combine numpy array "element-wise" - python

Currently I have two arrays: the shape of a1 is (5,4,6,3), the second one a2 is (5,4,6) and finally I want to get a merged array (5,4,6,4)
Currently I "for-loop" each (6,3) array and np.stack it with corresponding (6,1) to (6,4).
for i in range(a1.shape[0]):
for j in range(a1.shape[1]):
a = np.hstack((a1[i,j], a2[i,j].reshape(6,1)))
However, it's not pretty efficient if it's much bigger than 5*4.
Do you have a better way?

Is this what you want?
import numpy as np
a1 = np.ones((5, 4, 6, 3))
a2 = np.ones((5, 4, 6))
result = np.concatenate((a1, a2[..., np.newaxis]), axis=-1)
print(result.shape)
(5, 4, 6, 4)

Related

numpy array divide multiple columns by one column

Is it possible to divide multiple numpy array columns by another 1D column (row wise division)?
Example:
a1 = np.array([[1,2,3],[4,5,6],[7,8,9]])
array([[1, 2, 3],
[4, 5, 6],
[7, 8, 9]])
a2 = np.array([11,12,13])
array([11, 12, 13])
# that is divide all rows, but only columns 1 and 2 by a2 array
a1[:,:2] / a2
ValueError: operands could not be broadcast together with shapes (3,2) (3,)
I did try this, but this does not look elegant
(a1[:,:2].T / a2).T
array([[0.09090909, 0.18181818],
[0.33333333, 0.41666667],
[0.53846154, 0.61538462]])
Your a1 array is 2D and a2 is 1D, try expanding the dimension of a2 array to 2D before performing division:
>>> a1[:,:2]/np.expand_dims(a2, 1)
array([[0.09090909, 0.18181818],
[0.33333333, 0.41666667],
[0.53846154, 0.61538462]])
Apparently, you can just use a2[:, None] instead of calling expand_dims function for even cleaner code.

Reshaping each element of a multidimension 3D array to another Multidimension 3D array in Python

I'm working on a problem where I've to reshape a (63,16,3) array's each element to an array (4,4,3), and I'm stuck there.
I generated an array of (63,16,3) using the random function of NumPy. Please help me how to reshape that array's each element into a (4,4,3) and store those outputs into an array.
import numpy as np
a = np.random.rand(63, 16, 3)
return an array b whose each element is (4,4,3)
I have successfully converted the array (63, 16, 3) into (4, 4, 3) but elementwise. What I mean can be cleared using the below snippet of code.
a_resize_0th_element = a[0].reshape(4,4,3)
But I'm looking for a method where this element-wise operation of transforming a (16, 3) array into the shape of (4, 4, 3) and can be done for all the 63 elements of array a and store everything into array b.
You just need reshape(). The size of the array is 63 * 16 * 3 = 3,024 elements. If you want to divide that into 4x4x3 arrays, that's 3,024 / (4 * 4 * 3) = 63 elements.
So:
b = np.reshape(a, (63, 4, 4, 3))
print(b[0].shape)
Result:
(4, 4, 3)
So, b is an array with 63 shape (4, 4, 3) arrays.
Note: obviously, 4 * 4 = 16 here, but generally this works. However, if you don't want to do the math yourself, you can also just use this:
b = np.reshape(a, (-1, 4, 4, 3))
The -1 will cause numpy to figure it out itself and it will give you the same result.

Numpy Matrix Subtraction Different Dimensions

I currently have a 5D numpy array of dimensions 40 x 3 x 3 x 5 x 1000 where the dimensions are labelled by a x b x c x d x e respectively.
I have another 2D numpy array of dimensions 3 x 1000 where the dimensions are labelled by b x e respectively.
I wish to subtract the 5D array from the 2D array.
One way I was thinking of was to expand the 2D into a 5D array (since the 2D array does not change for all combinations of the other 3 dimensions). I am not sure what array method/numpy function I can use to do this.
I tend to start getting lost with nD array manipulations. Thank you for assisting.
In [217]: a,b,c,d,e = 2,3,4,5,6
In [218]: A = np.ones((a,b,c,d,e),int); B = np.ones((b,e),int)
In [219]: A.shape
Out[219]: (2, 3, 4, 5, 6)
In [220]: B.shape
Out[220]: (3, 6)
In [221]: B[None,:,None,None,:].shape # could also use reshape()
Out[221]: (1, 3, 1, 1, 6)
In [222]: C = B[None,:,None,None,:]-A
In [223]: C.shape
Out[223]: (2, 3, 4, 5, 6)
The first None isn't essential; numpy will add it as needed, but as a human it might help to see it.
IIUC, suppose your arrays are a and b:
np.swapaxes(np.swapaxes(a, 1, 3) - b, 1, 3)

