Python：行程编码

Question

I'm getting an error if the input only contains a character without a number attached to it. For example, if the user were to input "a2bc" the output should be "aabc". I have to meet the run-length format. The decode function works if its "a2b1c1". The single character doesn't recognize any of it. I played with the conditions and debugger. I can't seem to meet the format for run-length.

The code displayed below were my attempts. I commented on the block where I tried to fix my problems.

def decode(user_input):

    if not user_input:
        return ""

    else:
        char = user_input[0]
        num = user_input[1]

        if num.isdigit():
            result = char * int(num)

        # elif num.isalpha():
        #     # this should skip to the next two characters

        else:
            result = char * int(num)

        return result + decode(user_input[2:])

test1 = decode("a2b3c1") 
test2 = decode("a2b3c")
print(test1)
print(test2)

(Note: the output for test2 should be "aabbbc" )
Thank you so much.

Answer 1

This requires two changes: as you already figured out, if num is not actually a number, then you only use the char once and skip one character ahead. Otherwise you use the number and skip two characters ahead. But you also need to handle a single character without a number at the end of your string. You can solve this by not only checking if user_input is empty, but whether it only has one character - in both cases you can simply return the string.

def decode(user_input):
    if len(user_input) < 2:
        return user_input

    char = user_input[0]
    num = user_input[1]

    if num.isdigit():
        return char * int(num) + decode(user_input[2:])
    else:
        return char + decode(user_input[1:])

Answer 2

You should advance by 1 instead of 2 when the next character is not a digit (ie the 1 is implicit):

def decode(user_input):
    if len(user_input) < 2 : return user_input
    multiplier,skip = (int(user_input[1]),2) if user_input[1].isdigit() else (1,1)
    return user_input[0] * multiplier + decode(user_input[skip:])

note that doing this recursively will constrain the size of the input string that you can process because of the maximum recursion limit.