【Python】numpy安装+numpy.ndarray基本使用

numpy安装：

pip install numpy

numpy.ndarray基本使用：

import numpy as np

np.__version__    # 获取numpy的版本
# numpy.__version__  # name 'numpy' is not defined

nparr = np.array(list(range(10)))
nparr    # array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])

type(nparr)    # numpy.ndarray
nparr.dtype    # 数据类型，WIN和IOS可能不一样

nparr[3]    # 3
nparr[3] = 33
nparr    # array([ 0,  1,  2, 33,  4,  5,  6,  7,  8,  9])
# nparr[5] = 'hello'    ValueError: invalid literal for int() with base 10: 'hello'

nparr[5] = 55.8
nparr    # array([ 0,  1,  2, 33,  4, 55,  6,  7,  8,  9])

nparr2 = np.array([1, 2, 3.0])
nparr2.dtype    # dtype('float64')
nparr2    # array([1., 2., 3.])

nparr3 = np.array([1, 2, 3], dtype=float)
nparr3.dtype    # dtype('float64')

numpy与python性能比较：

import numpy as np

def python_test(n):
    a = [i**2 for i in range(n)]
    b = [i**3 for i in range(n)]
    c = []
    for i in range(n):
        c.append(a[i]+b[i])
    return c
python_test(10)


def numpy_test(n):
    a = np.arange(n) ** 2
    b = np.arange(n) ** 3
    c = a + b
    return c
numpy_test(10)

%time res = python_test(10000000)    # Wall time: 9.37 s

%time res = numpy_test(10000000)    # Wall time: 275 ms

注：代码来自《Python全栈工程师特训班》课程