ValueError: all input arrays must have the same number of dimensions

all input arrays must have the same number of dimensions

$ python - <<'PY'
import numpy as np
a = np.array([1,2,3])
b = np.array([[1,2,3]])
np.concatenate([a, b])
PY
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
ValueError: all input arrays must have the same number of dimensions

Why this happens

Functions like concatenate, stack, and hstack expect inputs to have identical dimensionality. Mixing 1D and 2D arrays leads to this error.

Fix

Reshape or use np.atleast_2d, np.ravel, or adjust axes so all inputs match in ndim.

Wrong code

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

Fixed code

import numpy as np
a = np.array([1,2,3])
b = np.array([[4,5,6]])
# make both 2D
combined = np.concatenate([a[np.newaxis, :], b])

# or flatten both
combined_flat = np.concatenate([a, b.ravel()])