How Functions are Applied to Matrices
- To: mathgroup at smc.vnet.net
- Subject: [mg66064] How Functions are Applied to Matrices
- From: Gregory Lypny <gregory.lypny at videotron.ca>
- Date: Fri, 28 Apr 2006 06:32:58 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
Hello everyone,
If I use functions, such as Mean, StandardDeviation, or Total, that
operate on lists, they work the way I expect when applied to a single
list. So, for example, the mean of data[[2]] below is 5.25.
However, when I apply Mean to the entire 3 x 4 matrix, which I
understand to be three lists, I expect to get three means. Instead I
get four because Mean is operating on the columns and not the rows,
that is, the four corresponding elements of each of the three lists.
Why is that?
Greg
data={{-9,8,3,1},{2,12,3,4},{-6,-9,-9,8}}
The mean of the second list:
In[182]:=
Mean[data[[2]]]//N
Out[182]=
5.25
Applying Mean to the whole matrix computes the mean of columns, not
rows.
In[181]:=
Mean[data]//N
Out[181]=
{-4.33333,3.66667,-1.,4.33333}
I need to Map it to have it applied to each list.
In[183]:=
Map[Mean,data]//N
Out[183]=
{0.75,5.25,-4.}
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- Re: How Functions are Applied to Matrices
- From: Andrzej Kozlowski <akoz@mimuw.edu.pl>
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