how to use an index variable for a recursive function?

Question

How can I use an index variable for a recursive function that increments every time the function is called so it can return a value without passing it as a parameter or using static or global variables as I want to call this function more than one time not only once.

Here's my code:

bool isAlphabetic(string s)
{
    static int i = 0;
    if (i==s.size())
    {
        return true;
    }
    else if (!isalpha(s[i])){
        return false;
    }
    i++;
    return isAlphabetic(s);
}

Does any body know how to fix this? I know i'm using a static variable but this makes the function run correctly the first time its called but not after that.

@2501 I don't want to pass it as an argument. is there any possible way to do this than passing an argument? — Youssef Ehab
– Youssef Ehab, Commented Dec 13, 2014 at 19:41
How the heck do you expect to hold status information without storing it somewhere either as a passed-in parameter or in some other variable? — Apprentice Queue
– Apprentice Queue, Commented Dec 13, 2014 at 19:42
What is the behaviour you see and how does it differ from what you expect? — Pradhan
– Pradhan, Commented Dec 13, 2014 at 19:42
@ApprenticeQueue this is a lab assignment and i am required to do this without passing 'i' as a parameter — Youssef Ehab
– Youssef Ehab, Commented Dec 13, 2014 at 19:45
You do not want to pass the position, but you use a string copy on each call. That's MUCH worst, even if the string buffer doesn't get copy each time. — Alexis Wilke
– Alexis Wilke, Commented Dec 13, 2014 at 19:51

Bill Lynch · Accepted Answer · 2014-12-13 19:53:22Z

4

Worth noting that the recursion may fail with larger strings due to stack overflow.

bool isAlphabetic(const string & s, int pos){
    if(i==s.size()) return true;
    else if(!isalpha(s[pos])) return false;
    return isAlphabetic(s, ++pos);
}

bool isAlphabetic(const string & s){
    return isAlphabetic(s, 0);
}

edited Dec 13, 2014 at 19:53

Bill Lynch

82.4k16 gold badges132 silver badges181 bronze badges

answered Dec 13, 2014 at 19:44

druckermanly

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3 Comments

Apprentice Queue Over a year ago

A smart compiler would prevent stack overflow because this algorithm is tail-recursive.

druckermanly Over a year ago

Not (necessarily) in debug mode! :)

Alexis Wilke Over a year ago

At least you use a reference to the string which alleviates the horrible copy.

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