Python学习笔记（4）：控制流
Python的控制语句有if、while、for、break、continue。
1. if语句
下面是一个猜数字游戏例子：
# Filename: if.py
number = 10
guess = int(input("Enter a integer:"))
if guess == number:
print("Congratulations, you guessed it.")
elif guess < number:
print("No, it is a little higher than that")
else:
print("No, it is a little lower than that")
注意if语句结尾处包含一个冒号（：），所有控制语句结尾处都需要冒号。Python没有switch语句，您可以用if...elif...else来完成相同功能。
2. while语句
您可能发现上面猜数字游戏，每次都要启动一下才能猜，下面可以用while语句来控制直到你猜中才退出游戏。
# Filename: while.py
number = 10
running = True
while running:
guess = int(input("Enter a integer:"))
if guess == number:
print("Congratulations, you guessed it.")
running = False
elif guess < number:
print("No, it is a little higher than that")
else:
print("No, it is a little lower than that")
else:
print("The game is over.")
其实与while相对的else是多余的，与直接把它的语句放在在while语句块后面的效果相同。
3. for语句
# Filename: for.py
for i in range(1, 5):
print(i)
else:
print("The for loop is over.")
输出结果为：
1
2
3
4
The for loop is over.
4. break语句
# Filename: break.py
while True:
s = input("Enter something:")
if s == "quit":
break
print("Length of the string is", len(s))
print("Done")
5. continue语句
# Filename: continue.py
while True:
s = input("Enter something:")
if s == "quit":
break
if len(s) < 3:
continue
print("Input is of sufficient length")