Update numpy array row by condition

Question

I need help) I have NumPy array:

False False False False
False True  True  False
True  True  True  True
False True  True  False
False False False False

How can I get this (take first and last rows, that contains True, and set all of them elements to False)?

False False False False
False False False  False
True  True  True  True
False False False  False
False False False False

unutbu · Accepted Answer · 2013-09-15 10:08:28Z

4

arr[arr.any(axis=1).nonzero()[0][[0,-1]]] = False

How it works:

In [19]: arr

Out[19]: 
array([[False, False, False, False],
       [False,  True,  True, False],
       [ True,  True,  True,  True],
       [False,  True,  True, False],
       [False, False, False, False]], dtype=bool)

arr.any(axis=1) finds which rows contains a True value:

In [20]: arr.any(axis=1)
Out[20]: array([False,  True,  True,  True, False], dtype=bool)

nonzero returns a tuple (one item for each axis) of indices of the True rows:

In [21]: arr.any(axis=1).nonzero()
Out[21]: (array([1, 2, 3]),)

We can use indexing to find the index of the first and last row containing a True value:

In [22]: arr.any(axis=1).nonzero()[0][[0,-1]]
Out[22]: array([1, 3])

And finally, we can set those rows to False with

In [23]: arr[arr.any(axis=1).nonzero()[0][[0,-1]]] = False

In [24]: arr
Out[24]: 
array([[False, False, False, False],
       [False, False, False, False],
       [ True,  True,  True,  True],
       [False, False, False, False],
       [False, False, False, False]], dtype=bool)

edited Sep 15, 2013 at 10:08

answered Sep 15, 2013 at 9:14

unutbu

886k197 gold badges1.9k silver badges1.7k bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

askewchan Over a year ago

@erthalion Does this answer the question generally? I interpret the question as: if a row has both True and False values, then set all values to False. Not just the first and last rows that fit that condition. If more than two rows fit that condition, this answer misses some. I would just do: arr[~a.all(1)] = False.

erthalion Over a year ago

@askewchan: Yes, in the my question I thought about the first/last rows only.

askewchan · Accepted Answer · 2013-09-15 17:18:05Z

1

In case you meant "first and last" only in reference to the particular example ...

If every row that contains both True and False values should be set to False, then you shouldn't restrict to the "first and last" of these rows, and the solution is much easier. Using the fact that ~a.all(1) will tell you which rows are not all True, you can set those rows to False with:

arr[~arr.all(1)] = False

or, to avoid redundantly setting rows of entirely False to False, use exclusive or, ^:

arr[arr.any(1) ^ arr.all(1)] = False

which will be faster in some circumstances.

answered Sep 15, 2013 at 17:18

askewchan

46.7k18 gold badges125 silver badges135 bronze badges

Collectives™ on Stack Overflow

Update numpy array row by condition

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related