Python基础-(3)条件与循环语句

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bool(None)
# False
bool('')
# False

bool("anything")
# True

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thisnum = 6
##层级1
if thisnum > 0:
    print("This number is positive.")
    
##层级2
if thisnum > 0:
    print("This number is positive.")
else:
    print("This number is negative.")
    
##层级3
if thisnum > 0:
    print("This number is positive.")
elif thisnum == 0:
    print("This number is zero.")
else:
    print("This number is negative.")

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thisnum = 5
if thisnum <= 9 and thisnum > 0:
    print("This is a natural number")

#也可以使用链式比较
if 0 < thisnum <= 9:
    print("This is a natural number")

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numebrs = [1,2,3,4,5,6]
#等价于
numbers = list(range(0,7))

for number in range(0,7):
    print(number)

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#(1)迭代字典
d = {'x':1, 'y':2, 'z':3}
##迭代字典的键
for key in d:
    print(key, 'value is', d[key])
##迭代字典的每一项
for key, value in d.items():
    print(key, 'value is', value)
    
#(2) zip()可缝合多个等长序列，用于迭代
name=['AA',"BB","CC"]
age=[12,15,11]
list(zip(name, age))
## [('AA', 12), ('BB', 15), ('CC', 11)]
for name,age in zip(name, age):
    print(name, 'is', age, 'years old')
    
    
#(3) enumerate()同时迭代索引
strs = ['A', 'B', 'C']
for index, st in enumerate(strs):
    print(index, st)
## 0 A
## 1 B
## 2 C

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from math import sqrt
for n in range(99,0,-1):
    root = sqrt(n) #平方根
    if root == int(root):
        print(n)
        break

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#打印奇数
for i in range(0,9):
    if i%2==0:
        continue
    print(i)

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[x*x for x in range(5)]
# [0, 1, 4, 9, 16]

##筛选符合条件的变量x
[x*x for x in range(5) if x%2==0]
# [0, 4, 16]

##组合两个列表，不同于zip()
As = [1,2]
Bs = ['a','b']
[(x, y) for x in As for y in Bs]
# [(1, 'a'), (1, 'b'), (2, 'a'), (2, 'b')]