需要帮助了解为什么我的find（）方法的使用没有正确执行

Question

Write a function called string_finder. string_finder should take two parameters: a target string and a search string. The function will look for the search string within the target string.

The function should return a string representing where in the target string the search string was found:

If search string is at the very beginning of target string, then return "Beginning". For example:

string_finder("Boston Common", "Boston") -> "Beginning"

If search string is at the very end of target string, then return "End". For example:

string_finder("Boston Common", "Common") -> "End"

If search string is in target string but not at the very beginning or very end, then return "Middle. For example:

string_finder("Boston Common", "ton") -> "Middle"

If search string is not in target string at all, then return "Not found". For example:

string_finder("Boston Common", "Idaho") -> "Not found"

Assume that we're only interested in the first instance of the search string if it appears multiple times in the target string, and that search string is definitely shorter than target string.

I am able to write a function that utilizes conditionals (if, elif, and else) and the find() method in Python 3. In some of my conditionals with the find() method, I'm using all three parameters for the find() method, which are string, starting index and ending index.

def string_finder(target_string, search_string):
    if search_string.find(target_string, 0, len(search_string)):
        return "Beginning"
    elif search_string.find(target_string, -1, -(len(target_string) - len(search_string))):
        return "End"
    elif search_string.find(target_string):
        return "Middle"
    else:
        return "Not found"

Below are some lines of code that will test your function. You can change the value of the variable(s) to test your function with different inputs.

If your function works correctly, this will originally print: Start, Middle, End, Not found, each on their own line.

print(string_finder("Boston Common", "Boston"))
print(string_finder("Boston Common", "ton"))
print(string_finder("Boston Common", "Common"))
print(string_finder("Boston Common", "Idaho"))

which should print

Beginning
Middle
End
Not found

However, with my current code I get:

Beginning
Beginning
Beginning
Beginning

I can't figure out how I messed up my conditionals and how to tweak. Also, I'm open to any better ways of approaching this problem.

Answer 1

First of all, your second if should be "elif". It works if u do it like this:

def string_finder(target_string, search_string):
    if search_string.find(target_string, 0, len(search_string)):
        print("Beginning")
    elif search_string.find(target_string, -1, -(len(target_string) - len(search_string))):
        print("End")
    elif search_string.find(target_string):
        print("Middle")
    else:
        print("Not found")
string_finder("Boston Common", "ton")

Answer 2

The find function ( docs here ) returns -1 when it does not find the substring (and returns the lowest indice where it found the string), and -1 is interpreted as True by python ( docs for truth/false in python here ), therefore the first if will always evaluate to true.

You should check if the value returned is different from -1 . Or if you want, you can check if the value return to tell where python found the string:

def string_finder(target_string, search_string):
    start_index = search_string.find(target_string)
    end_index = start_index + len(target_string)
    if start_index == -1: # Not found, find returns -1
      return "Not found"
    elif start_index == 0:
      return "Beginning"
    elif end_index == len(search_string):
      return "End"
    else:
      return "Middle"

Let me explain the code idea:

As the docs for find says, it "Return the lowest index in the string where substring sub is found". So, start_index represents where in the search_string the target_string starts. The end_index then represents where in the search_string the target_string ends. So, if you have start_index as 0 (first position), it is in the beginning of the string. If you have end_index equals the len(target_string) it goes until the end of the search_string .