Hello, How do i return the smallest positive value from a matrix?

Hello, I need to return the smallest value of a matrix but given a condition. So in the image i want to return the smallest value in Column 1( code name is m_dot) but only for a postive value of column 3 (code name is wd).

Thanks in advance

Answer 1

[John D'Errico]

So

   min(m_dot(wd > 0))

does not work? I'd try it anyway. :)

[Ieuan Lewis]

I'll try that out thanks. I actually have m_dot(k) and wd(k) . How do i then extract all other data on that k^th row of the matrix? This data will then be passed back to a script file that outputs to excel, but i only need he single row where m_dot is minimum

Thanks

[John D'Errico]

You did not ask for that.

min returns a second argument, the index of the element that was identified. However, then you would need to look back at find(wd>0), a reverse lookup to identify the original row.

Answer 2

[Image Analyst]

This code should do it. Repeat for wd. It's vectorized to operate on the whole column vector, so don't put (k) in there!

% Create sample positive and negative data.
m_dot = rand(10,1) - 0.5

% Find row of smallest magnitude.  The value may be negative.
[smallestAbsValue, rowOfSmallestValue] = min(abs(m_dot))
% Get the actual value in case it's negative.
smallestValue = m_dot(rowOfSmallestValue)

% Find row of smallest positive number.
positiveIndexes = m_dot > 0;
[smallestPosValue, rowOfSmallestValue] = min(m_dot(positiveIndexes))

If you have a 2D array of 3 columns, and want the whole row, you can extract just that row like this:

theRow = yourMatrix(rowOfSmallestValue, :);

[Ieuan Lewis]

Hello,

I've combined both answers to give , the following code.

m_dot = [0.001,0.02,0.004]' wd = [-1,1,2]'

[smallestAbsValue, rowOfSmallestValue] = min(m_dot(wd>0))

theRow = wd(rowOfSmallestValue, :)

This finds the smallest value of m_dot for when wd is positive but gives me the row when wd is smallest and not m_dot. (i.e returns row 2 instead of row 3) How can i fix that?

Just to confirm, i'm trying to find the value and row when m_dot is smallest as long as, in the same row, wd is postive. Thanks in advance.

[Image Analyst]

I see you've edited your question to make it more clear. So what you have to do is to find the criteria in each column, then AND them together to get where it holds for the combined criteria. So if you want "to return the smallest value in Column 1( code name is m_dot) but only for a positive value of column 3 (code name is wd)." then do this

% Find positive indexes in column 3
% which has already been extracted into wd = matrix(:, 3)
positiveCol3 = wd > 0; % Logical vector
% Now find the min of only those rows of m_dot
% which has already been extracted into m_dot = matrix(:, 1)
minWDotValue = min(m_dot(positiveCol3));
% Now find row(s) in original location.  There may be more than 1.
rowsWithMin = find(w_dot == minWDotValue);