CODEWITHSUNDEEP
pythonnumpymatrix
TriHard8
TriHard8

Reputation: 592

Find average of all columns of matrix filtered on last column

I'm new to numpy. I have an Nx4 matrix and I want to find the average of each column when the last column equals 1 for instance. In matlab I would do something like mean1 = mean(data[column 4] == 1). This would return a matrix (or vector) with the mean of the columns, with the mean of column 4 being equal to 1. I can't find any specific documentation that specifies how to handle this. This shows how to filter the matrix, but I shouldn't have to reassign the matrix to a new variable, doubling storage size. Thanks in advance.

Upvotes: 0

Views: 206

Answers (1)

Alan
Alan

Reputation: 9620

#make artificial data to match problem
data = np.random.random((100,4))
print( id(data) )
data[:,3] = data[:,3] < 0.5
print( id(data) )  #same object (memory location)
#get the filter
dfilter = data[:,3].astype(np.bool_)
#find the means
means = data[dfilter].mean(axis=0)

Upvotes: 1

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