Letter-based Strings not Recognized as Strings in Python Reversing

In summary: One note: there is no need for parentheses in the return statements. They work (since putting parentheses around any Python expression gives a valid Python expression), but they're not...necessary.
  • #1
WWGD
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TL;DR Summary
Algorithm correctly reverses numerical strings but does not recognize letter strings
Hi,
I have a simple Python algorithm in my Jupyter notebook , to reverse a string.
It works well on numerical strings, but when I try to reverse a letter string, I get an error message,
e.g., the string abc, I get the error message:
name 'abc' is not defined

This is my Python code

Python:
def reverse(x):
String=str(x)
if String[0]=='-':
return( '-' + str[:0:-1])
else:
return( str[::-1])
It works no problem with numerical input, e.g., if I enter:
reverse(-235), it returns -532

But if I try reverse(abc), I get the error message above:
"Name 'abc' not defined"

I have no idea what the issue could be. Any ideas?
 
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  • #2
Look at your return statements I think you conflated the str() function with the variable String.

Personally I would use something more descriptive and less looking like a keyword ie I would use something like s instead of String.

Python:
s="hello"
s[::-1]
 
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  • #3
jedishrfu said:
Look at your return statements I think you conflated the str() function with the variable String.

Personally I would use something more descriptive and less looking like a keyword ie I would use something like s instead of String.

Python:
s="hello"
s[::-1]
Good point, but in my original code I did not make that mistake.
It was:
[code = python]
def reverse(x):
string=str(x)
if string[0]=='-':
return('-' + string[:0:-1])
else:
return(string[::-1])
[/code]
 
  • #4
okay your last return is missing a right paren at the end of the statement. Maybe that was the cause of your error message.
 
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  • #5
WWGD said:
Good point, but in my original code I did not make that mistake.
It was:
[code =python]
def reverse(x):
string=str(x)
if string[0]=='-':
return('-' + string[:0:-1])
else:
return(string[::-1]
[/code]
WWGD said:
Summary:: Algorithm correctly reverses numerical strings but does not recognize letter strings

Hi,
I have a simple Python algorithm in my Jupyter notebook , to reverse a string.
It works well on numerical strings, but when I try to reverse a letter string, I get an error message,
e.g., the string abc, I get the error message:
name 'abc' is not defined

This is my Python code

Python:
def reverse(x):
String=str(x)
if String[0]=='-':
return( '-' + str[:0:-1])
else:
return( str[::-1])
It works no problem with numerical input, e.g., if I enter:
reverse(-235), it returns -532

But if I try reverse(abc), I get the error message above:
"Name 'abc' not defined"

I have no idea what the issue could be. Any ideas?

jedishrfu said:
okay your last return is missing a right paren at the end of the statement. Maybe that was the cause of your error message.
Really sorry for my carelessness, Jedi, but I did not make that mistake in the original either. I will be extra careful to type in code correctly. The one you suggested does work without a problem.
 
  • #6
jedishrfu said:
okay your last return is missing a right paren at the end of the statement. Maybe that was the cause of your error message.
Almost certainly that was the case.

This works:
Python:
def reverse(x):
    string=str(x)
    if string[0]=='-':
        return('-' + string[:0:-1])
    else:
        return(string[::-1])

str1 = 'abc'
str2 = reverse(str1)
print(str2)
 
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  • #7
Ok, my problem was fixed just by using the original code but entering strings within double quotes, i.e.
reverse("stringname")
Go figure.
Edit: @Mark44 suggestion also worked out. Thank you both.
 
  • #8
WWGD said:
Ok, my problem was fixed just by using the original code but entering strings within double quotes, i.e.
reverse("stringname")
Yes. The error is python telling you that it's never heard if a variable called abc. Adding quotes turns the input into the literal string you wanted. The program works with numeric input because numbers aren't legal variable names, so are always interpreted as literally what you typed. Your str(x) line turns it into the character string "532", which is then reversed.
 
