ValueError: zero-dimensional arrays cannot be concatenated

# ValueError: zero-dimensional arrays cannot be concatenated

The NumPy "ValueError: zero-dimensional arrays cannot be concatenated" occurs when you pass a single one-dimensional array to the numpy.concatenate() method.

To solve the error, wrap the argument in square brackets, when calling concatenate().

Here is an example of how the error occurs.

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import numpy as np

arr = np.array([1, 2, 3])

# ⛔️ ValueError: zero-dimensional arrays cannot be concatenated
new_array = np.concatenate(arr)

The numpy.concatenate() method joins a sequence of arrays along an existing axis.

We only passed a single one-dimensional array to the numpy.concatenate() method which caused the error.

You can solve the error by wrapping the array in square brackets.

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import numpy as np

arr = np.array([1, 2, 3])

new_array = np.concatenate([arr])

print(new_array)  # 👉️ [1 2 3]

Notice that we wrapped the array in square brackets.

Replace this:

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# ⛔️ Error
new_array = np.concatenate(arr)

With this:

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# ✅ Correct
new_array = np.concatenate([arr])

Here is an example of passing an inline list to the np.concatenate() method.

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import numpy as np

new_array = np.concatenate([[1, 2, 3]])

print(new_array)  # 👉️ [1 2 3]

Notice that we have 2 sets of square brackets.

The arrays you want to concatenate have to be placed in a sequence, e.g. a list.

The numpy.concatenate() method joins a sequence of arrays along an existing axis.

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import numpy as np

arr1 = np.array([1, 2, 3])
arr2 = np.array([4, 5, 6])

new_array = np.concatenate([arr1, arr2])

print(new_array)  # 👉️ [1 2 3 4 5 6]

Notice that we placed the arrays in a sequence (a list []) when passing them to numpy.concatenate().

You can also use a tuple () to achieve the same result.

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import numpy as np

arr1 = np.array([1, 2, 3])
arr2 = np.array([4, 5, 6])

new_array = np.concatenate((arr1, arr2))

print(new_array)  # 👉️ [1 2 3 4 5 6]

Here is an example of calling the numpy.concatenate() method with two-dimensional arrays.

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import numpy as np

arr1 = np.array([[1, 2], [3, 4]])
arr2 = np.array([[5, 6]])

new_arr = np.concatenate((arr1, arr2), axis=0)

# [[1 2]
#  [3 4]
#  [5 6]]
print(new_arr)

The example joins the two-dimensional arrays along the 0 axis.

0 is the default value for the axis parameter.

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