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Re: Map onto one column of a matrix

  • To: mathgroup at smc.vnet.net
  • Subject: [mg95566] Re: [mg95485] Map onto one column of a matrix
  • From: "David Park" <djmpark at comcast.net>
  • Date: Wed, 21 Jan 2009 06:50:06 -0500 (EST)
  • References: <30785635.1232448719772.JavaMail.root@m02>

Maybe some expert will know a direct method, but otherwise this is a case
where one could define a useful routine.

MapOnColumn::usage = 
  "MapOnColumn[f,n][matrix] will Map f onto the elements of column n \
in the matrix.";
SyntaxInformation[MapOnColumn] = {"ArgumentsPattern" -> {_ , _}};
MapOnColumn[f_, n_][matrix_?MatrixQ] /; 
  1 <= n <= Part[Dimensions[matrix], 2] := 
 Transpose[MapAt[f /@ # &, Transpose[matrix], n]]

data = {{a, b}, {c, d}};

data // MapOnColumn[f, 2]
{{a, f[b]}, {c, f[d]}}


David Park
djmpark at comcast.net
http://home.comcast.net/~djmpark/  



From: D. Grady [mailto:D.C.Grady at gmail.com] 


Hi, suppose we've got a matrix

data = {{a, b}, {c, d}};

and we want to map a function onto just the second column, so we want
to end up with

{{a, f[b]}, {c, f[d]}}.

The best way I've found to do this is with MapAt:

MapAt[f, data, Table[{i, 2}, {i, Length@data}]]

Is there a better way?  I keep hoping there's some notation similar to
what's used in Part that lets you refer to a whole column with All,
like data[[All,2]].  Thanks!

-Daniel




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