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python如何用零填充numpy數組

[英]python how to pad numpy array with zeros

我想知道如何使用帶有 numpy 版本 1.5.0 的 python 2.6.6 用零填充 2D numpy 數組。 但這些都是我的局限。 因此我不能使用np.pad 例如,我想用零填充a ,使其形狀與b匹配。 我想這樣做的原因是我可以這樣做:

b-a

這樣

>>> a
array([[ 1.,  1.,  1.,  1.,  1.],
       [ 1.,  1.,  1.,  1.,  1.],
       [ 1.,  1.,  1.,  1.,  1.]])
>>> b
array([[ 3.,  3.,  3.,  3.,  3.,  3.],
       [ 3.,  3.,  3.,  3.,  3.,  3.],
       [ 3.,  3.,  3.,  3.,  3.,  3.],
       [ 3.,  3.,  3.,  3.,  3.,  3.]])
>>> c
array([[1, 1, 1, 1, 1, 0],
       [1, 1, 1, 1, 1, 0],
       [1, 1, 1, 1, 1, 0],
       [0, 0, 0, 0, 0, 0]])

我能想到的唯一方法是追加,但這看起來很丑陋。 是否有可能使用b.shape的更清潔的解決方案?

編輯,謝謝 MSeiferts 的回答。 我不得不清理一下,這就是我得到的:

def pad(array, reference_shape, offsets):
    """
    array: Array to be padded
    reference_shape: tuple of size of ndarray to create
    offsets: list of offsets (number of elements must be equal to the dimension of the array)
    will throw a ValueError if offsets is too big and the reference_shape cannot handle the offsets
    """

    # Create an array of zeros with the reference shape
    result = np.zeros(reference_shape)
    # Create a list of slices from offset to offset + shape in each dimension
    insertHere = [slice(offsets[dim], offsets[dim] + array.shape[dim]) for dim in range(array.ndim)]
    # Insert the array in the result at the specified offsets
    result[insertHere] = array
    return result

NumPy 1.7.0(添加numpy.pad時)現在已經很老了(它於 2013 年發布)所以即使問題要求不使用該功能的方法,我認為了解如何使用該功能可能會很有用numpy.pad

其實很簡單:

>>> import numpy as np
>>> a = np.array([[ 1.,  1.,  1.,  1.,  1.],
...               [ 1.,  1.,  1.,  1.,  1.],
...               [ 1.,  1.,  1.,  1.,  1.]])
>>> np.pad(a, [(0, 1), (0, 1)], mode='constant')
array([[ 1.,  1.,  1.,  1.,  1.,  0.],
       [ 1.,  1.,  1.,  1.,  1.,  0.],
       [ 1.,  1.,  1.,  1.,  1.,  0.],
       [ 0.,  0.,  0.,  0.,  0.,  0.]])

在這種情況下,我使用0mode='constant'的默認值。 但它也可以通過顯式傳遞來指定:

>>> np.pad(a, [(0, 1), (0, 1)], mode='constant', constant_values=0)
array([[ 1.,  1.,  1.,  1.,  1.,  0.],
       [ 1.,  1.,  1.,  1.,  1.,  0.],
       [ 1.,  1.,  1.,  1.,  1.,  0.],
       [ 0.,  0.,  0.,  0.,  0.,  0.]])

以防第二個參數( [(0, 1), (0, 1)] )看起來令人困惑:每個列表項(在本例中為元組)對應一個維度,其中的項表示(第一個元素)之前之后的填充(第二個元素)。 所以:

[(0, 1), (0, 1)]
         ^^^^^^------ padding for second dimension
 ^^^^^^-------------- padding for first dimension

  ^------------------ no padding at the beginning of the first axis
     ^--------------- pad with one "value" at the end of the first axis.

在這種情況下,第一個軸和第二個軸的填充是相同的,因此也可以只傳入 2 元組:

>>> np.pad(a, (0, 1), mode='constant')
array([[ 1.,  1.,  1.,  1.,  1.,  0.],
       [ 1.,  1.,  1.,  1.,  1.,  0.],
       [ 1.,  1.,  1.,  1.,  1.,  0.],
       [ 0.,  0.,  0.,  0.,  0.,  0.]])

