2017.7.8 & numpy基础

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1.nump

二阶单位阵

d = np.eye(2)         # Create a 2x2 identity matrixprint(d)              # Prints "[[ 1.  0.]                     #          [ 0.  1.]]"

整型索引会降低矩阵的维度，切片索引却不会

row_r1 = a[1, :]    # Rank 1 view of the second row of arow_r2 = a[1:2, :]  # Rank 2 view of the second row of aprint(row_r1, row_r1.shape)  # Prints "[5 6 7 8] (4,)"print(row_r2, row_r2.shape)  # Prints "[[5 6 7 8]] (1, 4)"

numpy中关于array整型Index的一个点

两个矩阵，确定一个位置

a = np.array([[1,2], [3, 4], [5, 6]])# An example of integer array indexing.# The returned array will have shape (3,) andprint(a[[0, 1, 2], [0, 1, 0]])  # Prints "[1 4 5]"# The above example of integer array indexing is equivalent to this:print(np.array([a[0, 0], a[1, 1], a[2, 0]]))  # Prints "[1 4 5]"

numpy强制类型转换

x = np.array([1, 2], dtype=np.int64)   # Force a particular datatypeprint(x.dtype)                         # Prints "int64"

numpy中矩阵加减乘除开放运算

x = np.array([[1,2],[3,4]], dtype=np.float64)y = np.array([[5,6],[7,8]], dtype=np.float64)print(x + y)print(np.add(x, y))print(x - y)print(np.subtract(x, y))*这的乘积，是矩阵各元素的乘积，不是矩阵相乘，向量和*print(x * y)print(np.multiply(x, y))print(x / y)print(np.divide(x, y))print(np.sqrt(x))

dot用法：既可以用于vectors*vectors(各项之积的和)；用于matrix*vectors；还可用于matrix*matrix

import numpy as npx = np.array([[1,2],[3,4]])y = np.array([[5,6],[7,8]])v = np.array([9,10])w = np.array([11, 12])# Inner product of vectors; both produce 219print(v.dot(w))print(np.dot(v, w))# Matrix / vector product; both produce the rank 1 array [29 67]print(x.dot(v))print(np.dot(x, v))# Matrix / matrix product; both produce the rank 2 array# [[19 22]#  [43 50]]print(x.dot(y))print(np.dot(x, y))

遇到有趣的tile函数

#列方向上重复两次变为【1，2，1，2】a = np.tile([1,2], 2)#行方向重复三次，列方向重复2次，变为：#【1，2，1，2】#【1，2，1，2】#【1，2，1，2】a = np.tile([1,2], (3, 2))

x = np.array([[1,2,3], [4,5,6], [7,8,9], [10, 11, 12]])v = np.array([1, 0, 1])y = x + v  # Add v to each row of x using broadcastingprint(y)  # Prints "[[ 2  2  4]          #          [ 5  5  7]          #          [ 8  8 10]          #          [11 11 13]]"

今天就到这，明天是生日，出去玩耍

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