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Re: How to apply a list of functions

  • To: mathgroup at smc.vnet.net
  • Subject: [mg112888] Re: How to apply a list of functions
  • From: "Vincent N. Virgilio" <virgilio at ieee.org>
  • Date: Tue, 5 Oct 2010 05:32:52 -0400 (EDT)

Leonid,

Certainly.

(The timing difference between MapThread and Inner/Hold appears to be in the
noise. I didn't look at Inner/List as closely.)

Vince

On Mon, Oct 4, 2010 at 1:46 PM, Leonid Shifrin <lshifr at gmail.com> wrote:

> Vince,
>
> How about this:
>
> In[51]:=
> Inner[Compose,Hold[f1,f2,f3],Hold[{{1,2},{3,4}},{{5,6},{7,8}},{{9,10},{11,12}}],List]
>
> Out[51]= {f1[{{1,2},{3,4}}],f2[{{5,6},{7,8}}],f3[{{9,10},{11,12}}]}
>
> Basically, wrap your arguments in Hold rather than List, and you don't have
> this problem.
>
> Regards,
> Leonid
>
> On Mon, Oct 4, 2010 at 9:37 PM, Vincent N. Virgilio <virgilio at ieee.org>wrote:
>
>> Leonid,
>>
>> I notice that Inner/Compose is a little fragile if x is, say, a list of
>> matrices, and the goal is to apply each f to each whole matrix.
>> MapThread/Compose handles that transparently.
>>
>> Thanks again,
>>
>> Vince
>>
>> On Mon, Oct 4, 2010 at 1:07 PM, Leonid Shifrin <lshifr at gmail.com> wrote:
>>
>>> Hi Vince,
>>>
>>> First, you could use Compose in your code - perhaps it is a little
>>> faster:
>>>
>>> In[2]:= MapThread[Compose,{{f1,f2,f3},{x1,x2,x3}}]
>>>
>>> Out[2]= {f1[x1],f2[x2],f3[x3]}
>>>
>>> Using Inner can be a bit faster still:
>>>
>>> In[4]:= Inner[Compose,{f1,f2,f3},{x1,x2,x3},List]
>>>
>>> Out[4]= {f1[x1],f2[x2],f3[x3]}
>>>
>>> In most cases, you probably won't see the difference in performance,
>>> unless your functions
>>> do very little and you have lots of them.
>>>
>>> Regards,
>>> Leonid
>>>
>>>
>>>
>>>
>>> On Mon, Oct 4, 2010 at 2:06 PM, Vince Virgilio <blueschi at gmail.com>wrote:
>>>
>>>> On Oct 3, 3:39 am, Sam Takoy <sam.ta... at yahoo.com> wrote:
>>>> > Hi,
>>>> >
>>>> > As a follow up to my question about apply {Sin, Cos} to x, I came up
>>>> > with the tasteless
>>>> >
>>>> > Map[Apply[#, {x}] &, {Sin, Cos}]
>>>> >
>>>> > but I expect that the pros in this ng will be able to improve upon it.
>>>> >
>>>> > Thanks,
>>>> >
>>>> > Sam
>>>>
>>>> Tangentially,
>>>>
>>>> I've always wondered if there was a better way to apply a list of
>>>> functions to a list of arguments. Here's how I do it, where f and x
>>>> are the respective lists.
>>>>
>>>> MapThread[#1@#2 &, {f, x}]
>>>>
>>>> Seems performant.
>>>>
>>>> Anyone?
>>>>
>>>> Vince
>>>>
>>>>
>>>
>>
>



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