Re: Map[] and multiple args function
- To: mathgroup at smc.vnet.net
- Subject: [mg96230] Re: [mg96198] Map[] and multiple args function
- From: Sseziwa Mukasa <mukasa at jeol.com>
- Date: Tue, 10 Feb 2009 05:47:36 -0500 (EST)
- References: <200902091033.FAA12309@smc.vnet.net>
On Feb 9, 2009, at 5:33 AM, sagrailo at gmail.com wrote:
> I know this has to be FAQ, but I'm just not able to find an answer
> anywhere, so - here it goes: I have a multiple-arguments function,
> say something like:
> Foo[x_, y_, z_] := x + 2*y + 3*z
> Now, I want to call this function number of times, over different sets
> of arguments, and then to sum results. I thought about storing
> argument triplets into some kind of list, and then employing sequence
> of Map[] and Apply[] to get the job done. After number of trials and
> errors, I came up with following piece of code doing the thing:
> Plus @@ Foo /@ {{1, 2, 3}, {4, 5, 6}} /. List -> Sequence
> but I even don't fully understand what's going here, and also I'm
> wondering is there a "better" way to accomplish this. So - any
> suggestion?
Writing your code out longhand would help with interpretation,
longhand it's:
ReplaceAll[Apply[Plus, Map[Foo, List[List[1, 2, 3], List[4, 5, 6]]]],
Rule[List, Sequence]]
(obtained doing FullForm[Hold[Plus @@ Foo /@ {{1, 2, 3}, {4, 5,
6}} /. List -> Sequence]])
So the first thing that happens is Foo is mapped onto {{1, 2, 3}, {4,
5, 6}} giving
{Foo[{1, 2, 3}], Foo[{4, 5, 6}]}
Plus is applied giving:
Foo[{1, 2, 3}],+Foo[{4, 5, 6}]
then the lists are replaced by sequences leading to the evaluation of:
Foo[1, 2, 3],+Foo[4, 5, 6]
You can avoid the replace by applying Foo directly to the appropriate
level of the argument list, either using the shorthand
Plus@@Foo@@@{{1,2,3},{4,5,6}}
or
Apply[Plus,Apply[Foo,{{1,2,3},{4,5,6}},{1}]]
- References:
- Map[] and multiple args function
- From: sagrailo@gmail.com
- Map[] and multiple args function