Numpy - Combining Functions

Numpy Combine

Combining Arrays and Matrics

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Concatenate

np.concatenate((a1, a2, ...), axis=0, out=None, dtype=None, casting="same_kind")

Join a sequence of arrays along an exising axis.

concatenate_1 = array.concatenate((a1, a2), axis=0)

based arrays
Array a1:
[[1 2 3]
[4 5 6]]
Array a2:
[[7 8 9]]

Concatenate along axis zero:
Array:
[[1 2 3]
[4 5 6]
[7 8 9]]

concatenate_2 = array.concatenate((a1, a2), axis=1)

based arrays
Array a1:
[[1 2]
[3 4]]
Array a2:
[[5] [6]]

Concatenate along axis one:
Array:
[[1 2 5]
[3 4 6]]

Horizontal Stacking

np.hstack(tup)

Stask arrays in sequence horizontally (column-wise)

a1 = np.array([[1, 1, 1], [2, 2, 2]])
a2 = np.array([[3, 3, 3, 3], [4, 4, 4, 4]])

array_info(a1)
array_info(a2)

a_hstacked = np.hstack((a1, a2))

print('Horizontal stack:')

Horizontal stack:
Array:
[[1 1 1 3 3 3 3]
[2 2 2 4 4 4 4]]

Vertical Stacking

np.vstack(tup)

Stack arrays in sequence vertically (row-wise)

a1 = np.array([1, 2, 3])
a2 = np.array([4, 5, 6])

array_info(a1)
array_info(a2)

a_vstacked = np.vstack((a1, a2))

print('Vertical stack:')

Vertical stack:
[[1 2 3]
[4 5 6]]\

# Create three separate 1x4x4 arrays.
h1 = np.full((1, 4, 4), 1, dtype='float32')
h2 = np.full((1, 4, 4), 2, dtype='float32')
h3 = np.full((1, 4, 4), 3, dtype='float32')

h = np.vstack((h1, h2, h3))

Array:
[[[1. 1. 1. 1.]
[1. 1. 1. 1.]
[1. 1. 1. 1.]
[1. 1. 1. 1.]]\

[[2. 2. 2. 2.]
[2. 2. 2. 2.]
[2. 2. 2. 2.]
[2. 2. 2. 2.]]\

[[3. 3. 3. 3.]
[3. 3. 3. 3.]
[3. 3. 3. 3.]
[3. 3. 3. 3.]]]

Comparison: np.concatenate() vs np.stack()

Feature	np.concatenate()	np.stack()
Purpose	Joins arrays along an existing axis	Joins arrays by adding a new axis
Axis Behavior	Must use existing axis (e.g., 0 or 1)	Adds a new dimension (axis) before stacking
Changes Shape?	❌ No new axis added	✅ New axis added (dim increases by 1)
Input Requirement	Arrays must have same shape except along concat axis	Arrays must have exact same shape
Typical Use	Extend rows or columns	Create a stack of identical shapes (like a batch)
Example	(2, 3) + (1, 3) → (3, 3)	Two (2,) → (2, 2)