String being replaced by garbage before being printed

Question

I have this code:

#include <cs50.h>
#include <stdio.h>
#include <string.h>

string increment_str();

int  main() {
   string s = increment_str("xyz");
   printf("String: %s\n", s);
}

//Increment the characters of a string through lowercase and then uppercase alphabet
string increment_str(string s)
{
   int len = strlen(s);
   // Initialize an r var to make sure s_return has enough space in memory
   char r[len];
   string s_ret = r;

   //Remove the last character so the incremented one can be concatenated later
   strcpy(s_ret, s);
   s_ret[len - 1] = '\0';

   switch (s[len - 1])
   {
      //At the end of lowercase alphabet start over at uppercase
      case 'z':
         strcat(s_ret, "A");
         return s_ret;
         break;

      case 'Z':
         //If all the the first character is the last letter start over with an extra character
         if (len == 1)
         {
            strcpy(s_ret, "aa");
         }
         //Else refer to itself to increment the previous character in string
         else
         {
            strcpy(s_ret, increment_str(s_ret));
            strcat(s_ret, "a");
            return s_ret;
         }

      //Needs brackets because you can't declare a var after a case
      default:
      {
         //Concatenates the increased character with the string before it
         char c[2] = {s[len - 1] + 1, '\0'};
         strcat(s_ret, c);
         return s_ret;
      }
   }

}

That should run normally, and it does in fact, except when it gets to the part of printing. Through debug50 I can see step by step how everything is going but when it tries to print s it always prints "U" if run through debug50 or other garbage if run normally as seen in the screenshots bellow:

I am really not sure why this is happening or how to solve it so any help would be appreciated :)

Answer 1

char r[len];
string s_ret = r;

The issue is that s_ret is pointing to a local variable r, which goes out of scope once the function returns. Therefore, the "string" you are returning to main is no longer valid, so you will get garbage.

If this is for caesar, I'd suggest it is a too-complicated approach until you learn more about char pointers and the fact that there is no such thing as a "string" type in C.

If you are trying to "wrap around" the alphabet, it is far simpler to deal with it mathematically (bring the char down to the range of 0-25 and use %26 to wrap it around, such that 'z' + 1 will be 'a'. ( 'z'-'a' is 25, and 25+1 is 26, 26%26 is 0 and 0+'a' is 'a').