View on numpy array

Question

I have a matrix

 m = np.zeros((5,5))

and what to have a view on the last row

row = m[-1]

however if I add a column to m:

m = np.c_[m[:,:2],[0,0,1,1,1],m[:,2:]]

and print out row I don't get the new column.

Is there any way to get the change without use the line

row = m[-1]

again?

• Kasravnd: You want to modify your previous array in an in-place manner and that's not possible. Every time you change your array you'll end up creating a new object.
• mforpe: The question should be: "Why do you need to do that?" it makes no sense to me
• Guohua Cheng: I think maybe the problem lies in the method np.c_, which is different from ordinary operation on view of Numpy array, which can achieve in-place modification.
• Wikunia: @ManelFornos I have a simplex tableau in m and would like to get the objective row (last row) sometimes I need to add columns to the matrix.
• Mohamed Ali JAMAOUI: you want to get last row without running this row = m[-1] each time?

Answer 1

What you want to achieve here isn't currently possible within the bounds of the numpy library. See this answer on numpy arrays occupying a contiguous block of memory.

You can use the rx library to get around this by making a subject of the last row.

import numpy as np
from rx import Observable, Observer
from rx.subjects import Subject
m = np.zeros((5,5))
m_stream = Subject()
last_row_stream = Subject()
last_row = None
def update_last(v):
 global last_row
 last_row = v
last_row_stream.subscribe(on_next=update_last)
last_row_stream.on_next(m[-1])
print(last_row)
m_stream.map(
 lambda v: np.insert(v, 2, [0, 0, 1, 1, 1], axis=1)
).subscribe(lambda v: last_row_stream.on_next(v[-1]))
m_stream.on_next(m)
print(last_row)

• Wikunia: I thought of an observer pattern as well. I would like to observe on every change made so whenever m changed I would like to have row changed. Doesn't matter whether it really changed row or not. Therefore I would like to not change the insert or c_ row. Would like to have a subscribe to all changes of m.
• Oluwafemi Sule: m_stream is an observable of m. Any operations you want to carry out on m can be mapped on m_stream for a start. You can also dispose of observer(s) whenever you deem it fit. ...subscribe(lambda v: last_row_stream.on_next(v[-1])) sets a observer on m_stream that updates last_row_stream. You can dispose of this and set your very own observer.