I have a problem with array manipulation in NumPy. If I create two arrays x and y, and do
x = x - y
I get what I expect, that is each element of y is subtracted from the corresponding element of x, and thus x is modified.
However, if I put this in a loop:
m = np.array([[1,2,3],[1,2,3]])
y = array([1, 1, 1])
for i in m:
i = i - y
the matrix m remains unaltered. I am sure I am missing something very basic... How can I change the array m in a loop?
This is not related with numpy matrix, but how python deal with your
i = i - y
i - y produces a new reference of an array. When you assigns it to name i, so i is not referred to the one it was before, but the newly created array.
The following code will meet your purpose
for idx, i in enumerate(m):
m[idx] = i - y
Update: I realize that the easiest thing is to do
m = m-y
This does directly what I expected!
If working with an example where you are unable to avoid looping through the array but still want to change the row, do this
m = np.array([[1,2,3],[1,2,3]])
y = array([1, 1, 1])
for i in m:
i -= y
