MATLAB: Min value & correlating time from two different matrices

min valuetwo matrices

I have a 91×2 matrix, the first column is time the second is kinematic data of knee angles. I need to find the maximum knee flexion and save the time and angle separately for graphing. Any suggestions? I know how to find the maximum knee angle by using min (They're negative angles) but how do I get the correlating times?

Best Answer

Ask min for its second output, the index of the minimum value:

Example:

t = linspace(pi,2*pi,91);
y = sin(t);
data = [t' y'];
[minangl,idx] = min(data(:,2),[],1); 
out = [t(idx) minangl]

produces:

out =
       4.7124e+000    -1.0000e+000

Related Solutions

MATLAB: For loop using unique values

Indeed using find would be madness. For the record, find is over used by beginners and is rarely needed. For that matters, for loop are also overused and are also not needed in this case.

There are many ways to do this:

Using groupsummary:

maxdischarge = groupsummary(yourmatrix(:, 4), yourmatrix(:, 2), 'max');

Using splitapply:

[group, month] = findgroups(yourmatrix(:, 2));  %may not be needed if the month column is integers from 1 to 12
maxdischarge = splitapply(@max, yourmatrix(:, 4), group);  %maxdischarge(i) corresponds to month(i)

Using old fashioned accumarray:

[month, ~, group] = unique(yourmatrix(:, 2));  %works like findgroups, with different order of output. Again may not be needed
maxdischarge = accumarray(yourmatrix(:, 4), group, [], @max);

Or even using a loop:

maxdischarge = zeros(12, 1)
for month = 1:12
    maxdischarge = max(yourmatrix(yourmatrix(:, 2) == month, 4));
end

Note that I would recommend that you convert your matrix into a table:

t = array2table(yourmatrix, 'VariableNames', {'Day', 'Month', 'Year', 'Discharge'});

In which case, you definitively should use groupsummary:

groupsummary(t, 'Month', 'max', 'Discharge');  %calculate max of discharge grouped by month

Even better would be to convert the first 3 columns into a datetime array:

t = table(datetime(yourmatrix(:, 3), yourmatrix(:, 2), yourmatrix(:, 1)), yourmatrix(:, 4), 'VariableNames', {'Date', 'Discharge'});
groupsummary(t, 'Date', 'monthofyear', 'max', 'Discharge')

MATLAB: How to normalize matrix by specific columns matlab

One kind of normalization:

YourMatrix(:,1:33) = YourMatrix(:,1:33)./sum(YourMatrix(:,1:33));

Another kind:

YourMatrix(:,1:33) = mat2gray(YourMatrix(:,1:33));   %normalizes between min and max of the entire set of columns

Another kind:

min33 = min(YourMatrix(:,1:33));
max33 = max(YourMatrix(:,1:33));
range33 = max33 - min33;
YourMatrix(:,1:33) = (YourMatrix(:,1:33) - min33)./range33;   %normalizes each column to 0 to 1 independently

Best Answer

Related Solutions

MATLAB: For loop using unique values

MATLAB: How to normalize matrix by specific columns matlab

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