Thread Subject: averaging

I know it can be done in one line. But I am not sure how to do it. Anybody please?
I have a matrix. Say,
col1 col2
23 0.6
12 0.9
12 1.0
23 0.5
how to average col2 so that it averages by the same id number represented by col1 i.e it should give me the column of [(0.6+0.5)/2 ; (0.9+0.1)/2]. Thanks so much.

 

"jenya polyakova" <jenya56@yahoo.com> wrote in message <h9be7r$gh5$1@fred.mathworks.com>...
> I know it can be done in one line. But I am not sure how to do it. Anybody please?
> I have a matrix. Say,
> col1 col2
> 23 0.6
> 12 0.9
> 12 1.0
> 23 0.5
> how to average col2 so that it averages by the same id number represented by col1 i.e it should give me the column of [(0.6+0.5)/2 ; (0.9+1.0)/2]. Thanks so much.
>
>

Assuming col1 is exactly integer so we can use it in boolean tests:

>> mean([col2(col1==23), col2(col1==12)]) % assumes equal numbers of each id


A more robust method would use round(col1) for the boolean. If you want some truly horrendous notation that keeps everything on one line and references only one matrix:

>> A = [23 0.6 ; 12 0.9 ; 12 1.0 ; 23 0.5]
>> [mean(A(round(A(:, 1))==23, 2)), mean(A(round(A(:, 1))==12, 2))]
% mean called twice


Good luck parsing that. It would be wiser to explicitly pull out the indices instead:
>> id1 = round(23);
>> id2 = round(12);
>> col1 = round(A(:, 1));
>> col2 = A(:, 2);
>> k1 = col1 == id1;
>> k2 = col1 == id2;
>> [mean(col2(k1)), mean(col2(k2))]


Hope this helps.

--

"arich82 " <|a|r|i|c|8|2|@hotmail.com> wrote in message <h9brsm$r8s$1@fred.mathworks.com>...
> "jenya polyakova" <jenya56@yahoo.com> wrote in message <h9be7r$gh5$1@fred.mathworks.com>...
> > I know it can be done in one line. But I am not sure how to do it. Anybody please?
> > I have a matrix. Say,
> > col1 col2
> > 23 0.6
> > 12 0.9
> > 12 1.0
> > 23 0.5
> > how to average col2 so that it averages by the same id number represented by col1 i.e it should give me the column of [(0.6+0.5)/2 ; (0.9+1.0)/2]. Thanks so much.
> >
> >
>
> Assuming col1 is exactly integer so we can use it in boolean tests:
>
> >> mean([col2(col1==23), col2(col1==12)]) % assumes equal numbers of each id
>
>
> A more robust method would use round(col1) for the boolean. If you want some truly horrendous notation that keeps everything on one line and references only one matrix:
>
> >> A = [23 0.6 ; 12 0.9 ; 12 1.0 ; 23 0.5]
> >> [mean(A(round(A(:, 1))==23, 2)), mean(A(round(A(:, 1))==12, 2))]
> % mean called twice
>
>
> Good luck parsing that. It would be wiser to explicitly pull out the indices instead:
> >> id1 = round(23);
> >> id2 = round(12);
> >> col1 = round(A(:, 1));
> >> col2 = A(:, 2);
> >> k1 = col1 == id1;
> >> k2 = col1 == id2;
> >> [mean(col2(k1)), mean(col2(k2))]
>
>
> Hope this helps.
>
> --

but I do not know that id1=round(23). The matrix A was used just as example. My matrix A is HUGE. What I need is to sort by col 1 and then average col2 according to col1. Any suggestions?

Use similar technique from this thread. Call UNIQUE on the first column if needed:

http://www.mathworks.com/matlabcentral/newsreader/view_thread/261117

Bruno

> but I do not know that id1=round(23). The matrix A was used just as example. My matrix A is HUGE. What I need is to sort by col 1 and then average col2 according to col1. Any suggestions?

Does this work?

[dummy, dummy, i1] = unique(A(:,1));
avg = accumarray(i1, A(:,2), [], @mean);

--

"Bruno Luong" <b.luong@fogale.findmycountry> wrote in message <h9cgb0$g2s$1@fred.mathworks.com>...
> Use similar technique from this thread. Call UNIQUE on the first column if needed:
>
> http://www.mathworks.com/matlabcentral/newsreader/view_thread/261117
>
> Bruno

... And had I refreshed my browser before posting, I would have seen that Bruno already solved your problem.

Sorry for the duplicating the post (but at least I learned something in the process!).

--

"arich82 " <|a|r|i|c|8|2|@hotmail.com> wrote in message <h9cjpo$sma$1@fred.mathworks.com>...
> "Bruno Luong" <b.luong@fogale.findmycountry> wrote in message <h9cgb0$g2s$1@fred.mathworks.com>...
> > Use similar technique from this thread. Call UNIQUE on the first column if needed:
> >
> > http://www.mathworks.com/matlabcentral/newsreader/view_thread/261117
> >
> > Bruno
>
> ... And had I refreshed my browser before posting, I would have seen that Bruno already solved your problem.
>
> Sorry for the duplicating the post (but at least I learned something in the process!).
>
> --

Here is another "oneliner" using cells

% data
A = [23 0.6 ; 12 0.9 ; 12 1.0 ; 23 0.5] ;
% engine
R = cellfun(@mean,group2cell(A(:,2),A(:,1)))

My GROUP2CELL submission can be found here:
http://www.mathworks.com/matlabcentral/fileexchange/11192

Jos