Numpy append to an empty array

Question

1st case :

>>> import numpy as np
>>> x=np.array(0)
>>> x=np.append(x,1)
>>> x
array([0, 1])

x contains 2 elements. Why is that ?!

2nd case :

>>> x=np.array([])
>>> x=np.append(x,1)
>>> x
array([ 1.])

x contains 1 element, as expected.

What's the difference between np.array(0) and np.array([]) ?

np.array(0) is an array containing 0. whereas np.array([]) is an empty array containing nothing. — Srivatsan
– Srivatsan, Commented Nov 28, 2014 at 9:00
The real lesson to learn here is to check your results in between evaluations, it would have became obvious. — PascalVKooten
– PascalVKooten, Commented Nov 28, 2014 at 10:28

Srivatsan · Accepted Answer · 2015-08-01 10:18:43Z

7

In your first case, you are creating an array called x that will containing one value, which is 0.

In your second case you are creating an empty array called x that will contain no values, but is still an array.

FIRST CASE

So when you append x = np.append(x,1), the value 1 get's appended to your array (which already contains 0) i.e. it now contains 0 and 1

SECOND CASE

Since you have no values in the empty array, when you append x=np.append(x,1) the value 1 get's appended and the length of x becomes 1 (i.e. it now contains only 1)

P.S. I believe you might have thought that calling x = np.array(0) with the 0 would make it an empty array, it doesn't!! In Python, 0 is still taken to be a number and is appended to the array.

edited Aug 1, 2015 at 10:18

answered Nov 28, 2014 at 9:04

Srivatsan

9,38313 gold badges62 silver badges92 bronze badges

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

5 Comments

dpeng Over a year ago

Are you sure the 1st case create a 1-item array ? x.shape is ().

Srivatsan Over a year ago

@dplamp yes, you are true.. But as mentioned in my answer, when you create x = np.array(0) you are creating x with the value 0. So whatever you are going to append further to x will contain 0 along with the appended value

dpeng Over a year ago

That is not quite true, I think. My mistake is that array() takes an array-like object as its 1st argument, which is not the case in my '1st case'. Yet, it silently accepts it, which leads to a strange behaviour down the line. Hard to admit for a C++ guy...

sebix Over a year ago

@dplamp This is expected behavior. array(0) a 'zero-dimensional' array, while array([0]) is a one-dimensional array.

tttthomasssss Over a year ago

@dplamp x = np.array(0) creates a scalar, which is still an array (hence, type(x) would return numpy.ndarray), but is of empty shape. The whole handling of scalars vs. one-dimensional arrays seems a bit messy in numpy in general. Have a look at the docs or this answer to get a bit more insight.

Collectives™ on Stack Overflow

Numpy append to an empty array

1 Answer 1

5 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related