/* 
 * Program to copy text from input to output, changing case of letters.
 *
 * Input:  text file "in.txt".
 *
 * Output:  text file "out.txt", contents the same as "in.txt" except that
 *   uppercase letters have been changed to lowercase, and vice versa.
 */
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>

int main(void) {

	FILE * in;
	FILE * out;

	char input_char;

	/* open input file and check result */
	in = fopen("in.txt", "r");
	if (in == NULL) {
		printf("unable to open input file\n");
		/* exit program immediately */
		exit(1);
	}

	/* open output file and check result */
	out = fopen("out.txt", "w");
	if (out == NULL) {
		/* exit program immediately */
		printf("unable to open output file\n");
		exit(1);
	}

	/* read characters and copy to output, changing case */
	while (fscanf(in, "%c", &input_char) == 1) {
		/* if c is uppercase, convert to lowercase and write */
		if(isupper(input_char)) {
			fprintf(out, "%c", tolower(input_char));
		}
		/* if c is lowercase, convert to uppercase and write */
		else if (islower(input_char)) {
			fprintf(out, "%c", toupper(input_char));
		}
		/* otherwise just write */
		else {
			fprintf(out, "%c", input_char);
		}
	}

	/* close files */
	fclose(in);
	fclose(out);

	return 0;
}