Loop function question [duplicate]

Question

This question already has an answer here:

• Map a function over the columns of an M x N array 2 answers

I'm new to Mathematica. Below is the code I have trouble with.

Do[Print[
    Subscript[C, k][x_, y_]: = 
          Subscript[q, 2 k - 1] + x*Subscript[q, 2 k] - (Subscript[q, l] + y*Subscript[q, l + 1])
   ], 
 {k, 3, 5}, {l, 3}]

In this loop, I want k=3 map to l=1, k=4 map to l=2, and k=5 map to l=3. However in this loop function, it loops through l=1,2,3 when k=3 and so on. So is there an automatic way or code to stop the certain looping?

Answer 1

If you are aiming to use loop to define functions you could use MapThread:

MapThread[(Subscript[C, #1][x, y] := 
     Subscript[q, 2 #1 - 1] + 
      x*Subscript[q, 2 #1] - (Subscript[q, #2] + 
        y*Subscript[q, #2 + 1])) &, {Range[3, 5], Range[3]}];'

Now you can test functions are defined:

Table[Subscript[C, j][a, b], {j, Range[3, 5]}]

This yields:

{-Subscript[q, 1] - b Subscript[q, 2] + Subscript[q, 5] + 
  a Subscript[q, 6], -Subscript[q, 2] - b Subscript[q, 3] + Subscript[
  q, 7] + a Subscript[q, 8], -Subscript[q, 3] - b Subscript[q, 4] + 
  Subscript[q, 9] + a Subscript[q, 10]}