I'm getting a String index out of range error

I'm getting a String index out of range error - Printable Version

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I'm getting a String index out of range error - debian77 - Jun-26-2022

This function returns True if the first letter of the string is the same as the last letter of the string and False if they’re different.

I'm having trouble explaining why I have the following error with this code:

def first_and_last(message):
    if message[0] == message[-1]:
        return True
    return False
print(first_and_last("else"))
print(first_and_last("tree"))
print(first_and_last(""))

Error:Error on line 2:
    if message[0] == message[-1]:
IndexError: string index out of range

But if I just add a logical operator it doesn't have an error, my question is why? if both are basically the same, I think the same error should pop out because I'm not doing anything special to specify any input for the second one.

def first_and_last(message):
    if message == "" or message[0] == message[-1]:
        return True
    return False

print(first_and_last("else"))
print(first_and_last("tree"))
print(first_and_last(""))

Output:True
False

Thanks for reading.

RE: I'm getting a String index out of range error - rob101 - Jun-26-2022

Very strange. Your code (the first code block) works fine for me (Linux OS running Python 3.6.9).

The only thing I'd alter (not that it changes the operation on my system), is to have an 'else' clause in the 'if' branch.

def first_and_last(message):
    if message[0] == message[-1]:
        return True
    else:
        return False

I'm old school and as such I don't like to see an 'if' branch without an 'else' clause.

RE: I'm getting a String index out of range error - Larz60+ - Jun-26-2022

>>> message = 'a'
>>> message[0]
'a'
>>> message[-1]
'a'
>>> message = ""
>>> message[0]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: string index out of range
>>> message[-1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: string index out of range

if message == "" or message[0] == message[-1]:
in this example, the message == "" would test True, so conditional would be satisfied, and is done at that point.

RE: I'm getting a String index out of range error - menator01 - Jun-26-2022

One example

def f_and_l(msg):
    if msg[0] == msg[-1:]:
        return True
    return False

print(f_and_l('else'))
print(f_and_l('tree'))

Output:True
False

RE: I'm getting a String index out of range error - deanhystad - Jun-26-2022

Why use True or False when the if statement evaluates to True or False?

I can see no reason you should get an index error for the provided test cases. I think the test for an empty string is a good improvement, but it is not needed for "tree" or "else". Cannot explain how you got the error, but it is not from the code you posted.

RE: I'm getting a String index out of range error - debian77 - Jun-26-2022

Thank you, everyone. It was my fault, there was a final print function with an "" parameter at the end.

print(first_and_last(""))

So, I think that was the error.

RE: I'm getting a String index out of range error - rob101 - Jun-26-2022

(Jun-26-2022, 03:53 AM)debian77 Wrote: Thank you, everyone. It was my fault, there was a final print function with an "" parameter at the end.
print(first_and_last(""))
So, I think that was the error.

Easy done.

To mitigate the error, you could test for an empty string at the beginning of your function...

def first_and_last(message):
    if message:
        if message[0] == message[-1]:
            return True
        else:
            return False
    else:
        return False

... which will return 'False' if an empty string is passed to the function.

RE: I'm getting a String index out of range error - deanhystad - Jun-26-2022

If and else need not apply.

def first_and_last(message):
    return message == "" or message[0] == message[-1]