How to use sentinel values different from input & sort list by feature?

问题

I'm learning beginner python and there's one question that I'm stuck on.

The question involves asking user input for any amount of mushrooms that they have picked, entering a weight, and then sorting them according to the user input. For this, a list and a while loop is needed to append the inputs into the list.

Currently I am trying to implement a sentinel value that will stop the while loop after all of the user inputs have been entered, but setting the sentinel as "STOP" conflicts with the int() notification.

if __name__ == "__main__":
    STOP = "stop"
    mushroom = int(input("Enter a mushroom weight in grams, or STOP to end. "))
    total_list = []
    while total_list != STOP:
        total_list.append(mushroom)
        mushroom = int(input("Enter a mushroom weight in grams, or STOP to end. "))
    print(total_list)

The program runs well until entering "STOP", where a syntax error appears.

mushroom = int(input("Enter a mushroom weight in grams, or STOP to end. "))
ValueError: invalid literal for int() with base 10: 'STOP'

As you can see, the sentinel value STOP conflicts with my input suggestion, resulting in an error.

As for the second part of the problem, I need to sort the value inputs by weight. If everything has been done correctly, I should have a list with all the values. What kind of code can I use in order to sort the values? I need to sort each integer value based on small (<100), medium (100-1000), and large (>1000), then print the results in a statement. Am a little clueless regarding what I need to do here.

Thank you, just kind of stuck in one place.

回答1:

You can use the error produced by trying to convert a noninteger to an integer to your advantage with try and except in Python. Here is the documentation for that.

Rewriting your code, you might try this:

if __name__ == "__main__":
    STOP = "STOP"
    total_list = []
    while True:
        try:
            user_input = input("Enter a mushroom weight in grams, or STOP to end. ")
            total_list.append(int(user_input))  
        except ValueError:
            if user_input.strip().upper()==STOP:
                break
            else:
                print("Incorrect entry! '{}' is not an integer".format(user_input))

Then if you want to sort that list and put into categories, you might consider using a dictionary:

total_list.sort()       
di={'Small':[],'Medium':[],'Large':[]}
for e in total_list:
    key='Small' if e<100 else 'Medium' if 100<=e<=1000 else 'Large'
    di[key].append(e)

print(total_list, sum(total_list),di)

回答2:

The first crash is occurring because you are trying to cast "STOP" as an int. For example, if you open a python interpreter and type int("STOP") you will get the same crash. The interpreter does not know how to convert the string "STOP" to an integer. A better way might be to check if the input string can be converted to an integer with isdigit() or a try/except.

For the filtering, the easiest way would be to use a list comprehension on your lists. Something like this should work:

if __name__ == "__main__":
    mushroom = None
    total_list = []
    while mushroom != "STOP":
        mushroom = input("Enter a mushroom weight in grams, or STOP to end. ")
        if mushroom.isdigit():
            total_list.append(int(mushroom))
    sorted_total_list = sorted(total_list)
    small_values = [mushroom for mushroom in total_list if mushroom < 100]
    medium_values = [mushroom for mushroom in total_list if 100 <= mushroom <= 1000]
    large_values = [mushroom for mushroom in total_list if mushroom > 1000]
    print(sorted_total_list)
    print(small_values)
    print(medium_values)
    print(large_values)

来源：https://stackoverflow.com/questions/58244212/how-to-use-sentinel-values-different-from-input-sort-list-by-feature

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python

list

while-loop