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Re: How Functions are Applied to Matrices
*To*: mathgroup at smc.vnet.net
*Subject*: [mg66096] Re: [mg66064] How Functions are Applied to Matrices
*From*: Bob Hanlon <hanlonr at cox.net>
*Date*: Sat, 29 Apr 2006 03:40:52 -0400 (EDT)
*Reply-to*: hanlonr at cox.net
*Sender*: owner-wri-mathgroup at wolfram.com
m=Array[a,{3,4}];
Mean takes each row of the array as a single element of the outer list to be averaged.
Mean[m]==(m[[1]]+m[[2]]+m[[3]])/3
True
Bob Hanlon
---- Gregory Lypny <gregory.lypny at videotron.ca> wrote:
> 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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