Reverse Numbers function using recursion in C

Question

The following is a function that is meant to return the reverse of a number using recursion. However, it only returns the last digit of the number. I'd like to know why and how to fix it?

int rev(int number)
{
      int revNum=0, sum=100;

      if(number<=9) return(number);
      else if(number>0) 
      {
           return(rev(number/10)+revNum);
           revNum=(number%10)*sum; sum=sum/10;

      }
}

Thank you!!!

Answer 1

Here's some working code:

int rev (int number){
    int base = 1;

    while (number / (base * 10)){/*
        * This calculates the base of the number
        * ie number = 435
        *    base   = 100
        */
        base *= 10;
    }

    if (number <= 9){
        return number;
    } else if (number >= 10){ // notice different expression
        int revNum = (number % 10) * base; // this was out of order
        return rev (number / 10) + revNum;
    }
}

The main reason your code couldn't work, other than what I commented above, is that sum isn't preserved within the calls. This is a common problem in making recursive functions.

To remedy this, the "base" is calculated each function call, instead of having a fixed value. This also a bit better because it allows larger numbers to be passed, instead of ones not bigger than 100 (another limit of the code you've chosen).

Another implementation is to base the base as a second parameter, so that it doesn't have to be recalculated every function call. This can, however, be easily remedied by a simple macro. The call may be :

int rev_number (int number, int base){ .. }

But a can be conveniently placed in a macro (or other function call):

#define rev(num) rev_number (number, 0)

This is a little more efficient, but the difference may or may not be important.