Numpy Matrix Multiplication with Vectors

i wanna do a simple matrix multiplication with 2 Vectors: so that A * B.T = 3x3Matrix.
But somehow numpy returns a scalar or vector.
i already tried:
np.dot(a, b.transpose())
np.matmul(a, b.transpose())
a * b.transpose()
But nothins works, it seems like a simple operation to me, but i just cannot solve it
The reason why you are getting a scalar because you are multiplying two 1D vectors in numpy, which produces the inner product of 2 vectors. You need to reshape your vector to the shape (3,1), which turns them into a 2D shape and then you get the expected result upon performing the vector multiplication. Check the snippet below
>>> import numpy as np
>>> A = np.array([1,2,3])
>>> B = np.array([4,5,6])
>>> A.shape
(3,)
>>> B.shape
(3,)
>>> AA = A.reshape(3, 1)
>>> BB = B.reshape(3, 1)
>>> AA.shape
(3, 1)
>>> BB.shape
(3, 1)
>>> np.matmul(AA, np.transpose(BB))
array([[ 4, 5, 6],
[ 8, 10, 12],
[12, 15, 18]])
Using numpy.reshape works for me all the time.
Maybe you're stumbling on it because of your matrix's size.
A should be (3,1) dan B.transpose should be (1,3).
When using numpy.dot, both matrix should have the same inner size. In your case is (1). The inner should be 1 because the inner of AxA_transpose is (3,1)x(1,3). Result will be 3x3 matrix.
Do:
A_ = np.reshape(A,(1,-1)) # array (3,1)
B_ = np.reshape(B,(1,-1))
C = np.dot(A_,B_.T) # T for transpose

Multidimensional array in numpy

I have an array of shape (5,2) which each row consist of an array of shape (4,3,2) and a float number.
After I slice that array[:,0], I get an array of shape (5,) which each element has shape of (4,3,2), instead of an array of shape (5,4,3,2) (even if I'd use np.array()).
Why?
Edited
Example:
a1 = np.arange(50).reshape(5, 5, 2)
a2 = np.arange(50).reshape(5, 5, 2)
b1 = 15.0
b2 = 25.0
h = []
h.append(np.array([a1, b1]))
h.append(np.array([a2, b2]))
h = np.array(h)[:,0]
np.shape(h) # (2,)
np.shape(h[0]) # (5, 5, 2)
np.shape(h[1]) # (5, 5, 2)
h = np.array(h)
np.shape(h) # (2,) Why not (2, 5, 5, 2)?
You have an array of objects; You can use np.stack to convert it to the shape you need if you are sure all the sub elements have the same shape:
np.stack(a[:,0])
a = np.array([[np.arange(24).reshape(4,3,2), 1.]]*5)
a.shape
# (5, 2)
a[:,0].shape
# (5,)
a[:,0][0].shape
# (4, 3, 2)
np.stack(a[:,0]).shape
# (5, 4, 3, 2)
In [121]: a1.dtype, a1.shape
Out[121]: (dtype('int32'), (5, 5, 2))
In [122]: c1 = np.array([a1,b1])
In [123]: c1.dtype, c1.shape
Out[123]: (dtype('O'), (2,))
Because a1 and b1 are different shaped objects (b1 isn't even an array), an array made from them will have dtype object. And the h made from several continues to be object dtype.
In [124]: h = np.array(h)
In [125]: h.dtype, h.shape
Out[125]: (dtype('O'), (2, 2))
In [126]: h[:,1]
Out[126]: array([15.0, 25.0], dtype=object)
In [127]: h[:,0].dtype
Out[127]: dtype('O')
After the appends, h (as an array) is object dtype. The 2nd column is the b1 and b2 values, the 1st column the a1 and a2.
Some form of concatenate is required to combine those a1 a2 arrays into one. stack does it on a new axis.
In [128]: h[0,0].shape
Out[128]: (5, 5, 2)
In [129]: np.array(h[:,0]).shape # np.array doesn't cross the object boundary
Out[129]: (2,)
In [130]: np.stack(h[:,0]).shape
Out[130]: (2, 5, 5, 2)
In [131]: np.concatenate(h[:,0],0).shape
Out[131]: (10, 5, 2)
Turning the (2,) array into a list, does allow np.array to recombine the elements into a higher dimensional array, just as np.stack does:
In [133]: np.array(list(h[:,0])).shape
Out[133]: (2, 5, 5, 2)
You appear to believe that Numpy can magically divine your intent. As #Barmar explains in the comments, when you slice a shape(5,2) array with [:, 0] you get all rows of the first column of that array. Each element of that slice is a shape(4,3,2) array. Numpy is giving you exactly what you asked for.
If you want to convert that into a shape(5,4,3,2) array you'll need to perform further processing to extract the elements from the shape(4,3,2) arrays.

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