# Questions tagged [numpy-ndarray]

95 questions

4

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2

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19

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### Python NumPy - angled slice of 3D array

Working in NumPy, I understand how to slice 2D arrays from a 3D array using this article.
Depending on the axis I'd want to slice in:
array = [[[0 1 2]
[3 4 5]
[6 7 8]]
[[9 10 11]
[12 13 14]
[15 16 17]]
[[18 19 20]
[21 22 23]
[24 25 26]]]
Slicing would give me:
i_slice = array[0]
[[0 1 2]...

8

votes

2

answer

210

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### Fill Bounding Boxes in 2d array

I have a 2D numpy array which looks like
array([[0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.],
[0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.],
[0., 0., 1., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.],
[0., 0., 1., 1., 1., 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.],
[0....

0

votes

0

answer

4

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### Issue in shifting 3D array using numpy.roll()

I am having an issue while shifting 3D array using NumPy roll function. My array is of this dimension u((3, nx, ny, nz)).
Problem is that np. roll function is not working in the same way as it supposes to be. Generally, in roll function: axis = 0 defines the shifting of rows and axis =1 defines shi...

1

votes

1

answer

311

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### Are Numpy arrays hashable?

I've read that numpy arrays are hashable which means it is immutable but I'm able to change it's values so what does it exactly mean by being hashable?
c=pd.Series('a',index=range(6))
c
Out[276]:
0 a
1 a
2 a
3 a
4 a
5 a
dtype: object
This doesn't give me error then why it gives er...

0

votes

0

answer

4

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### Numpy ordered subtraction of elements with full vectorization

Imagine a mxn array a, and a 1xn array b, we want to subtract b from a so that b is subtracted from the first element of a, then maximum of zero and b-a[0] is subtracted from a[1], and so on...
So:
x = np.array([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
a = np.repeat(x, 100000).reshape(10, 100000)
b = np.repe...

1

votes

2

answer

40

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### Comparing object ids of two numpy arrays

I have been using numpy for quite a while but I stumbled upon one thing that I didn't understand fully:
a = np.ones(20)
b = np.zeros(10)
print(id(a)==id(b)) # prints False
print(id(a), id(b)) # prints (4591424976, 4590843504)
print(id(a[0])==id(b[0])) # prints True
print(id(a[0...

1

votes

2

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44

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### How to write a numpy ndarray to a textfile?

Say that I got this ndarray using numpy that I want to write to a text file. :
[[1 2 3 4]
[5 6 7 8]
[9 10 11 12]]
This is my code:
width, height = matrix.shape
with open('file.txt', 'w') as foo:
for x in range(0, width):
for y in range(0, height):
foo.write(str(matrix[x, y]))
foo...

1

votes

1

answer

42

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### Why is the use of numpy.nonzero enormously faster than looping through a numpy array?

Let's create a numpy ndarray of 10 million bools with all values initialized to True
n=10000000
sample = np.ones(n, dtype=bool)
Next we'll set a few values to False
sample[1] = sample[5] = sample[12] = sample[25] = sample[50] = False
The number of True values is now n-5 = 9999995
We can count the...

1

votes

1

answer

74

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### finding nearest neighbor for python numpy.ndarray in 3d-space

I have a numpy.ndarray of 3d-points, i.e. the np.shape of it is (4350,3) and such a second numpy.ndarray of 3d-points of np.shape (10510,3).
Now I am trying to find the right python-package to calculate the nearest neighbors in the second array of the points in the first array as quickly as possibl...

1

votes

1

answer

18

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### Python | cv2.imshow() loading arrays as BGR?

I have recorded some data as npy file. And I tried to diplay the image (data[0]) to check if it makes sense with the following code
import numpy as np
import cv2
train_data = np.load('c:/data/train_data.npy')
for data in train_data:
output = data[1]
# only take the height, width and channels of the...

0

votes

0

answer

7

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### Why numpy mixed basic / advanced indexing depends on slice adjacency?

I know similar questions have been asked before (e.g.), but AFAIK nobody has answered my specific question...
My question is about the numpy mixed advanced / basic indexing described here:
... Two cases of index combination need to be distinguished:
The advanced indexes are separated by a slice, ell...

0

votes

0

answer

9

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### Numpy array losing dimensions when applying mask of same shape

I have created a 2D mask appropriately named mask to have the same shape as the array data, the array I want to apply it upon. However when I do so, the data loses it's shape and becomes 1D.
I thought that as each level for axis 0 is identical (shown with creation of mask using loop comprehension) t...

-3

votes

1

answer

46

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### How to convert Numpy array to Panda DataFrame

I have a Numpy array that looks like this:
[400.31865662]
[401.18514808]
[404.84015554]
[405.14682194]
[405.67735105]
[273.90969447]
[274.0894528]
When I try to convert it to a Panda Dataframe with the following code
y = pd.DataFrame(data)
print(y)
I get the following output when printing it. Why do...

