Python Note6 (Numpy)

Install

After downloaded the Numpy into C:\Python27\Scripts.

pip2.7 install [path_of_numpy_whl]

Basic Knowledges

• Import all modules within Numpy into the current namespace.

from numpy import *

• Change the current directory within IDLE development environment.

import os  
os.getcwd()  # obtain the current directory
os.chdir("D:\\test") # change directory  
os.chdir('D:\\test')  # no difference between ' and ''

• axis
  For 2-D array, axis=0 means processing according to the vertical axis, while axis=1 means the horizontal axis.
  For N-D array, 'axis=i' means Numpy will process the array according to the i-th subscript.

• sort, sorted & argsort

• sort
  After sorting process, the original array will be sorted.
  >>> a = [1,2,1,4,3,5] >>> a.sort() >>> a [1, 1, 2, 3, 4, 5]

• sorted(iterable, cmp, key, reverse)
  After sorting process, the original array will not be influenced.
  >>> a = [1,2,1,4,3,5] >>> sorted(a) [1, 1, 2, 3, 4, 5] >>> a [1, 2, 1, 4, 3, 5]

• argsort
  Return index of array according to ascending values.

Advanced Knowledges

• Save a numpy matrix into a txt file.

# np.savetxt(FileName, saveMatdData, format, delimiter, newline)
np.savetxt('s12.txt', data, fmt='%f', delimiter=' ', newline='\n')

• Save a numpy matrix into a csv file.

# np.savetxt(FileName, saveMatdData, delimiter)
np.savetxt('s13.csv', data, delimiter=',')

• Load a csv file into a ndarray.

# np.loadtxt(open(csvFileName,'rb'), delimiter, skiprows)
np.loadtxt(open('s13.csv','rb'), delimiter=',', skiprows=0)

• Load a csv file into a numpy matrix.

np.mat(np.loadtxt(open('s13.csv','rb'), delimiter=',', skiprows=0))

Reference

Python IDLE或shell中切换路径
numpy模块之axis
Python中排序常用到的sort 、sorted和argsort函数