keep producing bits of code like the following:

stuff = Module[
    {curTarget = #},
    getRowsForUserAndTarget[u, #, curTarget] & /@ validUsers
] & /@ allTargets;

Basically, I'm iterating through all the targets and all the users. Using a For loop it would look something like this (in python):

res = []
for user in validUsers:
    for curTarget in allTargets:
        res.append(getRowsForUserAndTarget[u, user, curTarget])

It seems like I should be able to do this succinctly in map notation using something like:

getRowsForUserAndTarget[#userthing, #targetthing]& /@ validUsers /@ validTargets

but I don't know how to keep the mapped arguments from interfering with each other, or how to reference which one I mean; and I'm a little hazy about the order of iteration (validUsers would go first, then validTargets?)

Can someone set me straight? This would seem like a common pattern that I am abusing with my Module[] solution...

Answer 1

Is this what you want?

l1 = {a, b, c};
l2 = {aa, bb, cc};
sth[#1, #2] & @@@ Tuples[{l1, l2}]

{sth[a, aa], sth[a, bb], sth[a, cc], sth[b, aa], sth[b, bb], 
 sth[b, cc], sth[c, aa], sth[c, bb], sth[c, cc]}

Answer 2

A couple of other options:

allTargets = {"a", "b", "c", "d"};
validUsers = {1, 2, 3};

Table[getRowsForUserAndTarget[u, j, i], {i, allTargets}, {j, validUsers}]

Outer[getRowsForUserAndTarget[u, #2, #] &, allTargets, validUsers, 1]

Both of these methods produce nested lists separating output elements for each validUsers element, whereas Tuples produces a flattened list. This can be either advantagous or problematic. You can flatten one level using Join @@ e.g. Join @@ Table