如果之前和之后的填充相同,甚至可以省略元組(但在這種情況下不適用):

>>> np.pad(a, 1, mode='constant')
array([[ 0.,  0.,  0.,  0.,  0.,  0.,  0.],
       [ 0.,  1.,  1.,  1.,  1.,  1.,  0.],
       [ 0.,  1.,  1.,  1.,  1.,  1.,  0.],
       [ 0.,  1.,  1.,  1.,  1.,  1.,  0.],
       [ 0.,  0.,  0.,  0.,  0.,  0.,  0.]])

或者,如果軸之前和之后的填充相同但不同,您也可以省略內部元組中的第二個參數:

>>> np.pad(a, [(1, ), (2, )], mode='constant')
array([[ 0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.],
       [ 0.,  0.,  1.,  1.,  1.,  1.,  1.,  0.,  0.],
       [ 0.,  0.,  1.,  1.,  1.,  1.,  1.,  0.,  0.],
       [ 0.,  0.,  1.,  1.,  1.,  1.,  1.,  0.,  0.],
       [ 0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.]])

但是我傾向於總是使用顯式的,因為它很容易出錯(當 NumPys 的期望與你的意圖不同時):

>>> np.pad(a, [1, 2], mode='constant')
array([[ 0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.],
       [ 0.,  1.,  1.,  1.,  1.,  1.,  0.,  0.],
       [ 0.,  1.,  1.,  1.,  1.,  1.,  0.,  0.],
       [ 0.,  1.,  1.,  1.,  1.,  1.,  0.,  0.],
       [ 0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.],
       [ 0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.]])

這里 NumPy 認為你想在每個軸之前用 1 個元素和在每個軸之后用 2 個元素填充所有軸! 即使您打算在軸 1 中填充 1 個元素,在軸 2 中填充 2 個元素。

我使用元組列表進行填充,注意這只是“我的約定”,您也可以使用列表列表或元組元組,甚至數組元組。 NumPy 只檢查參數的長度(或者如果它沒有長度)和每個項目的長度(或者如果它有長度)!

非常簡單,您可以使用參考形狀創建一個包含零的數組:

result = np.zeros(b.shape)
# actually you can also use result = np.zeros_like(b) 
# but that also copies the dtype not only the shape

然后在需要的地方插入數組:

result[:a.shape[0],:a.shape[1]] = a

瞧,您已經填充了它:

print(result)
array([[ 1.,  1.,  1.,  1.,  1.,  0.],
       [ 1.,  1.,  1.,  1.,  1.,  0.],
       [ 1.,  1.,  1.,  1.,  1.,  0.],
       [ 0.,  0.,  0.,  0.,  0.,  0.]])

如果您定義左上角元素應插入的位置,也可以使其更通用

result = np.zeros_like(b)
x_offset = 1  # 0 would be what you wanted
y_offset = 1  # 0 in your case
result[x_offset:a.shape[0]+x_offset,y_offset:a.shape[1]+y_offset] = a
result

array([[ 0.,  0.,  0.,  0.,  0.,  0.],
       [ 0.,  1.,  1.,  1.,  1.,  1.],
       [ 0.,  1.,  1.,  1.,  1.,  1.],
       [ 0.,  1.,  1.,  1.,  1.,  1.]])

