python小技巧总结

1 math.ceil() #向上取整

math.floor() #向下取整

round（a,2） #a四舍五入,保留2位小数

int（） #向下取整，返回整数类型

math.sqrt() #开根号

pow（a,x） #求a的x次方

2 遍历字典

https://www.cnblogs.com/stuqx/p/7291948.html

3 字典根据value进行简单的排序

d = {'a':1,'b':4,'c':2}
a=sorted(d.items(),key = lambda x:x[1],reverse = True)
print(a)

输出：[('b', 4), ('c', 2), ('a', 1)]

4 复杂list排序

students = [('john', 'W', 1), ('jane', 'B', 12), ('dave', 'B', 1)]
a=sorted(students, key=operator.itemgetter(1,2))
print(a)

输出：[('dave', 'B', 1), ('jane', 'B', 12), ('john', 'W', 1)]

5 排列、组合

from itertools import permutations
from itertools import combinations

for i in permutations([1, 2, 3], 3):
    print(i)

for i in combinations([1, 2, 3], 2):
    print(i)

输出：(1, 2, 3)

(1, 3, 2)
(2, 1, 3)
(2, 3, 1)
(3, 1, 2)
(3, 2, 1)
(1, 2)
(1, 3)
(2, 3)

注意返回类型：

a=permutations([1, 2, 3])
print(type(a))
print(list(a))

输出：

[(1, 2, 3), (1, 3, 2), (2, 1, 3), (2, 3, 1), (3, 1, 2), (3, 2, 1)]

6 注意numpy广播机制

a = np.random.randn(2, 3) # a.shape = (2, 3)
b = np.random.randn(2, 1) # b.shape = (2, 1)
print(a)
print(b)
c = a + b
print(c)

aaa=[1,2]
bbb=[4,3]
print(aaa+bbb)

输出：

[[ 0.22439514 -1.46048723 -0.09989994]
[-0.16181647 0.89533506 -0.02419727]]
[[-1.98479857]
[-0.45767346]]
[[-1.76040344 -3.4452858 -2.08469852]
[-0.61948993 0.4376616 -0.48187073]]
[1, 2, 4, 3]

a = np.random.randn(3, 3)
b = np.random.randn(3, 1)
c = a*b
print(c)

输出

[[ 0.31930559 0.57259555 0.3371194 ]
[-0.55700812 -0.49222884 0.57338674]
[-0.67481986 -0.16179442 1.52381768]]

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