Python数据分析与机器学习-NumPy_1

import numpy

world_alcohol = numpy.genfromtxt("world_alcohol.txt",delimiter=",")
print(type(world_alcohol))

<class 'numpy.ndarray'>

# The numpy.array() function can take a list or list of lists as input. When we input a list, we get a one-dimensional arrat 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([[5,10,12],[20,25,30],[35,40,45]])
print(vector)
print(matrix)

[ 5 10 15 20]
[[ 5 10 12]
 [20 25 30]
 [35 40 45]]

# We can use the ndarray.shape property to figure out how many elements are in the array
vector = numpy.array([1,2,3,4,5])
print(vector.shape)
# For matrices, the shape property contains a tuple with 2 elements.
matrix = numpy.array([[5,10,15],[20,25,30]])
print(matrix.shape)

(5,)
(2, 3)

# Each value in a Numpy array has to have the same data type
# NumPy will automatically figure out an appropriate data type when reading in data or converting lists to arrays.
# You can check the date type of a NumPy array using the dtype property
numbers = numpy.array([1,2,'3',4.0])
print(numbers)
numbers.dtype

['1' '2' '3' '4.0']





dtype('<U21')

world_alcohol = numpy.genfromtxt("world_alcohol.txt",delimiter=",",dtype="U75", skip_header=1)
print(world_alcohol)

[['1986' 'Western Pacific' 'Viet Nam' 'Wine' '0']
 ['1986' 'Americas' 'Uruguay' 'Other' '0.5']
 ['1985' 'Africa' "Cte d'Ivoire" 'Wine' '1.62']
 ...
 ['1987' 'Africa' 'Malawi' 'Other' '0.75']
 ['1989' 'Americas' 'Bahamas' 'Wine' '1.5']
 ['1985' 'Africa' 'Malawi' 'Spirits' '0.31']]

uruguay_other_1986 = world_alcohol[1,4]
third_country = world_alcohol[2,2]
print(uruguay_other_1986)
print(third_country)

0.5
Cte d'Ivoire

vector = numpy.array([5,10,15,20])
print(vector[0:3])

[ 5 10 15]

matrix = numpy.array([[5,10,15],
                      [20,25,30],
                     [35,40,45]])
print(matrix[:,1])

[10 25 40]

matrix = numpy.array([[5,10,15],
                      [20,25,30],
                     [35,40,45]])
print(matrix[:,0:2])

[[ 5 10]
 [20 25]
 [35 40]]

matrix = numpy.array([[5,10,15],
                      [20,25,30],
                     [35,40,45]])
print(matrix[1:3,0:2])

[[20 25]
 [35 40]]