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Help to change a file extension

This is a discussion on Help to change a file extension within the C Programming forums, part of the General Programming Boards category; This is my code: Code: #include <stdio.h> #include <stdlib.h> int parse (char **buf, const char *src, const char *ext) { ...

  1. #1
    Registered User Joelito's Avatar
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    Help to change a file extension

    This is my code:
    Code:
    #include <stdio.h>
    #include <stdlib.h>
    
    int parse (char **buf, const char *src, const char *ext)
    {
    	const char *cpy = src; /* make a copy */
    	int len = 0; /* total size of buf */
    	int pos = 0; /* grab the last dot found */
    	int index = 0; /* current iterator character */
    	int stop = 1; /* stop the loop */
    	while (*cpy)
    	{
    		/* find the last dot in the string */
    		if (*cpy == '.')
    		{
    			pos = index; /* save the position */
    		}
    		cpy++; /* next char */
    		index++; /* len of the string */
    	}
    	len = pos; /* save the position of the last dot in the string */
    	index = 0;/* reset the counter */
    	for (;*src || !stop;pos++,src++)
    	{
    		/* now locate the position of the dot to ... */
    		if (pos == index)
    		{
    			/* copy the new file extension */
    			while (*ext)
    			{
    				**buf = *ext;
    				ext++;
    				*buf++;
    				len++;
    			}
    			**buf++ = '\0'; /* add null terminated char */
    			stop = 0; /* since we done, stop the loop */
    		}
    		**buf = *src; /* copying ... */
    		*buf++;
    	}
    	return len; /* return len of bufy */
    }
    
    int main (int argc, char *argv[])
    {
    	char *bufy = (char *) malloc (256);
    	int ret = parse (&bufy, "/home/joel/foo.flv", "mp3");
    	printf ("New path: %s\n", bufy);
    	free (bufy);
    	return 0;
    }
    "bufy", should return something like: /home/joel/foo.mp3, but instead it crashes
    Any ideas?
    * PC: Intel Core 2 DUO E6550 @ 2.33 GHz with 2 GB RAM: Archlinux-i686 with xfce4.
    * Laptop: Intel Core 2 DUO T6600 @ 2.20 GHz with 4 GB RAM: Archlinux-x86-64 with xfce4.

  2. #2
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    Holy crap... I generally just use...
    Code:
    char *x;
    
    x = strrchr(string,'.');
    strcpy(x,extension);
    Last edited by CommonTater; 04-28-2011 at 09:57 AM.

  3. #3
    Registered User Joelito's Avatar
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    Yes, I know...but I'm trying to do it for myself
    plus, learn about to use "&buffy"
    Last edited by Joelito; 04-28-2011 at 10:21 AM.
    * PC: Intel Core 2 DUO E6550 @ 2.33 GHz with 2 GB RAM: Archlinux-i686 with xfce4.
    * Laptop: Intel Core 2 DUO T6600 @ 2.20 GHz with 4 GB RAM: Archlinux-x86-64 with xfce4.

  4. #4
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    Quote Originally Posted by Joelito View Post
    Yes, I know...but I'm trying to do it for myself
    plus, learn about to use "&buffy"
    Then I suggest you learn to think in smaller blobs.... Write a simple function to find the dot and return a pointer. Write a second function to copy the extension to the pointer from the first function. Functions are pest kept to single purposes... one calculation, one operation, etc. Where you get into trouble is when you try to write one function that does 3245 different things all at once...

    For example, your find last dot function could look something like this...
    Code:
    char * FindLastDot(char* str)
      { char *ptr = str;
         char *dot = NULL;
    
         while(*ptr++)
           { if (*ptr == '.')
                dot = ptr; }
     
        return dot; }
    Smaller blobs make for easier coding and much easier debugging.
    Last edited by CommonTater; 04-28-2011 at 10:30 AM.

  5. #5
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    Congratulations Joelito for trying to do this by yourself. But there were a lot of problems with your code.
    - the parse() function should have char *buf as its first parameter. Not **
    - any assigns should be *buf = , not **buf =
    - There is no need to have a nested while loop inside the for loop.
    - if (pos == index) should be if (index > len)
    - you need to put the *buf = *src in an else block
    - you need to increment index in the loop
    - do not put null terminator inside the loop. Put *buf = '\0' outside at the end
    CommonTater likes this.

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