Wrong result from recursive func. in Python

Dev Shed Forums Sponsor:

December 15th, 2012, 10:48 AM

dandan2

Registered User

Join Date: Dec 2012

Posts: 3

Time spent in forums: 57 m 54 sec
Reputation Power: 0

Wrong result from recursive func. in Python

Dear all,

I am quite new in Python. I want to write very simple function which gets an integer "c" and a list of integers "li". I would like to recursively find first two items where sum of them will be equal to "c". I have written this function in Python to find second option "n" and then I can easily find the first number by finding "c-n".

My code is:

Code:

def shop(c, li):
        if ((c-li[0]) in li[1:]):
                return li[0];
        else:
                shop(c, li[1:]);

The problem is when I call function as below:

Code:

c=100;
li=[5, 75, 25];

ii=shop(c, li);

print ii, "\n";

The result is always "None", however it should return "25". When I put a "print" statement before "return" I can see that the function has found the item correctly, but I am not sure why it is not returned.

Anybody know why this happens?

Thanks a lot!

December 15th, 2012, 11:02 AM

b49P23TIvg

Contributing User

Join Date: Aug 2011

Posts: 3,460

Time spent in forums: 1 Month 2 Weeks 4 Days 6 h 56 m 42 sec
Reputation Power: 403

Repair

Code:

def shop(c, li):
    if ((c-li[0]) in li[1:]):
        return li[0]
    else:
        return shop(c, li[1:])

Python functions, by default, return None. When the test failed (in other words the value is not in list) the else block evaluated, and then the function returned None.

__________________
[code]Code tags[/code] are essential for python code!

December 15th, 2012, 11:32 AM

dandan2

Registered User

Join Date: Dec 2012

Posts: 3

Time spent in forums: 57 m 54 sec
Reputation Power: 0

Thanks a lot for your answer. But when I call the function as I mentioned before, at the first round the test if failed, since 100-5 is not in the list. But at the second round, when the function is called recursively, it should return a number since 100-75 exists in the list. Am i right or am I missing something?

Quote:

Originally Posted by b49P23TIvg

Code:

def shop(c, li):
    if ((c-li[0]) in li[1:]):
        return li[0]
    else:
        return shop(c, li[1:])

Python functions, by default, return None. When the test failed (in other words the value is not in list) the else block evaluated, and then the function returned None.

December 15th, 2012, 12:59 PM

dandan2

Registered User

Join Date: Dec 2012

Posts: 3

Time spent in forums: 57 m 54 sec
Reputation Power: 0

Hi again,

I solved the problem by rewriting the function as:

Code:

def shop(c, li):
        return li[0] if (c-li[0] in li[1:]) else shop(c, li[1:])

December 15th, 2012, 05:41 PM

b49P23TIvg

Contributing User

Join Date: Aug 2011

Posts: 3,460

Time spent in forums: 1 Month 2 Weeks 4 Days 6 h 56 m 42 sec
Reputation Power: 403

Hmm. I see they added the ternary if expression to python2.

Your new statement demonstrates, in my opinion, good style because there's one return statement.

Do you still want an explanation of the failure in your first attempt?

Wrong result from recursive func. in Python

Developer Shed Advertisers and Affiliates