Write Power in CForm[] as multiplication instead

Question

Is there a simple way to make Mathematica write CForm[p^2] not as Power[p,2] but rather as p*p and so on for other much more complicated expressions?

I think the best way is to try to make use of the SymbolicC package. See here. I am always struggling when trying to extend CForm to behave better. — Szabolcs, Mar 29 '17 at 16:52
One more suggestion: if you are dealing with polynomials, you can consider putting them into HornerForm for more efficient evaluation. And a shameless plug: if you evaluate many integer powers in C++ (not in C), you may be interested in my blog post on the topic. In fact you might want to generate power<n>(x) with a small extension to SymbolicC. — Szabolcs, Mar 29 '17 at 16:54

Carl Woll · Answer 1 · 2019-03-26T15:02:23.320

4

You can override the default CForm handling of Power as follows:

Unprotect[Power];
Format[Power[a_,n_Integer?Positive], CForm] := Distribute[
    ConstantArray[Hold[a],n],
    Hold, List, HoldForm, Times
]
Protect[Power];

Example:

CForm[p^3]

(* p*p*p *)

The only downside is that I don't know how to control parenthesization, so that:

CForm[x^2 y^3]

(* x*x*(y*y*y) *)

unnecessarily parenthesizes y*y*y.

edited Mar 26 '19 at 15:02

answered Mar 31 '17 at 01:47

Carl Woll

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Hello, I got weird results if the power is -1. Is there a way to fix it? I.e. CForm[a*p^-1] works fine but CForm[p^-1] does not. – Vsevolod A. Mar 26 '19 at 12:54
@VsevolodA. see update – Carl Woll Mar 26 '19 at 15:03
Thanks. Though now it's not working for negative powers (it previously worked fine): the only problem was with the -1 power expression when there are no constants around like p^-1. – Vsevolod A. Mar 26 '19 at 16:23
@VsevolodA. Do you have an example where it doesn't work for negative powers? It works for me. – Carl Woll Mar 26 '19 at 16:28
Sure, CForm[1*p^-2] – Vsevolod A. Mar 26 '19 at 16:34

score 3 · Answer 2 · answered Mar 29 '17 at 19:30

3

If the symbol you are using is known in advance, you can useTagSetDelayed and associate an UpValue with that symbol inside the Powerfunction:

p /: Power[p, 2] := HoldForm[p*p]

Then

CForm[p^2]

p*p

answered Mar 29 '17 at 19:30

Stitch

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This works BUT seems to break things -- Solve stopped working right, and I got strange results in some calculations. If you do this, I'd suggest doing it right before you use CForm, and then right afterwards undo it by doing p /: Power[p,2] = . – Dan Sandberg May 11 '20 at 20:19

Write Power in CForm[] as multiplication instead

2 Answers2