numpy学习笔记

1. numpy切片和整型数组访问

(1)
a = np.array([[1,2,3,4], [5,6,7,8], [9,10,11,12]])

b = a[1,:] b.shape = (4,), b = [1,2,3,4]
c = a[1:2, :], c.shape = (1,4), c = [[1,2,3,4]]

(2)
数组下标，可以使用数组来表示

a = np.array([[1,2], [3, 4], [5, 6]])

print a[[0, 1, 2], [0, 1, 0]]  # Prints "[1 4 5]"
print np.array([a[0, 0], a[1, 1], a[2, 0]])  # Prints "[1 4 5]"

a = np.array([[1,2,3], [4,5,6], [7,8,9], [10, 11, 12]])

# Create an array of indices
b = np.array([0, 2, 0, 1])

# Select one element from each row of a using the indices in b
print a[np.arange(4), b]  # Prints "[ 1  6  7 11]"

# Mutate one element from each row of a using the indices in b
a[np.arange(4), b] += 10

print a  # prints "array([[11,  2,  3],
         #                [ 4,  5, 16],
         #                [17,  8,  9],
         #                [10, 21, 12]])