Joining arrays

Join arrays end to end with concatenate, or stack them as rows or columns.

• np.concatenate([a, b]) joins along an existing axis.
• np.vstack stacks as rows; np.hstack joins side by side.
• The other dimensions must match.

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

[1, 2, 3, 4] [[1, 2], [3, 4]]

Remember: vstack = on top. hstack = side by side.

Common mistake: Stacking arrays whose other dimension differs, like rows of length 3 and 4.