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")