python之Numpy数组操作
数组操作
更改形状
numpy.ndarray.shape表示数组的维度，返回一个元组，这个元组的长度就是维度的数目，即 ndim 属性(秩)。
可以通过改变shape属性来改变数组的形状。
x = np.array([1, 2, 9, 4, 5, 6, 7, 8])
print(x.shape) # (8,)
x.shape = [2, 4]
print(x)
# [[1 2 9 4]
# [5 6 7 8]]
numpy.ndarray.flat将数组转换为一维的迭代器，可以用for访问数组每一个元素。
x = np.array([[11, 12, 13, 14, 15],
[16, 17, 18, 19, 20],
[21, 22, 23, 24, 25],
[26, 27, 28, 29, 30],
[31, 32, 33, 34, 35]])
y = x.flat
print(y)
#
for i in y:
print(i, end=' ')
# 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
numpy.ndarray.flatten([order='c'])将数组的副本转换为一维数组，并返回。
order：‘C’ – 按行，‘F’ – 按列，‘A’ – 原顺序，‘k’ – 元素在内存中的出现顺序。flatten()函数返回的是拷贝numpy.ravel(a, order='C')Return a contiguous flattened array.
返回的是视图
x = np.array([[11, 12, 13, 14, 15],
[16, 17, 18, 19, 20],
[21, 22, 23, 24, 25],
[26, 27, 28, 29, 30],
[31, 32, 33, 34, 35]])
y = np.ravel(x)
print(y)
# [11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
# 35]
y[3] = 0
print(x)
# [[11 12 13 0 15]
# [16 17 18 19 20]
# [21 22 23 24 25]
# [26 27 28 29 30]
# [31 32 33 34 35]]
numpy.reshape(a, newshape[, order='C'])在不更改数据的情况下为数组赋予新的形状。
reshape()函数当参数newshape = [rows,-1]时，将根据行数自动确定列数
x = np.arange(12)
y = np.reshape(x, [3, 4])
print(y.dtype) # int32
print(y)
# [[ 0 1 2 3]
# [ 4 5 6 7]
# [ 8 9 10 11]]
y = np.reshape(x, [3, -1])
print(y)
# [[ 0 1 2 3]
# [ 4 5 6 7]
# [ 8 9 10 11]]
reshape()函数当参数newshape = -1时，表示将数组降为一维
x = np.random.randint(12, size=[2, 2, 3])
print(x)
# [[[11 9 1]
# [ 1 10 3]]
#
# [[ 0 6 1]
# [ 4 11 3]]]
y = np.reshape(x, -1)
print(y)
# [11 9 1 1 10 3 0 6 1 4 11 3]
数组转置
-numpy.transpose(a, axes=None)Permute the dimensions of an array.
-numpy.ndarray.T Same as self.transpose(),except that self is returned if self.ndim < 2.转置
x = np.random.rand(5, 5) * 10
x = np.around(x, 2)
print(x)
# [[6.74 8.46 6.74 5.45 1.25]
# [3.54 3.49 8.62 1.94 9.92]
# [5.03 7.22 1.6 8.7 0.43]
# [7.5 7.31 5.69 9.67 7.65]
# [1.8 9.52 2.78 5.87 4.14]]
y = x.T
print(y)
# [[6.74 3.54 5.03 7.5 1.8 ]
# [8.46 3.49 7.22 7.31 9.52]
# [6.74 8.62 1.6 5.69 2.78]
# [5.45 1.94 8.7 9.67 5.87]
# [1.25 9.92 0.43 7.65 4.14]]
y = np.transpose(x)
print(y)
# [[6.74 3.54 5.03 7.5 1.8 ]
# [8.46 3.49 7.22 7.31 9.52]
# [6.74 8.62 1.6 5.69 2.78]
# [5.45 1.94 8.7 9.67 5.87]
# [1.25 9.92 0.43 7.65 4.14]]
更改维度
-numpy.newaxis = None None的别名，对索引数组很有用
x = np.array([1, 2, 9, 4, 5, 6, 7, 8])
print(x.shape) # (8,)
print(x) # [1 2 9 4 5 6 7 8]
y = x[np.newaxis, :]
print(y.shape) # (1, 8)
print(y) # [[1 2 9 4 5 6 7 8]]
y = x[:, np.newaxis]
print(y.shape) # (8, 1)
print(y)
# [[1]
# [2]
# [9]
# [4]
# [5]
# [6]
# [7]
# [8]]
numpy.squeeze(a, axis=None)从数组的形状中删除单维度条目，即把shape中为1的维度去掉。- axis用于指定需要删除的维度，但是指定的维度必须为单维度，否则将会报错；
数组组合
将两份数据组合到一起，需要拼接操作。
-numpy.concatenate((a1, a2, ...), axis=0, out=None)
-numpy.stack(arrays, axis=0, out=None)沿着新的轴加入一系列数组（stack为增加维度的拼接）
-numpy.vstack(tup)Stack arrays in sequence vertically (row wise).
