/*
 * Example of modular programming -- Chapter 6
 * 
 * Adds, subtracts, multiplies and divides common fractions, displaying 
 * results in reduced form.
 * 
 */

#include <stdio.h>

/* Function prototypes */
void scan_fraction(int *nump, int *denomp);
char get_operator(void); 
void add_fractions(int n1, int d1, int n2, int d2, int *n_ansp, int *d_ansp);
void multiply_fractions(int n1, int d1, int n2, int d2, 
                        int *n_ansp, int *d_ansp);
void reduce_fraction(int *nump, int *denomp);

/* Main function */ 
int
main(void)
{
    int  n1, d1, n2, d2, n_ans, d_ans; 
    char op, again; 

    do {
        /* Gets a fraction problem */
        scan_fraction(&n1, &d1);
        op = get_operator();
        scan_fraction(&n2, &d2);

        /* Computes the result  */
        switch (op) {
        case '+':  
            add_fractions(n1, d1, n2, d2, &n_ans, &d_ans);
            break;

        case '-':
            add_fractions(n1, d1, -n2, d2, &n_ans, &d_ans);
            break;

        case '*':
            multiply_fractions(n1, d1, n2, d2, &n_ans, &d_ans);
            break;

        case '/':
            multiply_fractions(n1, d1, d2, n2, &n_ans, &d_ans);
        }

        /* reduce fraction by the gcd */ 
        reduce_fraction(&n_ans, &d_ans);

        /* Displays result */
        printf("\n%d/%d %c %d/%d = %d/%d", n1, d1, op, n2, d2, n_ans, d_ans); 
        /*  Do another problem */ 
        printf("\nDo another problem? (y/n)> ");
        scanf(" %c", &again);

    } while (again == 'y'  ||  again == 'Y');

    return (0);
}