import numpy as np

Fancy Index

x = np.array(list('ABCDEFG')) x # array(['A', 'B', 'C', 'D', 'E', 'F', 'G'], dtype='<U1') x[1] # 'B' x[1:3] # array(['B', 'C'], dtype='<U1') x[1:5] # array(['B', 'C', 'D', 'E'], dtype='<U1') x[1:5:2] # 等步长 array(['B', 'D'], dtype='<U1') [x[1], x[2], x[4]] # ['B', 'C', 'E'] ind = [1, 2, 4] x[ind] # array(['B', 'C', 'E'], dtype='<U1') ind = np.arange(1, 5).reshape(2, -1) ind # array([[1, 2], [3, 4]]) x[ind] # array([['B', 'C'], ['D', 'E']], dtype='<U1') X = np.arange(16).reshape(4, -1) X # array([[ 0, 1, 2, 3], [ 4, 5, 6, 7], [ 8, 9, 10, 11], [12, 13, 14, 15]]) row = [0, 1, 3] col = [1, 2, 3] X[row, col] # array([ 1, 6, 15]) X[1:3, col] # array([[ 5, 6, 7], [ 9, 10, 11]]) col = [True, True, False, False] X[1:3, col] # array([[4, 5], [8, 9]])

 比较运算

x = np.arange(10) x # array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]) x < 5 # array([ True, True, True, True, True, False, False, False, False, False]) x[x<5] # array([0, 1, 2, 3, 4]) x >= 5 # array([False, False, False, False, False, True, True, True, True, True]) x == 5 # array([False, False, False, False, False, True, False, False, False, False]) x != 5 # array([ True, True, True, True, True, False, True, True, True, True]) x - 1 != x // 2 # array([ True, False, False, True, True, True, True, True, True, True]) x + 2 == x // 2 + 3 # array([ True, False, False, True, True, True, True, True, True, True]) x[x + 2 == x // 2 + 3] # array([1, 2]) X < 5 # array([[ True, True, True, True], [ True, False, False, False], [False, False, False, False], [False, False, False, False]]) x = x + 2 # 执行2次 x # array([ 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]) np.sum(x < 8) # 4 np.count_nonzero(x < 8) # 4 np.any(x < 4) # False np.any(x <= 4) # True np.all(x > 4) # False np.all(x >= 4) # True np.sum(X % 2 == 0) # 8 np.sum(X % 2 == 0, axis=0) # array([4, 0, 4, 0]) np.sum(X % 2 == 0, axis=1) # array([2, 2, 2, 2]) np.sum((X > 2) & (X < 10)) # 7 np.sum((X == 2) | (X == 10)) # 2 np.sum(~(X == 2)) # 15 X[(X > 2) & (X < 10)] # array([3, 4, 5, 6, 7, 8, 9]) X[:, -1] # array([ 3, 7, 11, 15])，最后1列 X[:, -2] # array([ 2, 6, 10, 14])，倒数第2列 X[:, -1] % 3 == 0 # array([ True, False, False, True]) X[X[:, -1] % 3 == 0] # array([[ 0, 1, 2, 3], [12, 13, 14, 15]]) X[X[:, -1] % 3 == 0, :] # array([[ 0, 1, 2, 3], [12, 13, 14, 15]]) X[X[:, -1] % 3 == 0, -1] # array([ 3, 15]) X[X[:, -1] % 3 == 0, 1:3] # array([[ 1, 2], [13, 14]])

注：代码来自《Python全栈工程师特训班》课程