recursive function

This is a discussion on recursive function within the C Programming forums, part of the General Programming Boards category; I am getting a Seg Fault; can anyone explain the problem?? I believe this simple program is to find the ...

  1. #1
    Registered User sballew's Avatar
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    Sep 2001
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    157

    recursive function

    I am getting a Seg Fault; can anyone explain the problem??

    I believe this simple program is to find the # of instances a letter s occurs in the string literal "system" (which is obviously 2 times).

    I am reviewing some programs for my final and I am obviously missing something quite simple but darned if I see it right now.


    Code:
    #include <stdio.h>
    
    int Example2(char ch, char *str)
    {
       int result, i;
    
       if (str[0] == '\0')
          result = 0;
       else if
          (ch == str[0])
             result = 1+Example2(ch,&str[i]);
       else
           result = Example2(ch,&str[i]);
       return result;
    }
    
    main()
    {
       char a = 's';
       char message[20] = "system";
    
       printf("returned from Example2: %d\n",
                 Example2(a,message));
    
       return 0;
    }
    Sue B.

    dazed and confused


  2. #2
    Registered User Strider's Avatar
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    Aug 2001
    Posts
    149
    You have a variable i that is never initialized and its value never changed. Try initializing i to 0 and increment its value in every recursive call.
    Code:
    #include <stdio.h>
    
    int Example2(char ch, char *str)
    {
       int result;
       int i = 0; // initialize to 0
    
       if (str[i] == '\0')
          result = 0;
       else if (ch == str[i])
           result = 1+Example2(ch,&(str[++i]));
       else
           result = Example2(ch,&(str[++i]));
       return result;
    }
    
    main()
    {
       char a = 's';
       char message[20] = "system";
    
       printf("returned from Example2: %d\n",
                 Example2(a,message));
    
       return 0;
    }
    David
    One Ring to rule them all, One Ring to find them,
    One Ring to bring them all and in the darkness bind them
    In the Land of Mordor where the Shadows lie.

  3. #3
    Registered User
    Join Date
    Sep 2001
    Posts
    752
    For the sake of redundancy....
    The lines that are actually recursive in your program access undefined memory addresses. Clearly, your Example2 function is meant to perform it's recursive step on the string that would start at the next letter...
    Code:
    int Example2(char ch, char *str)
    {
       int result;
    
       if (str[0] == '\0')
          result = 0;
       else if (ch == str[0])
           result = 1+Example2(ch, str + 1));
       else
           result = Example2(ch, str + 1));
       return result;
    }
    This is the effectively same code as Strider's of course, but I think it's important to note that all your recursive call is really supposed to do is advance the pointer by one (the terminating condition being when the pointer is at '/0').
    Callou collei we'll code the way
    Of prime numbers and pings!

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