python - Understanding argmax

Question

Let say I have the matrix

import numpy as np    
A = np.matrix([[1,2,3,33],[4,5,6,66],[7,8,9,99]])

I am trying to understand the function argmax, as far as I know it returns the largest value

If I tried it on Python:

np.argmax(A[1:,2])

Should I get the largest element in the second row till the end of the row (which is the third row) and along the third column? So it should be the array [6 9], and arg max should return 9? But why when I run it on Python, it returns the value 1?

And if I want to return the largest element from row 2 onwards in column 3 (which is 9), how should I modify the code?

I have checked the Python documentation but still a bit unclear. Thanks for the help and explanation.

Answer 1

No argmax returns the position of the largest value. max returns the largest value.

import numpy as np    
A = np.matrix([[1,2,3,33],[4,5,6,66],[7,8,9,99]])

np.argmax(A)  # 11, which is the position of 99

np.argmax(A[:,:])  # 11, which is the position of 99

np.argmax(A[:1])  # 3, which is the position of 33

np.argmax(A[:,2])  # 2, which is the position of 9

np.argmax(A[1:,2])  # 1, which is the position of 9