但是請注意,您的偏移量不要超過允許的值。 例如,對於x_offset = 2 ,這將失敗。


如果您有任意數量的維度,您可以定義一個切片列表來插入原始數組。 我發現玩一點很有趣,並創建了一個填充函數,只要數組和引用具有相同的維數並且偏移量不是太大,它就可以填充(帶有偏移量)任意形狀的數組。

def pad(array, reference, offsets):
    """
    array: Array to be padded
    reference: Reference array with the desired shape
    offsets: list of offsets (number of elements must be equal to the dimension of the array)
    """
    # Create an array of zeros with the reference shape
    result = np.zeros(reference.shape)
    # Create a list of slices from offset to offset + shape in each dimension
    insertHere = [slice(offset[dim], offset[dim] + array.shape[dim]) for dim in range(a.ndim)]
    # Insert the array in the result at the specified offsets
    result[insertHere] = a
    return result

還有一些測試用例:

import numpy as np

# 1 Dimension
a = np.ones(2)
b = np.ones(5)
offset = [3]
pad(a, b, offset)

# 3 Dimensions

a = np.ones((3,3,3))
b = np.ones((5,4,3))
offset = [1,0,0]
pad(a, b, offset)

我知道您的主要問題是您需要計算d=ba但您的數組有不同的大小。 不需要中間填充的c

您可以在不填充的情況下解決此問題:

import numpy as np

a = np.array([[ 1.,  1.,  1.,  1.,  1.],
              [ 1.,  1.,  1.,  1.,  1.],
              [ 1.,  1.,  1.,  1.,  1.]])

b = np.array([[ 3.,  3.,  3.,  3.,  3.,  3.],
              [ 3.,  3.,  3.,  3.,  3.,  3.],
              [ 3.,  3.,  3.,  3.,  3.,  3.],
              [ 3.,  3.,  3.,  3.,  3.,  3.]])

d = b.copy()
d[:a.shape[0],:a.shape[1]] -=  a

print d

輸出:

[[ 2.  2.  2.  2.  2.  3.]
 [ 2.  2.  2.  2.  2.  3.]
 [ 2.  2.  2.  2.  2.  3.]
 [ 3.  3.  3.  3.  3.  3.]]

如果您需要將 1 的柵欄添加到數組中:

>>> mat = np.zeros((4,4), np.int32)
>>> mat
array([[0, 0, 0, 0],
       [0, 0, 0, 0],
       [0, 0, 0, 0],
       [0, 0, 0, 0]])
>>> mat[0,:] = mat[:,0] = mat[:,-1] =  mat[-1,:] = 1
>>> mat
array([[1, 1, 1, 1],
       [1, 0, 0, 1],
       [1, 0, 0, 1],
       [1, 1, 1, 1]])

我知道我對此有點晚了,但是如果您想執行相對填充(也稱為邊緣填充),那么您可以通過以下方式實現它。 請注意,賦值的第一個實例會導致零填充,因此您可以將其用於零填充和相對填充(這是您將原始數組的邊緣值復制到填充數組的位置)。

def replicate_padding(arr):
    """Perform replicate padding on a numpy array."""
    new_pad_shape = tuple(np.array(arr.shape) + 2) # 2 indicates the width + height to change, a (512, 512) image --> (514, 514) padded image.
    padded_array = np.zeros(new_pad_shape) #create an array of zeros with new dimensions
    
    # perform replication
    padded_array[1:-1,1:-1] = arr        # result will be zero-pad
    padded_array[0,1:-1] = arr[0]        # perform edge pad for top row
    padded_array[-1, 1:-1] = arr[-1]     # edge pad for bottom row
    padded_array.T[0, 1:-1] = arr.T[0]   # edge pad for first column
    padded_array.T[-1, 1:-1] = arr.T[-1] # edge pad for last column
    
    #at this point, all values except for the 4 corners should have been replicated
    padded_array[0][0] = arr[0][0]     # top left corner
    padded_array[-1][0] = arr[-1][0]   # bottom left corner
    padded_array[0][-1] = arr[0][-1]   # top right corner 
    padded_array[-1][-1] = arr[-1][-1] # bottom right corner

    return padded_array

復雜性分析:

對此的最佳解決方案是 numpy 的 pad 方法。 在平均運行 5 次后,具有相對填充的 np.pad 僅比上面定義的函數好8% 這表明這是相對和零填充填充的最佳方法。


#My method, replicate_padding
start = time.time()
padded = replicate_padding(input_image)
end = time.time()
delta0 = end - start