2

votes

2

answer

71

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### Numpy array to Tensor

I am changing the code created by numpy to tensorflow code.
However, tensorflow does not support specifying each element, (eg x [i] = 7), boolean (eg.var [x

0

votes

2

answer

15

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### array is not callable in python “'numpy.ndarray' object is not callable”

I am working on a neural network and when i try to shuffle the two numpy.ndarray i get this error. I tried rechecking the shuffle function format and cannot find any faults with that. Please help
train_images,train_labels = shuffle(train_images,train_labels)
TypeError...

2

votes

0

answer

26

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### Why numpy.ndarray.data is toggle? [duplicate]

This question already has an answer here:
Behavior of ndarray.data for views in numpy
2 answers
Python numpy data pointer addresses change without modification
2 answers
I'm using python 3.6.7 create a 1-D array, and print its data attribute, and found it is toggle.
But why?
>>> a = np.arange(12)...

2

votes

2

answer

33

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### Numpy arithmetic

I have the following code :
import numpy as np
x=np.array([[3, 5, 1]])
print(x.shape) #get (1,3)
np.multiply(x.shape, 8) #get [ 8, 24]
print(*x.shape) # get 1 3
np.array((np.multiply(*x.shape), 8)) #get [3, 8]
Please explain why/how np.multiply(*x.shape, 8) get [3, 8] ?

19

votes

4

answer

7.1k

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### How does the axis parameter from NumPy work?

Can someone explain exactly what the axis parameter in NumPy does?
I am terribly confused.
I'm trying to use the function myArray.sum(axis=num)
At first I thought if the array is itself 3 dimensions, axis=0 will return three elements, consisting of the sum of all nested items in that same position....

5

votes

1

answer

65

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### 3-D Matrix Multiplication in Numpy

I have to multiply two 2-D matrices, bob and tim, in Numpy Python 3.x
bob.shape gives (2,4)
tim.shape gives (7,4)
This piece of code gives a 3-D matrix with a shape of (2,7,4)
np.array([foo*tim for foo in bob])
It gives the output I want. But, I was wondering if there was a more elegant/faster way t...

3

votes

2

answer

41

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### When is the size of an ndarray not fixed?

The numpy.ndarray documentation states that:
An ndarray is a (usually fixed-size) multidimensional container of items of the same type and size.
I'm surprised by the adjective usually here. I thought an ndarray is always of a fixed size. When is the size of an ndarray not fixed?

1

votes

2

answer

205

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### Group by with numpy.mean

How do I calculate the mean for each of the below workerid's? Below is my sample NumPy ndarray. Column 0 is the workerid, column 1 is the latitude, and column 2 is the longitude.
I want to calculate the mean latitude and longitude for each workerid. I want to keep this all using NumPy (ndarray),...

3

votes

1

answer

33

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### How to split a numpy array using a binary list?

If I have a numpy array say
A = [[1,2],[3,4],[5,6],[7,8]]
and a list
L = [1,0,1,1]
Is there a way to split A down axis0 based off of if they are a 1/0 in L? This would be my desired result:
A1 = [[1,2],[5,6],[7,8]]
A2 = [[3,4]]

2

votes

1

answer

20

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### ndarray rows are not displayed each one per line

I have this code:
import numpy as np
M = np.matrix([[-4.41991030e-05,-9.27712599e-04,3.75797779e-04,4.11804326e-04,1.08815444e-04],
[-3.58432112e-04,-6.11583291e-04,1.18565910e-03,4.10337098e-04,9.96854953e-05],
[-1.36865905e-03,1.19013259e-03,1.62785645e-03,1.85052363e-04,6.73256050e-05],
[-0.00292...

2

votes

1

answer

34

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### How can I add to each element of a numpy ndarray the sum of all its index values with python ?

I have a numpy ndarray, let's take an example (but it can be totally different):
[[[0 0 0]
[1 1 1]
[0 0 0]]
[[1 0 1]
[1 0 1]
[1 0 1]]]
[[1 0 0]
[1 1 0]
[1 1 1]]]
I want to add to each element the sum of its indexes in all dimensions.
The result here would be:
[[[ 0 1 2]
[ 4 5 6]
[ 6 7 8]]
[[10...

2

votes

3

answer

44

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### How do I in a sense “juggle” elements in ndarrays?

What I mean by this is imagine you have an ndarray a with shape (2,3,4). I want to define another ndarray b with shape (3,2,4) such that
b[i][j][k] = a[j][i][k]
Matrix operations only apply to the last 2 index places. If there is a way to make matrix operations act on any 2 chosen index places then...

2

votes

2

answer

30

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### Merge 2 numpy Arrays with timestamps

I have two numpy ndarrays - each with their own timestamp-dimension. I want to merge them together. However the interval of their timestamps is not necessarily the same. Here's an example of what I mean:
Array 1: names = ['timestamp', 'value']
a1 = [(1531000000, 0), (1532000000, 1), (1533000000, 2)...

