I have a 2 dimensional NumPy array. I know how to get the maximum values over axes:
>>> a = array([[1,2,3],[4,3,1]])
>>> amax(a,axis=0)
array([4, 3, 3])
How can I get the indices of the maximum elements? I would like as output array([1,1,0]) instead.
>>> a.argmax(axis=0)
array([1, 1, 0])
>>> import numpy as np
>>> a = np.array([[1,2,3],[4,3,1]])
>>> i,j = np.unravel_index(a.argmax(), a.shape)
>>> a[i,j]
4
a = np.array([[1,4,3],[4,3,1]]) to see that it returns i,j==0,1, and neglects the solution at i,j==1,0. For the indices of all the maxima use instead i,j = where(a==a.max().argmax() will only return the first occurrence for each row.
http://docs.scipy.org/doc/numpy/reference/generated/numpy.argmax.html
If you ever need to do this for a shaped array, this works better than unravel:
import numpy as np
a = np.array([[1,2,3], [4,3,1]]) # Can be of any shape
indices = np.where(a == a.max())
You can also change your conditions:
indices = np.where(a >= 1.5)
The above gives you results in the form that you asked for. Alternatively, you can convert to a list of x,y coordinates by:
x_y_coords = zip(indices[0], indices[1])
indices = np.where(a==a.max()) in line 3?
.max() instead of .argmax(). Please edit the answerx_y_coord = [(0, 2), (1, 1)] that does NOT match @eumiro answer, and is wrong. For example, try with a = array([[7,8,9],[10,11,12]]) to see that your code does not have any hit on this input. You also mention that this works better than unravel, but the solution posted by @blas answer the problem of the absolute maximum, not jsut along one axis.There is argmin() and argmax() provided by numpy that returns the index of the min and max of a numpy array respectively.
Say e.g for 1-D array you'll do something like this
import numpy as np
a = np.array([50,1,0,2])
print(a.argmax()) # returns 0
print(a.argmin()) # returns 2
And similarly for multi-dimensional array
import numpy as np
a = np.array([[0,2,3],[4,30,1]])
print(a.argmax()) # returns 4
print(a.argmin()) # returns 0
Note that these will only return the index of the first occurrence.
v = alli.max()
index = alli.argmax()
x, y = index/8, index%8
Allow me to give an up-to-date answer. We use the function argmax() with the parameter axis that can be defined as follows:
axis=0, the index of the max element per column will be returned.axis=1 the index of the max element per row will be returned.The following code will help you to better understand.
import numpy as np
a = np.array([[1,2,3],[4,3,1]])
#This will print the index of the max value for each column.
print(a.argmax(axis=0)) # output: [1 1 0]
#This will print the index of the max value for each row.
print(a.argmax(axis=1)) # output: [2 0]
