python replace values in 2d numpy array

Question

I want to replace the max values of each column of a 2d numpy array with -1:

b = numpy.array([[1,2,3,4],[5,6,7,8], [9,10,11,12]])
#get the max value of each column
maxposcol = b.argmax(axis = 0)
maxvalcol = b.max(axis = 0)
#replace max values with -1 
for i in numpy.arange(b.shape[1]):
    b[maxposcol[i]][i] = -1

Is there any other way to replace the max values whose positions are given by maxposcol[i]?

If I want to find the n maximal values of each column of my matrix what would you advise me to do? Using a sort? Repeating iteratively the search of max value and replace them at each step?

fraxel · Accepted Answer · 2012-06-11 17:47:30Z

3

You can do it like this:

>>> a=np.argmax(b, axis=0)
>>> b[a] = -1
>>> b
array([[ 1,  2,  3,  4],
       [ 5,  6,  7,  8],
       [-1, -1, -1, -1]])

answered Jun 11, 2012 at 17:47

fraxel

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2 Comments

lizzie Over a year ago

Thank you, What do you suggest to find the n maximal values of each column?

fraxel Over a year ago

@lizzie - [this link should help you ](stackoverflow.com/questions/6910641/…), also if you're not aware of: b[b>6]=-1 it may be of use to you :)

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