copy = concatenate ?

This is a discussion on copy = concatenate ? within the C Programming forums, part of the General Programming Boards category; As a newbye, I've justed started writing some code. Instead of writing 2 different functions for COPY and CONCATENATE string, ...

  1. #1
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    Question copy = concatenate ?

    As a newbye, I've justed started writing some code.
    Instead of writing 2 different functions for COPY and CONCATENATE string, I'm trying one that will do both:
    a) if destination is empty then it will be a simple copy from original string to destination,
    b) if destination is not empty, then a concatenation applies.

    So, my code is as follows:

    #include <stdio.h>


    // a CONCATENATE function is similar to a COPY function, with the only
    // difference that the copy to destination starts on first empty index

    Code:
    char *f_strcpy(char *v_string)
    
    {
    
      int length = 0; // determines what is actual length of destination string
      int i;
      char dest[100];
    
      // check for length, ignoring termination character
      for(i=0; dest[i]!='\0'; i++)
           ;
    
      length = i; // 
    
      // starts concatenation (or simple copy if destination is empty)
      {
        for(i=0; v_string[i]!='\0'; i++)
          dest[length+i]=v_string[i];  // leaves cycle as soon as ='\0'
      }
    
      dest[length+i] = '\0';     // assigns terminator after leaving cycle - otherwise
    
      return dest;
    }
    Now, if i run this code, it returns some strange characters.

    Any ideas?
    cheers,
    Arr

  2. #2
    Registered User SKeane's Avatar
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    You are returning a pointer to variable (dest) that is local to your function. If you insist on using a fixed length character array, make it static, otherwise use malloc to create a dynamically allocated character array which you can safely return.

    You are also scanning though dest when it isn't initialised.

  3. #3
    CSharpener vart's Avatar
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    1. dest is not initialized - undefined behavior of the loop checking for the length
    2. returning pointer to the local variable - memory is destroyed on function exit, pointer is dan
    gling
    3. You don't check the destination buffer length (possible memory overrun).
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  4. #4
    ATH0 quzah's Avatar
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    You are using dest uninitialized. Aren't you supposed to be counting v_string's length instead?
    You can't return local variables. You'll need to use something like malloc.

    [edit]
    Curses, foiled again!
    [/edit]

    [edit2]
    Your problem is the concept of your function, really. What exactly are you trying to copy / fill? If "it's" empty... What's "it"? You imply that you have some kind of pointer that you need to be adding to. If you already have a string you want to add to, why aren't you passing both the one you want to add to and the one you are adding from?

    You really need two arguments. If the first is NULL, duplicate the second. If it's not, then stick the two together.
    [/edit2]

    Quzah.
    Last edited by quzah; 11-03-2006 at 04:04 PM.
    Hope is the first step on the road to disappointment.

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