Staking and Splitting Arrays

Combining arrays

np.concatenate

Concatenate along an axis.

concat

import numpy as np
 
a = np.array([1, 2, 3])
b = np.array([4, 5, 6])
 
print(np.concatenate([a, b]))

For 2D arrays:

concat2d

import numpy as np
 
a = np.array([[1, 2], [3, 4]])
b = np.array([[5, 6], [7, 8]])
 
print(np.concatenate([a, b], axis=0))  # stack rows
print(np.concatenate([a, b], axis=1))  # stack cols

np.vstack and np.hstack

vstack-hstack

import numpy as np
 
a = np.array([1, 2, 3])
b = np.array([4, 5, 6])
 
print(np.vstack([a, b]))
print(np.hstack([a, b]))

np.stack (adds a new axis)

Unlike concatenate, stack creates a brand-new axis instead of joining along an existing one.

stack

import numpy as np
 
a = np.array([1, 2, 3])
b = np.array([4, 5, 6])
 
print(np.stack([a, b], axis=0).shape)  # (2, 3)
print(np.stack([a, b], axis=1).shape)  # (3, 2)

sketch Stacking two rows, then splitting back apart p5.js Pause Restart

vstack joins two 1D arrays into rows of a matrix; split cuts a matrix back into the original pieces.

Splitting arrays

np.split

split

import numpy as np
 
arr = np.arange(10)
a, b = np.split(arr, [6])
print(a)
print(b)

np.hsplit and np.vsplit

hsplit-vsplit

import numpy as np
 
mat = np.arange(16).reshape(4, 4)
left, right = np.hsplit(mat, 2)
print(left)
print(right)
 
top, bottom = np.vsplit(mat, 2)
print(top)
print(bottom)

Caution

Split points must match the array dimensions. For example, you can’t hsplit a 4-column matrix into 3 equal parts.

diagram Stack vs concatenate vs split mermaid

  flowchart LR
  A["Two arrays,
same shape"] --> B["np.vstack
join as rows"]
  A --> C["np.hstack
join as columns"]
  A --> D["np.stack
join on a NEW axis"]
  E["One matrix"] --> F["np.split / hsplit / vsplit
-> pieces back apart"]

Stacking functions join arrays along rows, columns, or a brand-new axis; split undoes that join.

Next

Continue to: NumPy Random Module for generating reproducible random numbers and sampling.

🧪 Try It Yourself

Exercise 1 – Stack Two Rows

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Exercise 2 – Split at an Index

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Exercise 3 – New Axis with stack

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