Expression manipulation: Explode sums and products?

Question

I wonder if the functionality exists to explode a sum, or product? What I mean by that is the following. Imagine, you have input data

data = a + b c - d e f ;

Is there a function ExplodeSum such that

res1 = ExplodeSum[data]
*out*
{a,b c,-d e f}

is the result (arbitrary but fixed order)? And then, is there a function, such that

res2 = ExplodeProduct[ res1[[2]] ]
*out*
{b,c}

is the result? If yes, please let me know how those functions are actually called. Or maybe one can implement these functions? Thanks for any suggestion!

I found a similar question: (25338). Unless someone argues for this question being unique I shall be closing it as a duplicate of that one. — Mr.Wizard, Oct 08 '14 at 01:02
Thanks to everyone for the solutions! My question has been answered. (@Mr.Wizard: The problem with these kind of questions is that one does not know exactly what to search for. So it is easier to ask again. - Now, I'm sure the next person searching for "explode sum" will find this question here, so it is not all in vain.) — Kagaratsch, Oct 08 '14 at 15:13
I did not mean to suggest that you should not have asked this Question. While it is always appreciated if one searches first I am fully aware that even extensive searching may not reveal existing questions that can be answered in a similar way. You are correct that your question, even though closed now, will help people find the answer they need, here or in the linked Q&A. — Mr.Wizard, Oct 08 '14 at 16:08

RunnyKine · Accepted Answer · 2014-10-07T23:55:06.187

9

res1 = data /. Plus -> List

 {a, b c, -d e f}

res1[[2]] /. Times -> List

{b, c}

As a function:

explodeOp[expr_, op_] := expr /. op -> List

Then:

res1 = explodeOp[data, Plus];
explodeOp[res1[[2]], Times]

edited Oct 07 '14 at 23:55

answered Oct 07 '14 at 23:49

RunnyKine

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DumpsterDoofus · Answer 2 · 2014-10-07T23:53:05.590

4

Try this:

List @@ (a + b c - d e ff)

Output is

{a, b c, -d e ff}

Likewise, to explode a product, do this:

List @@ %[[2]]

Output is

{b, c}

This works because List @@ deletes the head of (a + b c - d e ff), which is Plus, and replaces it with list, giving List[a, b c, -d e ff] (and similar for product explosion).

edited Oct 07 '14 at 23:53

answered Oct 07 '14 at 23:47

DumpsterDoofus

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score 1 · Answer 3 · answered Oct 08 '14 at 01:50

1

Cases[a + b c - d e f, _]
(*{a, b c, -d e f}*)

Level[a + b c - d e f, 1]
(*{a, b c, -d e f}*)

answered Oct 08 '14 at 01:50

Basheer Algohi

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kglr · Answer 4 · 2014-10-08T09:54:32.627

You can also set Part 0 of input list to List:

ClearAll[data, a, b, c, d, e, f];
data = a + b c - d e f;

data2=data;
data2[[0]] = List; 
data2
(* {a, b c, -d e f} *)

data2 = data[[2]];
data2[[0]] = List;
data2
(* {b,c} *)

or, define a function that Listifies the Heads:

ClearAll[lF1];
lF1 = Function[{h}, h[[0]] = List; h, HoldFirst];

data2 = data;
{lF1@data2, lF1@data2[[2]]}
(* {{a, b c, -d e f}, {b, c}}  *)

You can also use: MapAt or ReplacePart to change the Heads to List:

ClearAll[lF2];
lF2 = MapAt[List &, #, {0}] &;

data2 = data;
{lF2@data2, lF2@data2[[2]]}
(* {{a,b c,-d e f},{b,c}} *)

data2 = data;
{ReplacePart[data2, 0 -> List], ReplacePart[data2[[2]], 0 -> List]}
(* {{a, b c, -d e f}, {b, c}} *)

Expression manipulation: Explode sums and products?

4 Answers4