需要深入了解为什么我的 Python 函数没有对字符串进行正确的比较

Question

I'm completing an online course that utilizes Python 3 and below is the prompt I received:

Write a function called is_valid. is_valid should take two parameters: a string to check, and a string of all valid characters.

is_valid should return the boolean True if all the characters in the string to check are present in the string of valid characters. It should return False if any character in the checked string does not appear.

I have defined a function that takes in the two parameters mentioned in the problem, a string (which in my code I call string)), and a string of acceptable characters (which I called validcharacters).

I'm not supposed to use any of Python's built-in string functions, so I decided to write a for loop with my index variable being called character that indexes through string. After that for loop is written, I then wrote an if statement that looks to see if character (index variable) is in validcharacters. If it is, it returns True. Otherwise, I wrote an else statement which returns False.

def is_valid(string, validcharacters):
    for character in string:
        if character in validcharacters:
            return True
        else:
            return False

Below are some test cases to work with:

sample_valid_string = "1234-5678-9011-1111"
sample_invalid_string = "1234!5678.9011?1111"
valid_characters = "0123456789-"

print(is_valid(sample_valid_string, valid_characters))
print(is_valid(sample_invalid_string, valid_characters))

I expect the output (for the test cases I listed above) to be True and False respectively. However, the way my code is written, it returns True and True. I found out through debugging it only iterates through the first character of my code and fails to iterate the other characters. I'm not sure how I need to re-write my code in order to ensure it goes through other characters before returning True or False. Do I need to re-position my return statements? Are they causing the function to end too soon?

Answer 1

Given the way you've defined is_valid , you are directly returning True if the first character in string is in valid_characters . You should only be returning True once you've checked all characters are valid. You should instead be doing something like:

def is_valid(string, validcharacters):
    for character in string:
        if character not in validcharacters:
            return False
    return True

Note that from the docs, this is the proposed equivalent of all , which could be used to simplify the above to:

def is_valid(string, validcharacters):
    return all(i in valid_characters for i in string)

Where all will return True when all elements in the generator expression are also True .

---

Returning in this case:

print(is_valid(sample_valid_string, valid_characters))
print(is_valid(sample_invalid_string, valid_characters))

True
False

Answer 2

return exits the function immediately, so the way your loop is structured, it only ever runs once.

Reorganize your loop so it returns False if it finds an invalid character, and waits to return True only when it makes it all the way through the string without finding any invalid characters:

def is_valid(string, validcharacters):
    for character in string:
        if character not in validcharacters:
            return False
    return True

Answer 3

Your function is returning too early.

Right now, it sees that the first character, 1, is in your valid characters, and returns true. What you want is to ensure that the whole string is valid. Postpone your return True until you've reached the end of your loop.

def is_valid(string, validcharacters):
    for character in string:
        if character in validcharacters:
            continue
        else:
            return False
    return True

Side note: this has a pretty bad big O ( https://en.wikipedia.org/wiki/Big_O_notation ) of O(N^2), as it iterates through every character in validcharacters to look for a match. If you want to improve it, you could consider casting the validcharacters list as a set ( https://docs.python.org/3.7/library/stdtypes.html#set-types-set-frozenset ) of characters. The lookup for a match in a set is O(1) as compared to a list O(n).

Answer 4

def is_valid(string, validcharacters):
    #Use a variable to track unmatched characters
    flag = True 
    for character in string:
        #If you find a unmatched char, set var to flag to False and break
        if character not in validcharacters: 
            flag = False 
            break
    return flag

Answer 5

Aside from the other answers, you can refactor this function using all() . This function returns true if every single condition that was checked is true, otherwise it will return false.

def is_valid(string, validcharacters):
    # every character has to be in valid_characters for this to return 'True', 
    # otherwise it returns 'False'
    return all(character in valid_characters for character in string)