python - 限制语句中的循环数量？

Question

我的编程基础类(class)有一个作业。在这种情况下，在程序终止之前，我必须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的数量，因此我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 converted
miles = float(input('Enter the number of miles to convert to kilometers: '))
milesToKms = miles*1.6 #miles to kilometers formula
for 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

您不需要也不想要此处的 while 循环。您的外部 for 循环会限制其自身的尝试次数，并重新检查每个循环的条件，因此它会自行完成所有工作。

我已经对其进行了一些清理，以删除多余的 input 代码，这还提供了避免在最终尝试失败后提示输入的副作用，此时循环无论如何都会停止:

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.')