how to do a conditionnal mean

Question

alex monjambo 2014-9-23

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评论： alex monjambo 2014-9-23

采纳的回答： the cyclist

Hi all,

I have many variables in a dataset.

One of them is called : FYEAR and another one is called : DY

I would like to find the mean of DY for each FYEAR and plot them.

Is there anyway i could do this ? can i do it with a boucle (IF for example ) ?

Thanks.

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Matt J 2014-9-23

can i do it with a boucle

This kind of boucle?

http://en.wikipedia.org/wiki/Boucl%C3%A9

Guillaume 2014-9-23

'boucle' is french for 'loop'

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Answer 1

the cyclist 2014-9-23

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You can use accumarray() to do this:

FYEAR = [2001; 2001; 2002; 2002];
DY = [1; 2; 5; 6];
[uniqueYear,i,j] = unique(FYEAR);
meanDY = accumarray(j,DY,[],@mean)

uniqueYear is the list of years in your list, and meanDY is the mean values.

There are many way to plot them. Here is a simple one:

plot(uniqueYear,meanDY,'.-')

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alex monjambo 2014-9-23

Thanks to all :) !

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Answer 2

Jos (10584) 2014-9-23

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Something along these lines should work (assuming FYEAR and DY are numerical arrays)

FYEAR = [10 10 11 12 11 10 12 12 10]
DY =    [ 1  3  4  8  6  2 10  9  2]
[UniqueFYEAR,~,i] = unique(FYEAR,'stable')
MeanDY = accumarray(k,DY,[],@mean)
plot(UniqueFYEAR, MeanDY, 'bo')

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Answer 3

Matt J 2014-9-23

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编辑：Matt J 2014-9-23

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Here is yet another accumarray-based approach, but avoids a call to unique(), and also uses a faster 2-pass calculation,

FYEAR = [10 10 11 12 11 10 12 12 10];
DY =    [ 1  3  4  8  6  2 10  9  2];
counts=accumarray(FYEAR(:),1)
sums=accumarray(FYEAR(:),DY(:));
idx=logical(counts);
means(idx)=sums(idx)./counts(idx);