Thread: Help replacing words in a file

I'm trying to write a program that will open an existing file supplied by the command line argument and then replace words with "We" or "we" by "I" and "a" or "A" by "The". When I run the program it reads the file, changes the word but re writes it on a new line with only the replaced words not the whole sentence. Would anyone be able to help me?

Code:

 
#include        <stdio.h>
#include	<stdlib.h>
#include        <unistd.h>
#include        <fcntl.h>

#define BUFFERSIZE      4096

 main(int ac, char *av[])
{
        int     in_fd, n_chars;
        char    buf[BUFFERSIZE];
        char x;
                                        /* check args   */
        if ( ac != 2 ){
                fprintf( stderr, "usage: %s source destination\n", *av);
                exit(1);
        }
                                                /* open files   */

        if ( (in_fd=open(av[1], O_RDWR)) == -1 )
        	printf("Cannot open ", av[1]);


	while ( (n_chars = read(in_fd , buf, BUFFERSIZE)) > 0 ){


	for  (x=0; x<n_chars; x++)
	{


		if (buf[x] ==  'W') if (buf[x+1] == 'e') if (buf[x+2]=' ')
			{
				lseek(in_fd, 0, SEEK_CUR);
				write(in_fd, "I", strlen("I"));
			}
		if (buf[x] == 'w') if (buf[x+1] == 'e') if (buf[x+2]=' ')
			{
				lseek(in_fd, 0, SEEK_CUR);
				write(in_fd, "I", strlen("I"));
			}
		if (buf[x] == 'A') if (buf[x+1] == ' ')
			{
				lseek(in_fd, 0, SEEK_CUR);
				write(in_fd, "The", strlen("The"));
			}
		if (buf[x] == 'a') if (buf[x+2] == ' ')
			{
				lseek(in_fd, 0, SEEK_CUR);
				write(in_fd, "the", strlen("the"));
			}	
                write( in_fd, &buf[x], 1 );			
	}

	}
        if ( n_chars == -1 )
        	printf("Read error from ", av[1]);
        if ( close(in_fd) == -1)
                printf("Error closing files","");




}