Numpy Array -Stack

In this article, we will see how to join 2 Numpy arrays using built-in funcitons.

Numpy – Joining two numpy arrays

• stack — Joins arrays with given axis element by element
• hstack — Extends horizontally
• vstack — Extends vertically

Stack — Joins arrays with given axis element by element

• Both input arrays should be in same dimension/shape
• Axis parameter in Stack works as dimension here instead of horizontal/vertical manner.
• If Axis is 0, then it will join by first dimension
• If Axis is 1, the it will join by second dimension
• The maximum dimension that we can mention is dimension of input arrays (say n) + 1.
• If axis is given above n + 1, then “out of bounds for array of dimension” exception will be thrown

arr1 = np.array([1,5,2,10])
arr2 = np.array([10,50,20,100])
print(arr1)
print(arr2)

Output:

[ 1  5  2 10]
[ 10  50  20 100]

Stack by First Dimension:

[ 1  5  2 10]
[ 10  50  20 100]

Output:

array([[  1,   5,   2,  10],
       [ 10,  50,  20, 100]])

Stack by Second Dimension:

arr3 = np.stack((arr1, arr2), axis = 1)

Output:

array([[  1,  10],
       [  5,  50],
       [  2,  20],
       [ 10, 100]])

HStack — Stacks horizontally

• This function does not work with axis. It extends first array by second array Horizontally
• As it extends horizontally, both the arrays should have same number of rows else Value Error will be returned.

HStack for 1D Arrays:

arr1 = np.array([1,5,2,10])
arr2 = np.array([10,50,20,100])
np.hstack((arr1, arr2))

Output:

array([  1,   5,   2,  10,  10,  50,  20, 100])

HStack for 2D Arrays:

arr1 = np.array([[1,2,3],[4,5,6]])
arr2 = np.array([[10,20,30],[40,50,60]])
np.hstack((arr1, arr2))

Output:

array([[ 1,  2,  3, 10, 20, 30],
       [ 4,  5,  6, 40, 50, 60]])

VStack — Stacks vertically

• This function does not work with axis. It extends first array by second array Vertically

VStack by 2D Arrays:

arr1 = np.array([[1,2,3],[4,5,6]])
arr2 = np.array([[10,20,30],[40,50,60]])
np.vstack((arr1, arr2))

Output:

array([[ 1,  2,  3],
       [ 4,  5,  6],
       [10, 20, 30],
       [40, 50, 60]])

Conclusion:

In this article we experimented 1D and 2D arrays for stacking.

I will be adding another post with more demonstration for 3D arrays as it is little tricky to understand.

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