#np.pad with edge padding
start = time.time()
padded = np.pad(input_image, 1, mode='edge')
end = time.time()
delta = end - start


print(delta0) # np Output: 0.0008790493011474609 
print(delta)  # My Output: 0.0008130073547363281
print(100*((delta0-delta)/delta)) # Percent difference: 8.12316715542522%

Tensorflow 還實現了調整圖像大小/填充圖像的功能tf.image.pad tf.pad

padded_image = tf.image.pad_to_bounding_box(image, top_padding, left_padding, target_height, target_width)

padded_image = tf.pad(image, paddings, "CONSTANT")

這些功能就像 tensorflow 的其他輸入管道功能一樣工作,並且對於機器學習應用程序會更好地工作。

TL;博士

def pad_n_cols_left_of_2d_matrix(arr, n):
    """Adds n columns of zeros to left of 2D numpy array matrix.

    :param arr: A two dimensional numpy array that is padded.
    :param n: the number of columns that are added to the left of the matrix.
    """
    padded_array = np.zeros((arr.shape[0], arr.shape[1] + n))
    padded_array[:, n:] = arr
    return padded_array


def pad_n_cols_right_of_2d_matrix(arr, n):
    """Adds n columns of zeros to right of 2D numpy array matrix.

    :param arr: A two dimensional numpy array that is padded.
    :param n: the number of columns that are added to the right of the matrix.
    """
    padded_array = np.zeros((arr.shape[0], arr.shape[1] + n))
    padded_array[:, : arr.shape[1]] = arr
    return padded_array


def pad_n_rows_above_2d_matrix(arr, n):
    """Adds n rows of zeros above 2D numpy array matrix.

    :param arr: A two dimensional numpy array that is padded.
    :param n: the number of rows that are added above the matrix.
    """
    padded_array = np.zeros((arr.shape[0] + n, arr.shape[1]))
    padded_array[n:, :] = arr
    return padded_array


def pad_n_rows_below_2d_matrix(arr, n):
    """Adds n rows of zeros below 2D numpy array matrix.

    :param arr: A two dimensional numpy array that is padded.
    :param n: the number of rows that are added below the matrix.
    """
    padded_array = np.zeros((arr.shape[0] + n, arr.shape[1]))
    padded_array[: arr.shape[0], :] = arr
    return padded_array

輸出

Original array:
[[0.  0.5 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.5 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.3 0.7 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.3 0.7 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.3 0.7 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.3 0.7 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.3 0.7 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.3 0.7 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.3 0.7 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.3 0.7 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.2 0.5 0.8 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.3 0.7 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.2 0.5 0.8 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.  0.3 0.7 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.  0.2 0.5 0.8 1.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.  0.2 0.5 0.8 1.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.  0.  0.2 0.5 0.8 1.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.  0.  0.2 0.4 0.6 0.8 1.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.  0.  0.  0.2 0.5 0.8 1.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.  0.  0.  0.2 0.4 0.6 0.8 1.  0.  0.  0.  0.  0.  0. ]]
Pad left:
[[0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.5 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.5 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.3 0.7 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.3 0.7 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.3 0.7 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.3 0.7 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.3 0.7 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.3 0.7 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.3 0.7 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.3 0.7 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.2 0.5 0.8 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.3 0.7 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.2 0.5 0.8 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.3 0.7 1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.2 0.5 0.8 1.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
 [0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.2 0.5 0.8 1.  0.  0.  0.  0.  0.  0.  0.  0.  0. ]
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動機

我來這里是為了尋找如何填充數組並找到很多文本,盡管我的目標很簡單,比如問題很簡單:用n行或列填充二維數組。 我認為解釋更好,因為它們有助於建立理解。 但是,只是為了讓某些人願意節省一些時間,這里有一個可復制粘貼的功能。

代碼說明

每個函數將n行或列添加到傳入數組。 該代碼假定傳入數組是一個 2D numpy 數組。 填充的方向取決於函數定義/名稱。 有關更詳細的解釋,我想請讀者參考 MSeifert 的答案。

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