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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:

FromPackedArray messages

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

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    $\begingroup$ OTOH, coll = Reap[Nest[If[EvenQ[#], Sow[#]/2, 3 # + 1] &, 1000, 20]]; coll[[-1, 1]] // Developer`PackedArrayQ returns True. $\endgroup$ Commented May 22, 2017 at 14:40
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    $\begingroup$ I get no messages about packing (or unpacking) from the central block of code in Mathematica 10.1. $\endgroup$
    – Mr.Wizard
    Commented May 22, 2017 at 14:44
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    $\begingroup$ Ditto no messages in V11.1.1. $\endgroup$
    – Michael E2
    Commented May 22, 2017 at 14:54
  • $\begingroup$ No message in v9.0.1 $\endgroup$
    – xzczd
    Commented May 22, 2017 at 15:00
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    $\begingroup$ 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). $\endgroup$
    – Carl Woll
    Commented May 23, 2017 at 5:24

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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. >>

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