Tang机器学习课程笔记之五（Numpy库）

1:reading the data
the function of genfromtxt:
a:we read the data from the paper file
b:in order to need not to specify the path,we put the data and code in the same paper file

the common form of this function is:
genfromtxt(fname,dtype<type'float'>,delimiter=None,usecols=None)
#fname:the file name
#dtype<type'float'>:dtype optional.Data type of the resulting array.
#delimiter:str,int,or sequence,optional.The string used to separate values.
#usecols:sequence,optional.Which columns to read,with 0 being the first.For example,usecols=(1,4,5)will extract the 2nd,5th and 6th columns. 

the living example:
import numpy
data = genfromtxt('123.csv',dtype=str,delimiter=' ',usecols=(0,1,3,4)) 

the core operation:
numpy.array:building an array structure.
1:When you input an array like a list structure,the numpy will convert the list structure to the array structure.
2:Output the array.

#The numpy.array() function can take a list or list of lists as input. When we input a list, we get a one-dimensional array as a result:
vector = numpy.array([5, 10, 15, 20])
#When we input a list of lists, we get a matrix as a result:
matrix = numpy.array([[5, 10, 15], [20, 25, 30], [35, 40, 45]])
print(vector)
print(matrix)