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MathGroup Archive 2004

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RE: matrix operations

  • To: mathgroup at smc.vnet.net
  • Subject: [mg46343] RE: [mg46329] matrix operations
  • From: "David Park" <djmp at earthlink.net>
  • Date: Mon, 16 Feb 2004 08:59:46 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

Paolo,

The wonders of functional programming! Here's an example.

mat = Array[x, {4, 4}]

Partition[mat, 2, 1]
(answer = #2 - #1 & @@@ %) // MatrixForm

@@@ is the Apply function, mapped onto the first level of mat. #2 - #1& is a
pure function that subtracts the second argument from the first argument.

David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/


-----Original Message-----
From: paolo tarpanelli [mailto:tarpanelli at libero.it]
To: mathgroup at smc.vnet.net
Subject: [mg46343] [mg46329] matrix operations


If I have a matrix

a={x[[1,1]],x[[1,2]],...,x[[1,n]]}
    {x[[2,1]],x[[2,2]],...,x[[2,n]]}
            .
            .
            .
    {x[[m,1]],x[[m,2]],...,x[[m,n]]}

how can I compute the difference between any element and the previous for
each column :

aa={x[[2,1]]-x[[1,1]], x[[2,2]]-x[[1,2]],...,x[[2,n]]-x[[1,n]]}
      {x[[3,1]]-x[[2,1]], x[[3,2]]-x[[2,2]],...,x[[3,n]]-x[[2,n]]}
            .
            .
            .
      {x[[m,1]]-x[[m-1,1]],x[[m,2]]-x[[m-1,2]],...,x[[m,n]]-x[[m-1,n]]}

----------------------------------------------------------------------------
--------------------------

I built this code but it does not work

r=Array[0,{m,n}]
For[j=1,j=n,j++
       r[[i,j]]=Table[a[[i+1,j]]-a[[i,j]],{i,1,m-1,1}]]

thanks

Paolo

----------------



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