Transform a Sum into a List

Question

I want to turn a sum like this

sum =a-b+c+d

Into a List like this:

sumToList[sum]={a,-b,c,d}

How can I achieve this?

Answer 1

List @@ sum
(* {a, -b, c, d} *)

From the docs on Apply (@@):

f@@expr replaces the head of expr by f.

So List@@sum replaces Head[sum] (that is, Plus) with List.

Answer 2

Try :

a - b + c + d /. Plus -> List

You can have a look at a - b + c + d //FullForm to see why this works.

Answer 3

Still another route:

Last[CoefficientArrays[#]] Variables[#] &[a - b + c + d]
   {a, -b, c, d}