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?

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. — Kasravnd
– Kasravnd, Commented Aug 20, 2017 at 7:10
The question should be: "Why do you need to do that?" it makes no sense to me — mforpe
– mforpe, Commented Aug 20, 2017 at 7:35
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. — Guohua Cheng
– Guohua Cheng, Commented Aug 20, 2017 at 7:46
@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. — Wikunia
– Wikunia, Commented Aug 20, 2017 at 7:47
you want to get last row without running this row = m[-1] each time? — Mohamed Ali JAMAOUI
– Mohamed Ali JAMAOUI, Commented Aug 20, 2017 at 7:55

Oluwafemi Sule · Accepted Answer · 2017-08-20 08:23:03Z

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)

edited Aug 20, 2017 at 8:23

answered Aug 20, 2017 at 8:14

Oluwafemi Sule

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2 Comments

Wikunia Over a year ago

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 Over a year ago

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.

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