Strange behavior when using pattern match and HoldAll
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Newsgroups: comp.soft-sys.math.mathematica
From:
dnquark <dnqu... @gmail.com> Date: Sat, 7 Nov 2009 11:49:37 +0000 (UTC)
Local: Sat 7 Nov 2009 11:49
Subject: Strange behavior when using pattern match and HoldAll
I am using HoldAll with a function that I would like to modify a list http://forums.wolfram.com/student-support/topics/21247 Clear[test]; Remove[test];
This doesn't work:
gives {4,5,6}
However, test[list_, val_] := list[[2]] = val works.
Similarly,
doesn't work.
Thanks,
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Newsgroups: comp.soft-sys.math.mathematica
From:
Leonid Shifrin <lsh... @gmail.com> Date: Sun, 8 Nov 2009 11:50:42 +0000 (UTC)
Local: Sun 8 Nov 2009 11:50
Subject: Re: Strange behavior when using pattern match and HoldAll
Leo, Hold-attributes do affect the pattern-matching. Have a look at
http://www.mathprogramming-intro.org/book/node408.html
where I specifically discuss this topic using an example similar to yours.
Regards,
On Sat, Nov 7, 2009 at 3:50 AM, dnquark <dnqu
... @gmail.com> wrote:
> I am using HoldAll with a function that I would like to modify a list
> passed to it, as described on
>
http://forums.wolfram.com/student-support/topics/21247 > I wrote this:
> Clear[test]; Remove[test];
> This doesn't work:
> gives {4,5,6}
> However, test[list_, val_] := list[[2]] = val works.
> Similarly,
> doesn't work.
> Thanks,
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Newsgroups: comp.soft-sys.math.mathematica
From:
Albert Retey <a... @gmx-topmail.de> Date: Sun, 8 Nov 2009 12:27:20 +0000 (UTC)
Local: Sun 8 Nov 2009 12:27
Subject: Re: Strange behavior when using pattern match and HoldAll
dnquark schrieb:
> I am using HoldAll with a function that I would like to modify a list
> passed to it, as described on
>
http://forums.wolfram.com/student-support/topics/21247 > I wrote this:
> Clear[test]; Remove[test];
> This doesn't work:
> gives {4,5,6}
> However, test[list_, val_] := list[[2]] = val works.
> Similarly,
> doesn't work.
Clear[test];
If you want to test whether the value of the symbol is a list, you could
test[list_Symbol?(Head[#] == List &),val_]:= list[[2]]=val;
or:
test[list_Symbol /; Head[list] === List, val_] := list[[2]] = val;
hth,
albert
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