MATLAB: How to average only positive values in an array

MATLABmean

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?

Best Answer

x = rand(1000, 20) - 0.5;
Positive = x > 0;
x(~Positive) = 0;
MeanPositive = sum(x, 2) ./ sum(Positive, 2);

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MATLAB: Create a function to…

It's not bad actually. The lines

 positive=positive
 negative-negative

are useless, and you want to create vectors of sums, not scalars. So one mistake is that you don't index variables positive and negative with the row index. To make it more efficient, you could even prealloc these variables: change

 positive=0;
 negative=0;

for

 positive = zeros(size(x, 1));
 negative = zeros(size(x, 1));

Another mistake is that k and j should go from 1 to the number of rows and columns of x. You can obtain them using function SIZE.

MATLAB: Inversion of a boolean matrix

WRONG AGAIN

In testing Walter's suggestion about row and column sums I realized that isInvBool2 is wrong. Try B = [true true; false false]. The column sums are correct but the row sums are not. Here is my third attempt, which I hope is correct. It can be speeded up but I'm more interested in correctness than speed at the moment.

% --------------------------------------------------------------

  function answ = isInvBool3(B)
% --------------------------------------------------------------
% If the boolean matrix B is  invertible it must be a permutation
% matrix. Hence this check reduces to: Is B a permutation matrix?
% This is done by counting the number of TRUE values in each row
% and column. A permutation matrix has one TRUE value in each row
% and column.
% Derek O'Connor 24 Feb 2012. derekroconnor@eircom.net
    [n,n] = size(B);
    rowsum = zeros(1,n);
    colsum = zeros(1,n);
    answ = true;
    for i = 1:n
        for j = 1:n
            if B(i,j)
                rowsum(i) = rowsum(i)+1;
                colsum(j) = colsum(j)+1;
            end %if
        end %for j
    end %for i
    for k = 1:n
        if rowsum(k) ~= 1 || colsum(k) ~= 1
            answ = false;
            return
        end
    end
  % End isInvBool3

Best Answer

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MATLAB: Create a function to…

MATLAB: Inversion of a boolean matrix

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