-numpy.hstack(tup)Stack arrays in sequence horizontally (column wise)
hstack(),vstack()分别表示水平和竖直的拼接方式。在数据维度等于1时，比较特殊。而当维度大于或等于2时，它们的作用相当于concatenate，用于在已有轴上进行操作。
数组拆分
numpy.split(ary, indices_or_sections, axis=0)
x = np.array([[11, 12, 13, 14],
[16, 17, 18, 19],
[21, 22, 23, 24]])
y = np.split(x, [1, 3])
print(y)
# [array([[11, 12, 13, 14]]), array([[16, 17, 18, 19],
# [21, 22, 23, 24]]), array([], shape=(0, 4), dtype=int32)]
y = np.split(x, [1, 3], axis=1)
print(y)
# [array([[11],
# [16],
# [21]]), array([[12, 13],
# [17, 18],
# [22, 23]]), array([[14],
# [19],
# [24]])]
indices_or_sections这个数可以是整数或数组，整数的话就平均划分，数组的话，数组的每个值就是划分的索引。
numpy.vsplit(ary, indices_or_sections)Split an array into multiple sub-arrays vertically (row-wise)垂直切分是把数组按照高度切分
x = np.array([[11, 12, 13, 14],
[16, 17, 18, 19],
[21, 22, 23, 24]])
y = np.vsplit(x, 3)
print(y)
# [array([[11, 12, 13, 14]]), array([[16, 17, 18, 19]]), array([[21, 22, 23, 24]])]
y = np.vsplit(x, [1, 3])
print(y)
# [array([[11, 12, 13, 14]]), array([[16, 17, 18, 19],
# [21, 22, 23, 24]]), array([], shape=(0, 4), dtype=int32)]
y = np.split(x, [1, 3], axis=0)
print(y)
# [array([[11, 12, 13, 14]]), array([[16, 17, 18, 19],
# [21, 22, 23, 24]]), array([], shape=(0, 4), dtype=int32)]
numpy.hsplit(ary, indices_or_sections)Split an array into multiple sub-arrays horizontally (column-wise).水平切分是把数组按照宽度切分。
x = np.array([[11, 12, 13, 14],
[16, 17, 18, 19],
[21, 22, 23, 24]])
y = np.hsplit(x, 2)
print(y)
# [array([[11, 12],
# [16, 17],
# [21, 22]]), array([[13, 14],
# [18, 19],
# [23, 24]])]
y = np.hsplit(x, [1, 3])
print(y)
# [array([[11],
# [16],
# [21]]), array([[12, 13],
# [17, 18],
# [22, 23]]), array([[14],
# [19],
# [24]])]
数组平铺
numpy.tile(A, reps)Construct an array by repeating A the number of times given by reps.将原矩阵横向、纵向地复制。
A指待输入数组，reps则决定A重复的次数。整个函数用于重复数组A来构建新的数组.
x = np.array([[1, 2], [3, 4]])
print(x)
# [[1 2]
# [3 4]]
y = np.tile(x, (1, 3))
print(y)
# [[1 2 1 2 1 2]
# [3 4 3 4 3 4]]
y = np.tile(x, (3, 1))
print(y)
# [[1 2]
# [3 4]
# [1 2]
# [3 4]
# [1 2]
# [3 4]]
y = np.tile(x, (3, 3))
print(y)
# [[1 2 1 2 1 2]
# [3 4 3 4 3 4]
# [1 2 1 2 1 2]
# [3 4 3 4 3 4]
# [1 2 1 2 1 2]
# [3 4 3 4 3 4]]
numpy.repeat(a, repeats, axis=None)Repeat elements of an array.
axis=0，沿着y轴复制，实际上增加了行数。
axis=1，沿着x轴复制，实际上增加了列数。
repeats，可以为一个数，也可以为一个矩阵。
axis=None时就会flatten当前矩阵，实际上就是变成了一个行向量。
x = np.repeat(3, 4)
print(x) # [3 3 3 3]
x = np.array([[1, 2], [3, 4]])
y = np.repeat(x, 2)
print(y)
# [1 1 2 2 3 3 4 4]
y = np.repeat(x, 2, axis=0)
print(y)
# [[1 2]
# [1 2]
# [3 4]
# [3 4]]
y = np.repeat(x, 2, axis=1)
print(y)
# [[1 1 2 2]
# [3 3 4 4]]
y = np.repeat(x, [2, 3], axis=0)
print(y)
# [[1 2]
# [1 2]
# [3 4]
# [3 4]
# [3 4]]
y = np.repeat(x, [2, 3], axis=1)
print(y)
# [[1 1 2 2 2]
# [3 3 4 4 4]]
数组的话，就是分别重复n次
添加和删除元素
-numpy.unique(ar, return_index=False, return_inverse=False,return_counts=False, axis=None) 对于一维数组或者列表，unique函数去除其中重复的元素，并按元素由大到小返回一个新的无元素重复的元组或者列表
return_index：the indices of the input array that give the unique values
return_inverse：the indices of the unique array that reconstruct the input array
return_counts：the number of times each unique value comes up in the input array*
举个例子：
查找数组的唯一元素：
a=np.array([1,1,2,3,3,4,4])
b=np.unique(a,return_counts=True)
print(b[0][list(b[1]).index(1)])
#2