how to copy numpy array value into higher dimensions

Question

I have a (w,h) np array in 2d. I want to make a 3d dimension that has a value greater than 1 and copy its value over along the 3rd dimensions. I was hoping broadcast would d

Answer 1

Another simple approach is to use matrix multiplication - multiplying by a matrix of ones that will essentially copy the values across the new dimension:

a=np.random.randn(4,4)    #a.shape = (4,4)
a = np.expand_dims(a,-1)  #a.shape = (4,4,1)
a = a*np.ones((1,1,3))
a.shape                   #(4, 4, 3)

Answer 2

This would work. (I think this would not a recommended way :-) But maybe this is the most closest way you thought.)

np.array([img, img, img]).transpose(1,2,0)

just stacking targets(img) any time you want(3), and make the channel(3) go to the last axis.

Answer 3

I'd suggest you to use the barebones numpy.concatenate() simply because the below piece of code shows that it's the fastest among all other suggested answers:

# sample 2D array to work with
In [51]: arr = np.random.random_sample((12, 34))

# promote the array `arr` to 3D and then concatenate along `axis 2`
In [52]: arr3D = np.concatenate([arr[..., np.newaxis]]*3, axis=2)

# verify for desired shape
In [53]: arr3D.shape
Out[53]: (12, 34, 3)

You can see the timings below to convince yourselves. (ordered: best to worst):

In [42]: %timeit -n 100000 np.concatenate([arr[..., np.newaxis]]*3, axis=2)
1.94 µs ± 32.9 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

In [43]: %timeit -n 100000 np.repeat(arr[..., np.newaxis], 3, axis=2)
4.38 µs ± 46.7 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

In [44]: %timeit -n 100000 np.dstack([arr]*3)
5.1 µs ± 57.6 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

In [49]: %timeit -n 100000 np.stack([arr]*3, -1)
5.12 µs ± 125 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

In [46]: %timeit -n 100000 np.tile(arr[..., np.newaxis], 3)
7.13 µs ± 85.1 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

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Having said that, if you're looking for shortest piece of code, then you can use:

# wrap your 2D array in an iterable and then multiply it by the needed depth
arr3D = np.dstack([arr]*3)

# verify shape
print(arr3D.shape)
(12, 34, 3)

Answer 4

Another approach that works:

x_train = np.stack((x_train,) * 3, axis=-1)

Answer 5

Not sure if I understood correctly, but broadcasting seems working to me in this case:

>>> a = numpy.array([[1,2], [3,4]])
>>> c = numpy.zeros((4, 2, 2))
>>> c[0] = a
>>> c[1:] = a+1
>>> c
array([[[ 1.,  2.],
        [ 3.,  4.]],

       [[ 2.,  3.],
        [ 4.,  5.]],

       [[ 2.,  3.],
        [ 4.,  5.]],

       [[ 2.,  3.],
        [ 4.,  5.]]])

Answer 6

You can push all dims forward, introducing a singleton dim/new axis as the last dim to create a 3D array and then repeat three times along that one with np.repeat, like so -

arr3D = np.repeat(arr[...,None],3,axis=2)

Here's another approach using np.tile -

arr3D = np.tile(arr[...,None],3)