三、流程控制
3.1 判斷 if
第一種：
age=28
if age > 18:
print(‘表白‘)
第二種if...else...：
if age > 18 and age < 22:
print(‘表白‘)
else:
print(‘阿姨好‘)
第三種 #if多分支：
score=input(‘>>: ‘)
score=int(score)
if score >= 90:
print(‘優秀‘)
elif score >= 80:
print(‘良好‘)
elif score >= 60:
print(‘合格‘)
else:
print(‘滾犢子‘)
if嵌套：
age=19
is_pretty=True
success=True
if age > 18 and age < 22 and is_pretty:
if success:
print(‘表白成功，在一起‘)
else:
print(‘去他媽的愛情‘)
else:
print(‘阿姨好‘)
3.2 條件循環 while
3.2.1 格式一：while：條件循環
while 條件:
循環體
count=0
while count <= 10:
print(count)
count+=1
嵌套循環
count=1
while True:
if count > 3:
print(‘too many tries‘)
break
name=input(‘name>>: ‘)
password=input(‘password>>: ‘)
if name == ‘egon‘ and password == ‘123‘:
print(‘login successfull‘)
break
else:
print(‘user or password err‘)
count+=1
例2：登陸，只允許登陸三次，使用tag
count = 1
tag=True
while tag:
if count > 3:
print(‘too many tries‘)
break
name = input(‘name>>: ‘)
password = input(‘password>>: ‘)
if name == ‘egon‘ and password == ‘123‘:
print(‘login successfull‘)
while tag:
cmd=input(‘cmd>>: ‘) #q
if cmd == ‘q‘:
tag=False
continue
print(‘run %s‘ %cmd)
else:
print(‘user or password err‘)
count += 1
3.2.2 格式二：while+else
count=0
while count <= 5:
if count == 3:
break
print(count)
count+=1
else:
print(‘當while循環在運行過程中沒有被break打斷，則執行我‘)
3.3 for叠代式循環
3.3.1 格式一：
for i in range(1,10,2):
print(i)
例1：
l1=[‘a‘,‘b‘,‘c‘,‘d‘,‘e‘]
for i in range(len(l1)):
print(i,l1[i])
3.3.2 格式二：for+else:
for i in range(5):
if i == 3:break
print(i)
else:
print(‘當循環在運行過程中沒有被break打斷，則執行我‘)
3.4 break和continue關鍵字
while+break
count=0
while True:
if count == 5:
break #跳出本層循環
print(count)
count+=1
while+continue
#1,2,3,4,5,7
count=1
while count <= 7:
if count == 6:
count += 1
continue #跳出本次循環
print(count)
count+=1
三、流程控制