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Re: Map onto a column

  • To: mathgroup at smc.vnet.net
  • Subject: [mg125449] Re: Map onto a column
  • From: Bill Rowe <readnews at sbcglobal.net>
  • Date: Wed, 14 Mar 2012 00:40:03 -0500 (EST)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com

On 3/13/12 at 3:02 AM, slypandemia at gmail.com (Sly Pandemia) wrote:

>Is there a neat way to map a function onto a specific column in an
>array?

>For example if I have

>a = {{1,2,3},{4,5,6},{7,8,9}}

>and I want to map f onto the second element in each row. I would
>normally use

>b = Map[MapAt[f,#,2]&,a]

>but this seems a bit awkward. I would expect to be able to use

>b = MapAt[f,a,{All,2}]

>but this appears to be illegal syntax.

>Is there any way to do what I want with a single Map/MapAt function?

Here is one way. But I don't that it will be any more
satisfactory to you

MapAt[f, a, Table[{n, 2}, {n, 3}]]

And to show this is the same

In[4]:= MapAt[f, a, Table[{n, 2}, {n, 3}]] == Map[MapAt[f, #, 2]
&, a]

Out[4]= True

Or how about re-writing your solution as

MapAt[f,#,2]&/@a




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