Reap unpacks packed arrays

Question

I noticed that Reap unpacks packed arrays. I've looked at previous answers but am still an unsure whether this a serious issue.

E.g. consider Ex 47.3 from EIWL:

Use Sow and Reap to get a list of all cases where #/2 is used in Nest[If[EvenQ[#], #/2, 3#+1]&, 1000, 20].

One solution is

On["Packing"]
Reap @ Nest[If[EvenQ[#], Sow[#]/2, 3#+1]&, 1000, 20]

but one gets multiple (FromPackedArray) messages about unpacking arrays due, I assume, to the fact that intermediate results are irregular structures.

Since other users don't appear to see this, here's a screenshot:

BTW, since $3x+1$ is always even, a more efficient coding of the Collatz problem is to replace 3#+1 by (3#+1)/2.

OTOH, coll = Reap[Nest[If[EvenQ[#], Sow[#]/2, 3 # + 1] &, 1000, 20]]; coll[[-1, 1]] // Developer`PackedArrayQ returns True. — J. M.'s missing motivation, May 22 '17 at 14:40
I get no messages about packing (or unpacking) from the central block of code in Mathematica 10.1. — Mr.Wizard, May 22 '17 at 14:44
You are using TraditionalForm as your default output format, and creating those boxes is generating the unpacking messages. For example, adding a ; at the end of the Reap will prevent the messages (until you display the output). — Carl Woll, May 23 '17 at 05:24
So should using TraditionalForm as the default output format do that? Perhaps a more useful FromPackedArray message would have been helpful here. — TheDoctor, May 24 '17 at 09:59
Even in StandardForm I get the similar behaviour with Reap@Plot[x,{x,0,1},EvaluationMonitor :> Sow[x]];. — TheDoctor, May 25 '17 at 07:06

score 3 · Answer 1 · answered May 30 '17 at 10:39

_{Posting the comment by Carl Woll as an answer:}

You are using TraditionalForm as your default output format, and creating those boxes is generating the unpacking messages. For example, adding a ; at the end of the Reap will prevent the messages (until you display the output).

Proof:

On["Packing"]
ToBoxes[RandomReal[1, 10], TraditionalForm];

Developer`FromPackedArray::unpack: Unpacking array in call to LeafCount. >>

Reap unpacks packed arrays

1 Answers1