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  • #9
WWGD said:
I have a simple Python algorithm in my Jupyter notebook , to reverse a string.
Why would you want to keep a "-" at the beginning for a string that doesn't contain all numeric characters?
 
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  • #10
PeterDonis said:
Why would you want to keep a "-" at the beginning for a string that doesn't contain all numeric characters?
True, but it seems it is used just in case it is numeric and ignored if it is not.
 
  • #11
WWGD said:
True, but it seems it is used just in case it is numeric and ignored if it is not.
No, it isn't. There is no test anywhere in the code for an all-numeric string. It just tests for the first character being "-". So we would get, for example, reverse("-xyz") == "-zyx".
 
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  • #12
Mark44 said:
This works
One note: there is no need for parentheses in the return statements. They work (since putting parentheses around any Python expression gives a valid Python expression), but they're not necessary.
 
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  • #13
PeterDonis said:
No, it isn't. There is no test anywhere in the code for an all-numeric string. It just tests for the first character being "-". So we would get, for example, reverse("-abc") == "-cba".
To address the example you brought up; that of -abc, -3ab, etc. , which may or may not be numeric.
 
  • #14
Are you using a, b, c... as hexadecimal digits? If so:
  • This is not the way to do it, you want -0x3ab
  • Reversing digits of a decimal number and a hexadecimal number are different operations so mixing them in the same program is undesirable
What are you actually trying to do?
 
  • #15
pbuk said:
Are you using a, b, c... as hexadecimal digits? If so:
  • This is not the way to do it, you want -0x3ab
  • Reversing digits of a decimal number and a hexadecimal number are different operations so mixing them in the same program is undesirable
What are you actually trying to do?
These are interview questions. This question was about an algorithm to reverse strings; no more context was given.
 
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  • #16
WWGD said:
To address the example you brought up; that of -abc, -3ab, etc. , which may or may not be numeric.
Yes, but "-xyz" is clearly not, yet it behaves the same way; see post #11.
 
  • #17
If it's an interview question presumably you are expected to both diagnose the immediate error (abc is interpreted as a variable which is undefined) and point out that the function behaviour is peculiar if you are not feeding it a number, perhaps leading into a discussion on validating your inputs. So Peter gets thr job...
 
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  • #18
PeterDonis said:
One note: there is no need for parentheses in the return statements. They work (since putting parentheses around any Python expression gives a valid Python expression), but they're not necessary.
And this note is not just a quibble. The parentheses make the statement syntactically indistinguishable from a function invocation. This matters if we just happen to fat-finger the spelling of “return” in an infrequent code path that isn’t executed by our test suite - the parentheses have turned a compile-time syntax error during development into a run-time crash of a production system.
 

Related to Letter-based Strings not Recognized as Strings in Python Reversing

1. What is meant by "Letter-based Strings not Recognized as Strings in Python Reversing"?

When working with strings in Python, it is important to note that the letters of a string are not recognized as individual strings. This means that when trying to reverse a string, the letters will not be reversed as separate strings, but rather as one complete string.

2. How do you reverse a string in Python?

To reverse a string in Python, you can use the built-in function reversed(). This function takes in an iterable object, such as a string, and returns a reversed version of it.

3. Can you provide an example of reversing a letter-based string in Python?

Yes, for example, if we have a string "hello", using the reversed() function would return 'olleh' as the reversed string.

4. How do you handle special characters or numbers when reversing a string in Python?

When using the reversed() function, special characters and numbers will also be reversed. If you want to preserve the original order of these characters, you can use the join() method to join the reversed string with the original string.

5. Are there other ways to reverse a string in Python besides using the reversed() function?

Yes, there are other ways to reverse a string in Python. One way is to use slicing, where you can specify the start and end index of a string and use a negative step to reverse it. Another way is to use a loop to iterate through the string and append the characters in reverse order to a new string.

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