/* This program tries to demonstrate the use of the command line arguments and file I/O.
 * Command line syntax is:
 *
 *  ./a.out N [input-file [output-file]]
 *
 * where N is the number of strings to read, input-file is the optional name of the input file (defaults to stdin).  If
 * output-file is specified, the input file must also be specified. The output file defaults to stdout if not specified.
 *
 * For example:
 *
 *  ./a.out 3
 *  ./a.out 3 inputfile1
 *  ./a.out 3 inputfile1 outputfile1
 */
#include <stdio.h>

int main( int argc, char *argv[] ) {
    char buffer[20];
    FILE * in = stdin;    /* in is initially set to standard input   */
    FILE * out= stdout;   /* out is initially set to standard output */
    int number = 0;
    int i;

    if ( argc > 4 ) {
	fprintf(stderr,"Specified too many arguments\n");
	exit(-2);
    } else if ( argc < 2 ) {
	fprintf(stderr,"Did not specify required argument\n");
	exit(-2);
    } /* if */
    
    /* Note how the "break" statement only occurs in the lowest switch statement.
    * This means that if argc = 4, the statements for case 4, 3, and 2 will be executed.
     * If argc = 3, the statements for case 3 and 2 will be executed, etc.
     */
    switch (argc) {
      case 4:
	out = fopen( argv[3], "w" );
	if ( out == NULL ) {
	    fprintf(stderr,"Error! Could not open the output file %s\n", argv[3]);
	    exit(-3);
	} /* if */
      case 3:
	in = fopen( argv[2], "r" );
	if ( in == NULL ) {
	    fprintf(stderr,"Error! Could not open the input file %s\n", argv[2]);
	    exit(-3);
	} /* if */
      case 2:
	number = atoi( argv[1] );
	break;	
    } /* switch */

    if ( in == stdin ) {
	printf("Enter a list of words, each under 20 characters in length.\n");
	printf("Press <Ctrl>-d to signal EOF\n");
    } /* if */
    
    for ( i = 0; i < number; i += 1 ) {
	if ( fscanf(in, "%s", buffer) == EOF ) break;
	fprintf(out, "%s\n", buffer);
    } /* for */
}