How to copy a 2D array into a 3rd dimension, N times?

Question

Probably the cleanest way is to use np.repeat:

a = np.array([[1, 2], [1, 2]])
print(a.shape)
# (2,  2)

# indexing with np.newaxis inserts a new 3rd dimension, which we then repeat the
# array along, (you can achieve the same effect by indexing with None, see below)
b = np.repeat(a[:, :, np.newaxis], 3, axis=2)

print(b.shape)
# (2, 2, 3)

print(b[:, :, 0])
# [[1 2]
#  [1 2]]

print(b[:, :, 1])
# [[1 2]
#  [1 2]]

print(b[:, :, 2])
# [[1 2]
#  [1 2]]

---

Having said that, you can often avoid repeating your arrays altogether by using broadcasting. For example, let's say I wanted to add a (3,) vector:

c = np.array([1, 2, 3])

to a. I could copy the contents of a 3 times in the third dimension, then copy the contents of c twice in both the first and second dimensions, so that both of my arrays were (2, 2, 3), then compute their sum. However, it's much simpler and quicker to do this:

d = a[..., None] + c[None, None, :]

Here, a[..., None] has shape (2, 2, 1) and c[None, None, :] has shape (1, 1, 3)*. When I compute the sum, the result gets 'broadcast' out along the dimensions of size 1, giving me a result of shape (2, 2, 3):

print(d.shape)
# (2,  2, 3)

print(d[..., 0])    # a + c[0]
# [[2 3]
#  [2 3]]

print(d[..., 1])    # a + c[1]
# [[3 4]
#  [3 4]]

print(d[..., 2])    # a + c[2]
# [[4 5]
#  [4 5]]

Broadcasting is a very powerful technique because it avoids the additional overhead involved in creating repeated copies of your input arrays in memory.

---

* Although I included them for clarity, the None indices into c aren't actually necessary - you could also do a[..., None] + c, i.e. broadcast a (2, 2, 1) array against a (3,) array. This is because if one of the arrays has fewer dimensions than the other then only the trailing dimensions of the two arrays need to be compatible. To give a more complicated example:

a = np.ones((6, 1, 4, 3, 1))  # 6 x 1 x 4 x 3 x 1
b = np.ones((5, 1, 3, 2))     #     5 x 1 x 3 x 2
result = a + b                # 6 x 5 x 4 x 3 x 2

Another way is to use numpy.dstack. Supposing that you want to repeat the matrix a num_repeats times:

import numpy as np
b = np.dstack([a]*num_repeats)

The trick is to wrap the matrix a into a list of a single element, then using the * operator to duplicate the elements in this list num_repeats times.

For example, if:

a = np.array([[1, 2], [1, 2]])
num_repeats = 5

This repeats the array of [1 2; 1 2] 5 times in the third dimension. To verify (in IPython):

In [110]: import numpy as np

In [111]: num_repeats = 5

In [112]: a = np.array([[1, 2], [1, 2]])

In [113]: b = np.dstack([a]*num_repeats)

In [114]: b[:,:,0]
Out[114]: 
array([[1, 2],
       [1, 2]])

In [115]: b[:,:,1]
Out[115]: 
array([[1, 2],
       [1, 2]])

In [116]: b[:,:,2]
Out[116]: 
array([[1, 2],
       [1, 2]])

In [117]: b[:,:,3]
Out[117]: 
array([[1, 2],
       [1, 2]])

In [118]: b[:,:,4]
Out[118]: 
array([[1, 2],
       [1, 2]])

In [119]: b.shape
Out[119]: (2, 2, 5)

At the end we can see that the shape of the matrix is 2 x 2, with 5 slices in the third dimension.

Use a view and get free runtime! Extend generic n-dim arrays to n+1-dim

Introduced in NumPy 1.10.0, we can leverage numpy.broadcast_to to simply generate a 3D view into the 2D input array. The benefit would be no extra memory overhead and virtually free runtime. This would be essential in cases where the arrays are big and we are okay to work with views. Also, this would work with generic n-dim cases.

I would use the word stack in place of copy, as readers might confuse it with the copying of arrays that creates memory copies.

Stack along first axis

If we want to stack input arr along the first axis, the solution with np.broadcast_to to create 3D view would be -

np.broadcast_to(arr,(3,)+arr.shape) # N = 3 here

Stack along third/last axis

To stack input arr along the third axis, the solution to create 3D view would be -

np.broadcast_to(arr[...,None],arr.shape+(3,))

If we actually need a memory copy, we can always append .copy() there. Hence, the solutions would be -

np.broadcast_to(arr,(3,)+arr.shape).copy()
np.broadcast_to(arr[...,None],arr.shape+(3,)).copy()

Here's how the stacking works for the two cases, shown with their shape information for a sample case -

# Create a sample input array of shape (4,5)
In [55]: arr = np.random.rand(4,5)

# Stack along first axis
In [56]: np.broadcast_to(arr,(3,)+arr.shape).shape
Out[56]: (3, 4, 5)

# Stack along third axis
In [57]: np.broadcast_to(arr[...,None],arr.shape+(3,)).shape
Out[57]: (4, 5, 3)

Same solution(s) would work to extend a n-dim input to n+1-dim view output along the first and last axes. Let's explore some higher dim cases -

3D input case :

In [58]: arr = np.random.rand(4,5,6)

# Stack along first axis
In [59]: np.broadcast_to(arr,(3,)+arr.shape).shape
Out[59]: (3, 4, 5, 6)

# Stack along last axis
In [60]: np.broadcast_to(arr[...,None],arr.shape+(3,)).shape
Out[60]: (4, 5, 6, 3)

4D input case :

In [61]: arr = np.random.rand(4,5,6,7)

# Stack along first axis
In [62]: np.broadcast_to(arr,(3,)+arr.shape).shape
Out[62]: (3, 4, 5, 6, 7)

# Stack along last axis
In [63]: np.broadcast_to(arr[...,None],arr.shape+(3,)).shape
Out[63]: (4, 5, 6, 7, 3)

and so on.

Timings

Let's use a large sample 2D case and get the timings and verify output being a view.

# Sample input array
In [19]: arr = np.random.rand(1000,1000)

Let's prove that the proposed solution is a view indeed. We will use stacking along first axis (results would be very similar for stacking along the third axis) -

In [22]: np.shares_memory(arr, np.broadcast_to(arr,(3,)+arr.shape))
Out[22]: True

Let's get the timings to show that it's virtually free -

In [20]: %timeit np.broadcast_to(arr,(3,)+arr.shape)
100000 loops, best of 3: 3.56 µs per loop

In [21]: %timeit np.broadcast_to(arr,(3000,)+arr.shape)
100000 loops, best of 3: 3.51 µs per loop

Being a view, increasing N from 3 to 3000 changed nothing on timings and both are negligible on timing units. Hence, efficient both on memory and performance!

This can now also be achived using np.tile as follows:

import numpy as np

a = np.array([[1,2],[1,2]])
b = np.tile(a,(3, 1,1))

b.shape
(3,2,2)

b
array([[[1, 2],
        [1, 2]],

       [[1, 2],
        [1, 2]],

       [[1, 2],
        [1, 2]]])

A=np.array([[1,2],[3,4]])
B=np.asarray([A]*N)

Edit @Mr.F, to preserve dimension order:

B=B.T

Here's a broadcasting example that does exactly what was requested.

a = np.array([[1, 2], [1, 2]])
a=a[:,:,None]
b=np.array([1]*5)[None,None,:]

Then b*a is the desired result and (b*a)[:,:,0] produces array([[1, 2],[1, 2]]), which is the original a, as does (b*a)[:,:,1], etc.