c++ 递归 function 条件满足时不会退出

Question

I need some help. I have a recursive function, in c++, that simply breaks apart a string(all numbers) and adds up the characters to see if its total is less than 9. if not it calls itself again until the condition is meet. So far everything it is working untill the end when my condition is meet it calls the function one more time. This obviously messes up the value of the variable that I am trying to assign it to. I referred to many references and it seems like my syntax is correct? Any help on why my function calls its self one last time.

/////////////////////////////////////////////////////////

int finalNumber =0;

// set finalNumber

finalNumber = sumTotal(sumInput);

int sumTotal(int sumInputToString)
{

    stringstream strToInt;
    string convertedInt;

    strToInt << sumInputToString;
    convertedInt= strToInt.str();

    int sum = 0;

    for(int i = 0; i < convertedInt.length(); i++)
    {
        sum += (int)convertedInt[i] - 48;
    }

    if (sum > 9)
    {
        sumTotal(sum);
    }
    return sum; //ONCE SUM IS LESS THAN 9,which is what i want, 
                //MY PROGRAM AUTOMATICALLY JUMPS UP TWO LINES does it one 
                //more time than it will escape.
}

I have tried everything from putting my return in a else{} and swapping my if else conditions so my return would be in my if. Any help would be greatly appreciated.

Thanks.

Answer 1

Shouldn't the statement inside the condition be:

if (sum > 9)
{
    sum = sumTotal(sum);
}

Otherwise, you're just calling

if (sum > 9)
{
    sumTotal(sum);
}
return sum;

which does nothing (ie doesn't contribute to the final output).

Answer 2

Based on this comment

MY PROGRAM AUTOMATICALLY JUMPS UP TWO LINES

I may suggest you see this in the debugger. Jumping up 2 lines itself is correct -- you move up on the function stack (ie return to the function which called yours -- and it is again sumTotal ). So are you sure that after everything is done function really starts from the beginning?

One more thing to think about: if initial value of sumInputToString is less than 10 your code will be executed anyway. Do you want it really?

And few advices not related with your question directly. It's very difficult to understand what your function should really do. For any complex algorithm (and this may be considered as complex because you use resursion) you must have a good function comment. I would suggest something like this:

This function first calculates the sum of all digits in the given number. If the result is less or equal than 9 it is returned. If not, we continue with this sum as a given number.

You may also include short sentense why do you need it (because it may seem strange at the first glance):

This function can be used to figure out whether some number is divisible by 3 or not.

After wtirting such a comment you are starting to understand what the logic should be and may realize that you don't need a recursion here. And transform your code into something like this:

// Function comment
int CalculateRecursiveSumOfDigits(int val) {
    while (val > 9) {
        val = CalculateSumOfDigits(val); // TODO: implement this function
    }

    return val;
}

Answer 3

That's the least efficient way to steal your neighbor's electricity but sure..

Your program isn't jumping back two lines, your program popped the last recursion instance from the stack, you have to make sure that it would successfully exit the function call per nested function call.