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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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