Numpy 简明教程

NumPy - Arithmetic Operations

用于执行加法、减法、乘法和除法等算术运算的输入数组必须具有相同的形状，或者应遵守数组广播规则。

Input arrays for performing arithmetic operations such as add(), subtract(), multiply(), and divide() must be either of the same shape or should conform to array broadcasting rules.

Example

import numpy as np
a = np.arange(9, dtype = np.float_).reshape(3,3)

print 'First array:'
print a
print '\n'

print 'Second array:'
b = np.array([10,10,10])
print b
print '\n'

print 'Add the two arrays:'
print np.add(a,b)
print '\n'

print 'Subtract the two arrays:'
print np.subtract(a,b)
print '\n'

print 'Multiply the two arrays:'
print np.multiply(a,b)
print '\n'

print 'Divide the two arrays:'
print np.divide(a,b)

它将生成如下输出：

It will produce the following output −

First array:
[[ 0. 1. 2.]
 [ 3. 4. 5.]
 [ 6. 7. 8.]]

Second array:
[10 10 10]

Add the two arrays:
[[ 10. 11. 12.]
 [ 13. 14. 15.]
 [ 16. 17. 18.]]

Subtract the two arrays:
[[-10. -9. -8.]
 [ -7. -6. -5.]
 [ -4. -3. -2.]]

Multiply the two arrays:
[[ 0. 10. 20.]
 [ 30. 40. 50.]
 [ 60. 70. 80.]]

Divide the two arrays:
[[ 0. 0.1 0.2]
 [ 0.3 0.4 0.5]
 [ 0.6 0.7 0.8]]

下面我们将讨论 NumPy 中提供的一些其他重要算术函数。

Let us now discuss some of the other important arithmetic functions available in NumPy.

numpy.reciprocal()

该函数逐个元素返回参数的倒数。对于绝对值大于 1 的元素，由于 Python 处理整数除法的方式，结果始终为 0。对于整数 0，则会发出溢出警告。

This function returns the reciprocal of argument, element-wise. For elements with absolute values larger than 1, the result is always 0 because of the way in which Python handles integer division. For integer 0, an overflow warning is issued.

Example

import numpy as np
a = np.array([0.25, 1.33, 1, 0, 100])

print 'Our array is:'
print a
print '\n'

print 'After applying reciprocal function:'
print np.reciprocal(a)
print '\n'

b = np.array([100], dtype = int)
print 'The second array is:'
print b
print '\n'

print 'After applying reciprocal function:'
print np.reciprocal(b)

它将生成如下输出：

It will produce the following output −

Our array is:
[   0.25    1.33    1.      0.    100.  ]

After applying reciprocal function:
main.py:9: RuntimeWarning: divide by zero encountered in reciprocal
  print np.reciprocal(a)
[ 4.         0.7518797  1.               inf  0.01     ]

The second array is:
[100]

After applying reciprocal function:
[0]

numpy.power()

此函数将第一个输入数组中的元素视为基，并返回它提高到第二个输入数组中相应元素的幂次。

This function treats elements in the first input array as base and returns it raised to the power of the corresponding element in the second input array.

import numpy as np
a = np.array([10,100,1000])

print 'Our array is:'
print a
print '\n'

print 'Applying power function:'
print np.power(a,2)
print '\n'

print 'Second array:'
b = np.array([1,2,3])
print b
print '\n'

print 'Applying power function again:'
print np.power(a,b)

它将生成如下输出：

It will produce the following output −

Our array is:
[  10  100 1000]

Applying power function:
[    100   10000 1000000]

Second array:
[1 2 3]

Applying power function again:
[        10      10000 1000000000]

numpy.mod()

该函数返回输入数组中对应元素除法的余数。 numpy.remainder() 函数也会产生相同的结果。

This function returns the remainder of division of the corresponding elements in the input array. The function numpy.remainder() also produces the same result.

import numpy as np
a = np.array([10,20,30])
b = np.array([3,5,7])

print 'First array:'
print a
print '\n'

print 'Second array:'
print b
print '\n'

print 'Applying mod() function:'
print np.mod(a,b)
print '\n'

print 'Applying remainder() function:'
print np.remainder(a,b)

它将生成如下输出：

It will produce the following output −

First array:
[10 20 30]

Second array:
[3 5 7]

Applying mod() function:
[1 0 2]

Applying remainder() function:
[1 0 2]

以下函数用于对具有复数的数组执行运算。

The following functions are used to perform operations on array with complex numbers.

  1. numpy.real() − returns the real part of the complex data type argument.

  2. numpy.imag() − returns the imaginary part of the complex data type argument.

  3. numpy.conj() − returns the complex conjugate, which is obtained by changing the sign of the imaginary part.

  4. numpy.angle() − returns the angle of the complex argument. The function has degree parameter. If true, the angle in the degree is returned, otherwise the angle is in radians.

import numpy as np
a = np.array([-5.6j, 0.2j, 11. , 1+1j])

print 'Our array is:'
print a
print '\n'

print 'Applying real() function:'
print np.real(a)
print '\n'

print 'Applying imag() function:'
print np.imag(a)
print '\n'

print 'Applying conj() function:'
print np.conj(a)
print '\n'

print 'Applying angle() function:'
print np.angle(a)
print '\n'

print 'Applying angle() function again (result in degrees)'
print np.angle(a, deg = True)

它将生成如下输出：

It will produce the following output −

Our array is:
[ 0.-5.6j 0.+0.2j 11.+0.j 1.+1.j ]

Applying real() function:
[ 0. 0. 11. 1.]

Applying imag() function:
[-5.6 0.2 0. 1. ]

Applying conj() function:
[ 0.+5.6j 0.-0.2j 11.-0.j 1.-1.j ]

Applying angle() function:
[-1.57079633 1.57079633 0. 0.78539816]

Applying angle() function again (result in degrees)
[-90. 90. 0. 45.]