Timeline for Why does list assignment with a packed array result in unpacked values?
Current License: CC BY-SA 3.0
19 events
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| Apr 17, 2015 at 17:01 | comment | added | Mr.Wizard | @Oleksandr Good point. I was only going off Tali's "I've filed a bug for this" but perhaps it is more of a limitation than a bug? | |
| Apr 17, 2015 at 10:20 | comment | added | Oleksandr R. | Just a small comment on an old question. Since packed arrays were only introduced in version 5.0, I would think that if it exists in 5.2 it probably was a simple omission in the (AIUI, involved) operation of grafting packed arrays into the language in the first place. I am not quite sure what this meant about its "bug-ness" before the report was filed. | |
| Apr 15, 2015 at 22:30 | history | edited | Mr.Wizard | CC BY-SA 3.0 |
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| Feb 10, 2015 at 6:50 | vote | accept | Mr.Wizard | ||
| Feb 10, 2015 at 6:50 | history | edited | Mr.Wizard |
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| Feb 9, 2015 at 18:42 | answer | added | Taliesin Beynon | timeline score: 7 | |
| Aug 3, 2014 at 9:47 | comment | added | Leonid Shifrin | @Mr.Wizard Well, I think this was kind of easy to guess, since the pattern matcher has no way to not unpack for this particular rule. | |
| Aug 3, 2014 at 4:42 | comment | added | Mr.Wizard |
@Leonid I see from these comments that a year ago I was unaware that this also unpacks one level: /. h_@x__ :> h@x
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| Jul 24, 2013 at 14:22 | comment | added | Leonid Shifrin |
@Mr.Wizard Map only unpacked because the length of the list was smaller than "MapCompileLength" setting. And Outer is rather inelegant, I agree.
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| Jul 24, 2013 at 14:01 | comment | added | Mr.Wizard |
@Leonid I believe you could also use #& /@ packed, and even other functions such as Outer, though inelegantly: Outer[# &, packed, {1}, 1][[All, 1]]
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| Jul 23, 2013 at 9:10 | comment | added | Leonid Shifrin |
@MichaelE2 I would agree that the special case of List was probably considered not important. In fact, the only reason one would want to do List @@ packed would be if one wants to unpack one level. It also seems to be the only way to do this (i.e. using Apply), if one wants to only unpack one level.
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| Jul 23, 2013 at 3:49 | comment | added | Michael E2 |
I had thought @m_goldberg's question was why is Apply designed that way. That Apply always unpacks the first level is not an explanation why it does it when it's unnecessary. I would think that it's because List @@ List[..] is not important enough to make a special case. Or it's so that {a, b, c} = List @@ tensor is a workaround for Set's behavior. Or something else like that.
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| Jul 23, 2013 at 3:22 | history | edited | Mr.Wizard | CC BY-SA 3.0 |
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| Jul 23, 2013 at 3:19 | comment | added | Mr.Wizard |
@m_goldberg Yes, that is what I tried to express that with the final example given. Another way to look at this problem is that apparently when Set gets a packed array on the RHS and a list on the LHS, it fully unpacks the RHS rather that only unpacking it to the level of the LHS. This seems like an unfortunate choice, but I have often learned that there are good reasons for such choices once I asked about them.
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| Jul 23, 2013 at 2:16 | comment | added | m_goldberg |
@LeonidShifrin. So, although it's not his main point, one thing Mr.Wizard is telling us is that when Set is given tensor with its top-level unpacked, the lower levels will not be disturbed.
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| Jul 22, 2013 at 22:59 | comment | added | Leonid Shifrin |
@m_goldberg Because Apply does unpack the top level of the packed array. It does it always, even if the head to be applied is also a List.
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| Jul 22, 2013 at 19:58 | comment | added | m_goldberg |
Your question raises another in my mind: since tensor already has the head List, why does List @@ tensor have any effect on Set at all, considering it's evaluated before Set even sees it?
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| Jul 22, 2013 at 14:35 | history | tweeted | twitter.com/#!/StackMma/status/359320701003513857 | ||
| Jul 22, 2013 at 14:16 | history | asked | Mr.Wizard | CC BY-SA 3.0 |