# Grouping by multiple coefficients

Good day! I have an output equation that has a very large amount of coeff. b(x,y), like so:

8*b(0,0) + 2*x*b(0,0) + 3*y*b(0,0) + 5*x*y*b(0,0) + 4*b(0,1) + 3*x*b(0,1) + ...


I need to group by b, so each b(x,y) was seperate. I can Collect[] by b(0,0), but what abut the other coeff.? I'm trying to get something like this:

(8 + 2*x + 3*y + 5*x*y) * b(0,0) + (4 + 3*x + ...


Any ideas about how to do this?

To clarify: each x,y in b(x,y) is written as a Subscript. Not all b(x,y) have a "full" group.The grouping i'm trying to achieve will correspond to a cell in a matrix. The equation is a matrix row, and there are several equations which will form an equation group. I'm new to Wolfram, so any general advice is very welcome.

• In the future would you please give your input as valid Mathematica code? Jun 3, 2015 at 17:37
• Take a look at CoefficientList[ ] Jun 3, 2015 at 17:46
• What is your purpose in group the terms together? For instance, if you're trying to construct the matrix, there are probably other ways to go. Jun 3, 2015 at 18:33

expr = 8*Subscript[b, 0, 0] + 2*x*Subscript[b, 0, 0] - 3*y*Subscript[b, 0, 0] +

(8 + 2 x - 3 y + 5 x y) Subscript[b, 0, 0] + (-4 + 3 x) Subscript[b, 0, 1]

( _Subscript is the Blank pattern that matches any expression with head Subscript. )