How to average only positive values in an array?

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Ute123
Ute123 2011-7-18
My problem: I have a matrix M with 20 columns and thousands of rows. Each column in the matrix has positive and negative values (and each column has a different ratio of positive to negative numbers). I want to find the average value of only the positive numbers in each column in the matrix, and put those averages into an array (so a 1 x 20 array). How can I code this?

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Jan
Jan 2011-7-18
x = rand(1000, 20) - 0.5;
Positive = x > 0;
x(~Positive) = 0;
MeanPositive = sum(x, 2) ./ sum(Positive, 2);
  1 个评论
Ute123
Ute123 2011-7-18
This almost did it. I wanted the average of the columns, not the rows. I had to look up the sum function in Matlab help, the 2nd argument needs to be 1, not 2. So I just changed your last line to: MeanPositive = sum(x, 1) ./ sum(Positive, 1);, and that worked.
Thanks!

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更多回答（2 个）

Walter Roberson
Walter Roberson 2011-7-18
arrayfun(@(K) mean(M(:,M(:,K)>0)), 1:size(M,2))
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Walter Roberson
Walter Roberson 2011-7-18
I did have an error: it should have been
arrayfun(@(K) mean(M(M(:,K)>0),K), 1:size(M,2))
However, if your R2009B does not have arrayfun over function handles then something is wrong with it. The documentation for that version is
http://www.mathworks.com/help/releases/R2009b/techdoc/ref/arrayfun.html
Notice that according to the documentation the "fun" parameter is a function handle.
There may have been versions of arrayfun too old to support function handles, but anything closer to the 2009 releases support them for arrayfun()

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Derek O'Connor
Derek O'Connor 2011-7-19
At the risk of offending the "vectorizers", the function below is 2 to 3 times faster than Jan's answer:
function MeanP = MeanPos(A)
% Calculate the mean of the positive values in each column of A.
% Fast if m (rows) >> n (cols). Slow if m << n.
% Derek O'Connor 19 July 2011. derekroconnor@eircom.net
%
[m,n] = size(A);
MeanP(1:n) = 0;
for j = 1:n
npos = 0;
for i = 1:m
if A(i,j) > 0
npos = npos+1;
MeanP(j) = MeanP(j) + A(i,j);
end
end
if npos > 0
MeanP(j) = MeanP(j)/npos;
end
end
Here are some run times (secs), with n = 20 cols., m = 10^4, ..., 10^7 rows
m Tj Td Tj/Td
-----------------------------------
10^4 0.0091 0.0044 2.0776
10^5 0.1356 0.0437 3.1001
10^6 0.9935 0.4350 2.2839
10^7 9.9366 4.3983 2.2592
-----------------------------------
Dell Precision 690, Intel 2xQuad-Core E5345 @ 2.33GHz, 16GB RAM
Windows7 64-bit Prof., MATLAB R2008a, 7.6.0.324
Derek O'Connor

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