python学习旅程笔记3-控制流
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Python的基本运算表达式
(1)判断语句
关于if语句的规则我不再介绍,只在这里提出Python下if语句的用法,以及特点。
- #!/usr/bin/python
- # Filename: if.py
- number = 23
- guess = int(raw_input('Enter an integer : '))
- if guess == number:
- print 'Congratulations, you guessed it.' # New block starts here
- print "(but you do not win any prizes!)" # New block ends here
- elif guess < number:
- print 'No, it is a little higher than that' # Another block
- # You can do whatever you want in a block ...
- else:
- print 'No, it is a little lower than that'
- # you must have guess > number to reach here
- print 'Done'
- # This last statement is always executed, after the if statement is executed
以下为输出
- $ python if.py
- Enter an integer : 50
- No, it is a little lower than that
- Done
- $ python if.py
- Enter an integer : 22
- No, it is a little higher than that
- Done
- $ python if.py
- Enter an integer : 23
- Congratulations, you guessed it.
- (but you do not win any prizes!)
- Done
在这里,我们使用了一个函数raw_input(),用以获取用户的输入。
我们为内建的raw_input
函数提供一个字符串,这个字符串被打印在屏幕上,然后等待用户的输入。一旦我们输入一些东西,然后按回车键之后,函数返回输入。对于raw_input
函数来说是一个字符串。我们通过int
把这个字符串转换为整数,并把它存储在变量guess
中。事实上,int
是一个类,不过你对它所需了解的只是它把一个字符串转换为一个整数。
注意:我们使用了缩进层次来告诉Python每个语句分别属于哪一个块。这就是为什么缩进在Python如此重要的原因。
在Python中没有switch
语句。不过你可以使用if..elif..else
语句来完成同样的工作。
(2)循环语句
1.while语句
Python的while语句,与其他语言的while语句区别不大。
- #!/usr/bin/python
- # Filename: while.py
- number = 23
- running = True
- while running:
- guess = int(raw_input('Enter an integer : '))
- if guess == number:
- print 'Congratulations, you guessed it.'
- running = False # this causes the while loop to stop
- 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 while loop is over.'
- # Do anything else you want to do here
- print 'Done'
输出为:
- $ python while.py
- Enter an integer : 50
- No, it is a little lower than that.
- Enter an integer : 22
- No, it is a little higher than that.
- Enter an integer : 23
- Congratulations, you guessed it.
- The while loop is over.
- Done
注意:在Python中,你可以在while
循环中使用一个else
从句。
2.for语句
for语句,在这里,我通过一个例子在作出说明
- #!/usr/bin/python
- # Filename: for.py
- for i in range(1, 5):
- print i
- else:
- print 'The for loop is over'
输出为:
- $ python for.py
- 1
- 2
- 3
- 4
- The for loop is over
注意:else
部分是可选的。如果包含else,它总是在for
循环结束后执行一次,除非遇到break语句。
与C/C++语言相比,Python的if语句,无疑简单了许多。
(3)其他语句
1.break语句
break语句在for循环和while循环中使用。
- while True:
- s = raw_input('Enter something : ')
- if s == 'quit':
- break
- print 'Length of the string is', len(s)
- print 'Done'
输出为
- $ python break.py
- Enter something : Programming is fun
- Length of the string is 18
- Enter something : When the work is done
- Length of the string is 21
- Enter something : if you wanna make your work also fun:
- Length of the string is 37
- Enter something : use Python!
- Length of the string is 12
- Enter something : quit
- Done
2.continue语句
在这里,我只通过一个例子来说明。
- #!/usr/bin/python
- # Filename: continue.py
- while True:
- s = raw_input('Enter something : ')
- if s == 'quit':
- break
- if len(s) < 3:
- continue
- print 'Input is of sufficient length'
- # Do other kinds of processing here...
输出为:
- $ python continue.py
- Enter something : a
- Enter something : 12
- Enter something : abc
- Input is of sufficient length
- Enter something : quit
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