2

votes

2

answer

32

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### overriding partially an numpy array does not work

I tried to override a numpy array partially
does anyone know how to do that in such comfort indexing way?
Thanks!

4

votes

2

answer

47

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### numpy conditional list membership element wise

I have a 2D numpy array:
a = np.array([[0,1],
[2,3]])
I have a list of values to keep:
vals_keep = [1,2]
I want to test for list membership for each element in the array. Something like:
mask = a in vals_keep
The result I want:
array([[False, True],
[True, False]])

6

votes

0

answer

79

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### Numpy: strange different behavior of inplace and explicit operation

I want to operate on numpy arrays to use their indexing, and I want to include the 0-dimensional case. Now I came across a strange situation, where a type conversion appears, if I don't use in-place multiplication:
In [1]: import numpy as np
In [2]: x = 1.*np.array(1.)
In [3]: y = np.array(1.)
In [4...

4

votes

2

answer

819

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### Perform numpy exp function in-place

As in title, I need to perform numpy.exp on a very large ndarray, let's say ar, and store the result in ar itself. Can this operation be performed in-place?

2

votes

3

answer

1k

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### Python Numpy repeating an arange array

so say I do this
x = np.arange(0, 3)
which gives
array([0, 1, 2])
but what can I do like
x = np.arange(0, 3)*repeat(N=3)times
to get
array([0, 1, 2, 0, 1, 2, 0, 1, 2])

2

votes

3

answer

651

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### How to delete a row based on a condition from a numpy array?

From the following array :
test = np.array([[1,2,'a'],[4,5,6],[7,'a',9],[10,11,12]])
How can I delete the rows that contain 'a' ?
Expected result :
array([[ 4, 5, 6],
[10, 11, 12]])

7

votes

2

answer

7.3k

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### Numpy zip function

If I have two numpy 1D arrays, for example
x=np.array([1,2,3])
y=np.array([11,22,33])
How can I zip these into Numpy 2D coordinates arrays?
If I do:
x1,x2,x3=zip(*(x,y))
The results are of type list, not Numpy arrays. So I have do
x1=np.asarray(x1)
and so on..
Is there a simpler method, where I do n...

4

votes

4

answer

12.9k

Views

### Transforming a row vector into a column vector in Numpy

Let's say I have a row vector of the shape (1, 256). I want to transform it into a column vector of the shape (256, 1) instead. How would you do it in Numpy?

4

votes

4

answer

1.9k

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### Compare a matrix against a column vector

Arrays 'A' and vector 'B' below are part of pandas dataframe.
I have a large array A of form:
28 39 52
77 80 66
7 18 24
9 97 68
I have a vector B of form:
32
5
42
17
How do I compare pythonically each column of A against B. I am trying to get True/False values for A < B comparison to get t...

3

votes

0

answer

71

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### Partitioning the rows of a numpy array into buckets

I am writing code to process a large point cloud. I have been able to successfully vectorize most of my code to make it efficient, however, I cannot think of a good way of achieving this:
I have an nx3 numpy array that represents points in 3d (x,y,z) with z >= 0. I want to create a list of length k,...

0

votes

2

answer

26

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### Slicing a column from a 2D array

I have the following code :
import numpy as np
a = np.array([[ 1, 2, 3, 4, 5, 6],
[ 7, 8, 9, 10, 11, 12]])
a[:, 2:3] #get [[3],[9])
a[:, 2] #get [3, 9]
The last line returns a 1D array. The only way to get a 2D back is do a[:, 2:3]. Intuitively, I'd think slicing 1 column out of a 2D array ge...

3

votes

3

answer

71

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### NumPy ufuncs are 2x faster in one axis over the other

I was doing some computation, and measured the performance of ufuncs like np.cumsum over different axes, to make the code more performant.
In [51]: arr = np.arange(int(1E6)).reshape(int(1E3), -1)
In [52]: %timeit arr.cumsum(axis=1)
2.27 ms ± 10.5 µs per loop (mean ± std. dev. of 7 runs, 100 loops...

3

votes

2

answer

91

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### Why NumPy creates a view for x[[slice(None), 1, 2]]

In NumPy documentation for advanced indexing, it is mentioned that
Also recognize that x[[1, 2, 3]] will trigger advanced indexing, whereas x[[1, 2, slice(None)]] will trigger basic slicing.
A matrix is stored sequentially into the memory. I understand that it makes sense to make a view of x[[1, 2,...

3

votes

2

answer

69

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### Numpy […,None]

I have found myself needing to add features to existing numpy arrays which has led to a question around what the last portion of the following code is actually doing:
np.ones(shape=feature_set.shape)[...,None]
Set-up
As an example, let's say I wish to solve for linear regression parameter estimates...