Pythonのnumpy.append()

Pythonのnumpyのappend()関数は、2つの配列を結合するために使用されます。この関数は新しい配列を返し、元の配列は変更されません。

NumPyのappend()の構文

関数の構文は次のようになります。

numpy.append(arr, values, axis=None)

• The arr can be an array-like object or a NumPy array. The values are appended to a copy of this array.
• The values are array-like objects and it’s appended to the end of the “arr” elements.
• The axis specifies the axis along which values are appended. If the axis is not provided, both the arrays are flattened.

Pythonのnumpy.append()の例

NumPyのappend()関数のいくつかの例を見てみましょう。

1. 2つの配列を平坦化する

import numpy as np

arr1 = np.array([[1, 2], [3, 4]])
arr2 = np.array([[10, 20], [30, 40]])

# no axis provided, array elements will be flattened
arr_flat = np.append(arr1, arr2)

print(arr_flat)  # [ 1  2  3  4 10 20 30 40]

2. 軸に沿って統合

import numpy as np

arr_merged = np.append([[1, 2], [3, 4]], [[10, 20], [30, 40]], axis=0)
print(f'Merged 2x2 Arrays along Axis-0:\n{arr_merged}')

arr_merged = np.append([[1, 2], [3, 4]], [[10, 20], [30, 40]], axis=1)
print(f'Merged 2x2 Arrays along Axis-1:\n{arr_merged}')

出力：

Merged 2x2 Arrays along Axis-0:
[[ 1  2]
 [ 3  4]
 [10 20]
 [30 40]]
Merged 2x2 Arrays along Axis-1:
[[ 1  2 10 20]
 [ 3  4 30 40]]

• When the 2×2 arrays are merged along the x-axis, the output array size is 4×2.
• When the 2×2 arrays are merged along the y-axis, the output array size is 2×4.

3. 異なる形状の配列の結合

append()関数は、軸を除いた場合に両方の配列の形状が異なる場合にValueErrorを送出します。単純な例を使ってこのシナリオを理解しましょう。

arr3 = np.append([[1, 2]], [[1, 2, 3], [1, 2, 3]])
print(arr3)

arr3 = np.append([[1, 2]], [[1, 2], [3, 4]], axis=0)
print(arr3)

• In the first example, the array elements are flattened. So even if they have completely different size – 1×2 and 2×3, the append() works fine.
• In the second example, the array shapes are 1×2 and 2×2. Since we are appending along the 0-axis, the 0-axis shape can be different. The other shapes should be the same, so this append() will also work fine.

結果：

[1 2 1 2 3 1 2 3]

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

別の例を見てみましょう。ValueErrorが発生する場合を考えましょう。

>>> import numpy as np
>>> 
>>> arr3 = np.append([[1, 2]], [[1, 2, 3]], axis=0)
Traceback (most recent call last):
  File "", line 1, in 
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/numpy/lib/function_base.py", line 4528, in append
    return concatenate((arr, values), axis=axis)
ValueError: all the input array dimensions except for the concatenation axis must match exactly
>>> 

配列の形状は1×2と2×3です。axis-1の形状が異なるため、ValueErrorが発生します。参照：APIドキュメント