numpy.stack() in Python

The numpy.stack() function is used to join multiple arrays by creating a new axis in the output array. This means the resulting array always has one extra dimension compared to the input arrays. To stack arrays, they must have the same shape, and NumPy places them along the axis you specify.

Example: This example stacks two 1D arrays along a new axis to form a 2D array.

[[1 2 3]
 [4 5 6]]

Explanation: np.stack((a, b), axis=0) creates a new 0th axis and places arrays one below another.

Syntax

numpy.stack(arrays, axis=0, out=None)

Parameters:

• arrays: Sequence of input arrays with the same shape.
• axis: Position of the new axis where arrays will be stacked (default: 0).
• out(Optional): output array to store the result.

Examples

Example 1: This example shows how stacking the same 1D arrays along axis 0, 1, and -1 changes the output shape.

[[1 2 3]
 [4 5 6]]
[[1 4]
 [2 5]
 [3 6]]
[[1 4]
 [2 5]
 [3 6]]

Explanation:

• axis=0: a and b become rows.
• axis=1: a and b become columns.
• axis=-1: same as axis 1 because it refers to the last dimension.

Example 2: This example stacks two 2D arrays along axis 0, 1, and 2 to show how the new 3D structure changes.

[[[ 1 2 3]
 [ 4 5 6]]

 [[ 7 8 9]
 [10 11 12]]]
[[[ 1 2 3]
 [ 7 8 9]]

 [[ 4 5 6]
 [10 11 12]]]
[[[ 1 7]
 [ 2 8]
 [ 3 9]]

 [[ 4 10]
 [ 5 11]
 [ 6 12]]]

Explanation:

• axis=0: stacks arrays as two “layers” of a 3D array.
• axis=1: stacks row-wise.
• axis=2: stacks column-wise forming 3D structure.

Example 3: This example stacks two 3D arrays along axis 0, 1, 2, and 3 to demonstrate how stacking works with higher-dimension data.

Output

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

[[ 5 6]
[ 7 8]]]

[[[10 20]
[30 40]]

[[50 60]
[70 80]]]]
[[[[ 1 2]
[ 3 4]]

[[10 20]
[30 40]]]

[[[ 5 6]
[ 7 8]]

[[50 60]
[70 80]]]]
[[[[ 1 2]
[10 20]]

[[ 3 4]
[30 40]]]

[[[ 5 6]
[50 60]]

[[ 7 8]
[70 80]]]]
[[[[ 1 10]
[ 2 20]]

[[ 3 30]
[ 4 40]]]

[[[ 5 50]
[ 6 60]]

[[ 7 70]
[ 8 80]]]]

Explanation:

• axis=0: stacks arrays as two 3D layers.
• axis=1: stacks "planes" together.
• axis=2: stacks each corresponding row.
• axis=3: stacks each corresponding element as a new last-axis pair.