Re: Question on Thread
- To: mathgroup at smc.vnet.net
- Subject: [mg64459] Re: [mg64418] Question on Thread
- From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
- Date: Fri, 17 Feb 2006 04:12:59 -0500 (EST)
- References: <200602160805.DAA29888@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
On 16 Feb 2006, at 09:05, Matt wrote:
> Hello,
>
> Clear[f]
> Thread[(f[#1, #2] & )[{1, 2, 3}, {{a, 1}, {b, 2}, {c, 3}}]]
>
> Gives the expected output, i.e. {f[1,{a,1}], f[2,{b,2}],...}
>
> but
>
> Clear[f, g]
> Thread[(f[g[#1], #2] & )[{1, 2, 3}, {{a, 1}, {b, 2}, {c, 3}}]]
>
> gives me:
>
> f[g[{1, 2, 3}], {a, 1}], f[g[{1, 2, 3}], {b, 2}], ..., etc.
>
> whereas I would have expected:
>
> f[g[1], {a, 1}], f[g[2], {b, 2}], ..., etc.
>
> I've looked at the 'Thread' documentation and in the Mathematica Book,
> but I can't see why the second case doesn't work.
>
> BTW, I used MapThread as follows, which does work:
>
> MapThread[f[g[#1], #2] & , {{1, 2, 3}, {{a, 1}, {b, 2}, {c, 3}}}
>
> Thanks for your time,
>
> Matt
>
The reason is that Thread evaluates its arguments. Use
In[7]:=
Thread[Unevaluated[(f[g[#1], #2] & )[{1, 2, 3}, {{a, 1}, {b, 2}, {c,
3}}]]]
Out[7]=
{f(g(1),{a,1}),f(g(2),{b,2}),f(g(3),{c,3})}
Andrzej Kozlowski
- References:
- Question on Thread
- From: "Matt" <anonmous69@netscape.net>
- Question on Thread