I am having problem to add a subarray to an existing 2D array. I am actually new to numpy and python and coming from MATLAB this was something trivial to do. Note that usually a is a big matrix in my problems.
import numpy as np
a = np.array(arange(16)).reshape(4,4) # The initial array
b = np.array(arange(4)).reshape(2,2) # The subarray to add to the initial array
ind = [0,3] # The rows and columns to add the 2x2 subarray
a[ind][:,ind] += b #Doesn't work although does not give an error
I saw looking around that the following can work
a[:4:3,:4:3] += b
but how can I define ind beforehand? In addition how to define ind if it consists of more than two numbers that cannot be expressed with a stride? for example ind = [1, 15, 34, 67]
a[ind][:,ind] += bdoesn't work, it's becausea[ind](whereindis a sequence of coordinates) makes a copy. Therefore, it does work, but the+=is applied to the copy, which is not saved.awinds up being unchanged. Basically, it's because you have use "fancy" indexing twice. @DSM's answer combines it into one "fancy" indexing expression, so+=works as expected. Another way to write it isa[ind[:,None], ind[None,:]] += b, but that requiresindto be a numpy array instead of a list. – Joe Kington yesterday