限制语句中的循环次数？

Question

我的编程基础课有一个作业。在这种情况下，我必须在程序终止之前接受无效输入一定次数（在本例中为五次）。这本书似乎没有解释在这种情况下限制循环运行次数，所以我已经包含了我的代码作为指导。正如所写，它继续循环并且不会停止。max = 5#Input miles to be convertedmiles = float(input('Enter the number of miles to convert to kilometers: '))milesToKms = miles*1.6 #miles to kilometers formulafor counter in range(max):    if miles >= 0:        print ('Miles converted: ', miles)        print ('Kilometers: ', milesToKms) #display kilometers result        break    elif miles <0:        while miles < 0:            print ('Invalid value entered.')            miles = float(input('Enter a valid number of miles to convert: '))else:    print ('Too many invalid entries submitted.')    exit ()         

Answer 1

您不需要或不想要while这里的循环。您的外for循环会自行限制尝试次数，并重新检查每个循环的条件，因此它会自行完成工作。

我已经清理了一点以删除冗余input代码，这也提供了避免在最终尝试失败后提示输入的附带好处，当循环无论如何都会停止时：

import sys

max = 5

for counter in range(max):

    # Input miles to be converted

    miles = float(input('Enter the number of miles to convert to kilometers:'))

    if miles >= 0:

        # May as well defer calculation of milesToKms until we know

        # the miles value is valid

        milesToKms = miles * 1.6 # miles to kilometers formula

        print('Miles converted:', miles)

        print('Kilometers:', milesToKms) #display kilometers result

        break

    else:  # No need for elif miles < 0; anything not valid is invalid

        print('Invalid value entered.')

else:

    # sys.exit is correct way to exit program, and it can output error for you

    sys.exit('Too many invalid entries submitted.')

Answer 2

继续计算直到5给出错误输入的解决方案：

import sys

invalids = 0

maxWrongInputs = 5

# loop until too many wrongs were done

while invalids < maxWrongInputs:

    miles = float(input('Enter the number of miles to convert to kilometers:'))

    if miles >= 0:

        milesToKms = miles * 1.6 

        print('Miles converted:', miles)

        print('Kilometers:', milesToKms) 

    else:  

        # add another to your counter of invalid attempts

        invalids += 1

        print('Invalid value entered.')

else:

    sys.exit('Exited after {} invalid entries.'.format(invalids))

输入： 3.  4.  5.  6.  7.  8.  -1.  -1.  -1.  4.  -1.  -1.

输出：

Enter the number of miles to convert to kilometers:3

Miles converted: 3.0

Kilometers: 4.800000000000001

Enter the number of miles to convert to kilometers:4

Miles converted: 4.0

Kilometers: 6.4

Enter the number of miles to convert to kilometers:5

Miles converted: 5.0

Kilometers: 8.0

Enter the number of miles to convert to kilometers:6

Miles converted: 6.0

Kilometers: 9.600000000000001

Enter the number of miles to convert to kilometers:7

Miles converted: 7.0

Kilometers: 11.200000000000001

Enter the number of miles to convert to kilometers:8

Miles converted: 8.0

Kilometers: 12.8

Enter the number of miles to convert to kilometers:-1    # 1.

Invalid value entered.

Enter the number of miles to convert to kilometers:-1    # 2.

Invalid value entered.

Enter the number of miles to convert to kilometers:-1    # 3.

Invalid value entered.

Enter the number of miles to convert to kilometers:4

Miles converted: 4.0

Kilometers: 6.4

Enter the number of miles to convert to kilometers:-1    # 4.

Invalid value entered.

Enter the number of miles to convert to kilometers:-1    # 5.

Invalid value entered.

Exited after 5 invalid entries.

Answer 3

您的内部 while 循环导致了问题：

max = 5

#Input miles to be converted

for counter in range(max):

    miles = float(input('Enter the number of miles to convert to kilometers: '))

    milesToKms = miles*1.6 #miles to kilometers formula

    if miles >= 0:

        print ('Miles converted: ', miles)

        print ('Kilometers: ', milesToKms) #display kilometers result

        break

    elif miles <0:

        print ('Invalid value entered.')

if counter == (max - 1):

    print ('Too many invalid entries submitted.')

    exit ()

有效输出：

Enter the number of miles to convert to kilometers: 10

('Miles converted: ', 10.0)

('Kilometers: ', 16.0)

无效输出：

Enter the number of miles to convert to kilometers: -1

Invalid value entered.

Enter the number of miles to convert to kilometers: -2

Invalid value entered.

Enter the number of miles to convert to kilometers: -3

Invalid value entered.

Enter the number of miles to convert to kilometers: -4

Invalid value entered.

Enter the number of miles to convert to kilometers: -5

Invalid value entered.

Too many invalid entries submitted.