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Re: take square of the second and third column of a table

  • To: mathgroup at smc.vnet.net
  • Subject: [mg124189] Re: take square of the second and third column of a table
  • From: Helen Read <readhpr at gmail.com>
  • Date: Thu, 12 Jan 2012 04:19:20 -0500 (EST)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <jel20o$lkl$1@smc.vnet.net>

On 1/11/2012 5:21 PM, hanciong awesome wrote:
> Hello, suppose I have a very long table like this:
>
> 1 2 3
> 4 5 6
> 7 8 9
> .......
> 2 3 4
>
> how can I take square of the 2nd and 3rd column? I always do it by
> making a new table. so let's say the above table is A and it has 100
> lines, then I make the new table as the following:
>
> B=Table[{A[[n]][[1]],A[[n]][[2]]^2,A[[n]][[3]]^2},{n,1,100}]
>
> But if the length of the table is unknown, this way is impractical.
> Could anyone suggests better way? thank you
>

Well, Mathematica knows the length of your table.

table = RandomInteger[{-10, 10}, {50, 3}]

Table[{table[[n]][[1]], table[[n]][[2]]^2, table[[n]][[3]]^2}, {n, 1, 
Length[table]}]


That said, Map would be a lot easier. Make a little function for what 
you want to do to one row, and Map it across your table, like so.

f[{x_, y_, z_}] := {x, y^2, z^2}
Map[f, table]


Even quicker, once you get the hang of working with pure functions:

Map[{#[[1]], #[[2]]^2, #[[3]]^2} &, table]



-- 
Helen Read
University of Vermont



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