Are variables shared in recursive functions

Question

I'm trying to calculate the number of success cases within a recursive function in C#, but I'm astonished by the fact that my variable is shared between all the function calls!

[update 2]

More than strange this time. doing so

i = i + validTreesFun(tree.Nodes, newWords.ToList()) ;

resets i to 0

doing this

i = validTreesFun(tree.Nodes, newWords.ToList()) + i ;

gives some results (I'm not sure if it's correct)

[updated : the full code]

public static  int validTreesFun(List<Tree<char>> nodes, List<string> words)
    {
        int i = 0;
        if (nodes == null && (words == null || words.Count == 0 || (words.Count == 1 && words.First() == "")))
            return 1;
        else
            if (nodes == null)
                return 0;

        foreach (Tree<char> tree in nodes)
        {
            var validWords = words.Where(w => w.ToCharArray()[0] == tree.Root)
                .Select(w => w);
            if (validWords.Count() == 0)
                return 0;
            else
            {
                var newWords = validWords.Select(w => join( w.ToCharArray().Skip(1).ToArray()));
                i += validTreesFun(tree.Nodes, newWords.ToList());
            }
        }
        return i;
    }

when debuging the variable i take the value 1 but it resets to 0 on the next iteration!! despite the use of

i = i + ....

What is the problem in that piece of code?

Thank you

Answer 1

No local variables are not at all shared between recursive calls, you should consider some other design problem, inside and after your foreach loop, I dont see any return statements, can you post full code.

Ok, in debugging you will always observe i's current method's value, debugging is not good in recursive functions, its little hard to understand, you will have to move your control down in Call Stack in order to actually observe value of earlier caller of current function.

I would advice you to output Trace or on log file with your level of node, that will help you actual debugging.

Please use TRACE Statement as follow..

Trace.WriteLine(string.Format("{0},{1}",tree.Name,i));

Answer 2

Local variables are not being shared. What you are seeing (the reset to 0) is the value of i in the (recursively) called function validTreesFun (i gets set to 0 at the start of the function).

Just looking at your code, I think a possible bug might be in someTestHere - if that is never true, then i will stay 0 in the outer scope. Otherwise it should increment by 1 for each true test.

Answer 3

When you are in debug mode, you indeed see that the i is reseted for the call but remain to the wanted value for the caller. Eg , the stack :

validTreesFun --i = 0 for this one

validTreesFun --i = x for this one, but if you do not go trow the calling stack, you will see 0, which is the good value for the top of the stack

Answer 4

if (validWords.Count() == 0)
    return 0;

Should be

if (validWords.Count() == 0)
    continue;

Also, in general, I personally think it is nicer looking to only send in one element at a time to a recursive function.

public static  int validTreesFun(Tree<char> node, List<string> words)

That way you don't get the same kind of mistake like above. Finally, a minor note.

w => w.ToCharArray()[0] == tree.Root

can be written as

w => w[0] = tree.Root