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MathGroup Archive 1998

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Re: sort and find in MATHEMATICA like in MATLAB


  • To: mathgroup@smc.vnet.net
  • Subject: [mg11035] Re: sort and find in MATHEMATICA like in MATLAB
  • From: Paul Abbott <paul@physics.uwa.edu.au>
  • Date: Wed, 18 Feb 1998 20:31:34 -0500
  • Organization: University of Western Australia
  • References: <6catkj$fsn@smc.vnet.net>

Alexej Jerschow wrote:

> in MATLAB I would write:
> 
> [dummy,index]=sort(a(:,i));
> sorta=a(index,:);
> 
> to sort the matrix a by it's i-th column.
> 
> What's the easiest way to do the same in MATHEMATICA ?

In[1]:= a=Table[Random[Integer,{0,4}],{4},{4}] Out[1]= {{4, 3, 3, 1},
{0, 1, 4, 4}, {4, 3, 4, 3}, {4, 0, 4, 3}}

Sort by the 2nd column:

In[2]:= Sort[a,#1[[2]]<#2[[2]]&]
Out[2]= {{4, 0, 4, 3}, {0, 1, 4, 4}, {4, 3, 4, 3}, {4, 3, 3, 1}}

> Another thing, again MATLAB:
> 
> index=find(a(:,i) == 0);
> sorta=a(index,:);
> 
> to remove lines, which have zeros in the i-th column.

Remove rows with zeros in the 2nd column:

In[3]:= Select[a, #[[2]]=!=0&]
Out[3]= {{4, 3, 3, 1}, {0, 1, 4, 4}, {4, 3, 4, 3}}> 

Cheers,
	Paul 


____________________________________________________________________ 
Paul Abbott                                   Phone: +61-8-9380-2734
Department of Physics                           Fax: +61-8-9380-1014
The University of Western Australia            Nedlands WA  6907       
mailto:paul@physics.uwa.edu.au  AUSTRALIA                            
http://www.pd.uwa.edu.au/~paul

            God IS a weakly left-handed dice player
____________________________________________________________________



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