Python Sentinel不工作

Question

我不明白为什么这个前哨不能工作。我找遍了所有地方都找不到答案。谢谢你的帮助!

gas = 0
miles = 0
miles = int(input("Enter number of miles driven(enter -11 to exit): "))
while miles != -11:
    gas = int(input("Enter the number of gallons of gas used: "))
    print ("The number of miles entered was:", miles)
    print ("The number of gallons of gas used was:", gas)
    mpg = miles/gas
    print ("The MPG was:", mpg)

这就是结果：

Enter number of miles driven(enter -11 to exit): 100
Enter the number of gallons of gas used: 10
The number of miles entered was: 100
The number of gallons of gas used was: 10
The MPG was: 10.0
Enter the number of gallons of gas used: -11
The number of miles entered was: 100
The number of gallons of gas used was: -11
The MPG was: -9.090909090909092
Enter the number of gallons of gas used:

Answer 1

您还应该包括循环中的里程数。

gas = 0
miles = 0
while miles != -11:
    miles = int(input("Enter number of miles driven(enter -11 to exit): "))
    gas = int(input("Enter the number of gallons of gas used: "))
    print ("The number of miles entered was:", miles)
    print ("The number of gallons of gas used was:", gas)
    mpg = miles/gas
    print ("The MPG was:", mpg)

即使在这种情况下，程序在检查前哨条件之前也会询问气体量，因此更好的版本应该是：

gas = 0
miles = 0
while True:
    miles = int(input("Enter number of miles driven(enter -11 to exit): "))
    if miles==-11:
        break
    gas = int(input("Enter the number of gallons of gas used: "))
    print ("The number of miles entered was:", miles)
    print ("The number of gallons of gas used was:", gas)
    mpg = miles/gas
    print ("The MPG was:", mpg)

Answer 2

用一个空行代替“前哨”...我也会使用iter的两个表单版本重写它，如下所示：

for miles in iter(lambda: input('Enter miles (or enter to exit): '), ''):
    try:
        miles = int(miles)
        gas = int(input('Enter gas used: '))
        print('MPG was: ', miles/gas)
    except (ValueError, ZeroDivisionError): # int type conversion failed or div by 0
        print('You must enter valid numbers and gas used must be > 0 or press enter to exit')