1.c

#include <stdio.h>
#include <assert.h>
#include <stdbool.h>

# define LEN(x) (sizeof(x) / sizeof(*x))

enum Product { COFFE = 0, TEA = 1, CHOCOLATE = 2, CAPPUCCINO = 3, LAST_PRODUCT };
const char *const products_str[] = { "caffe'", "the", "cioccolata", "cappuccino", };
const int products_price[] = { 50, 40, 60, 70 };

const int coins[] = { 1, 5, 10, 20, 50 };

void print_numbered_strings(const char *const products[], size_t len) {
    for (size_t i = 0; i < len; ++i) {
        printf("%zu. %s\n", i, products[i]) ;
    }
}

int input_int() {
    int ret;
    scanf("%d", &ret);
    return ret;
}

int input_int_range(int lower, int upper) {
    int ret;
    while ((ret = input_int()) < lower || ret > upper) {
        puts("Valore non ammesso");
    }
    return ret;
}

bool is_in_array(int needle, const int haystack[], size_t len) {
    bool found = false;
    // could use memmem(3) instead
    for (size_t i = 0; i < len && !found; i++) {
        if (haystack[i] == needle) { found = true; }
    }
    return found;
}

int input_int_filter(const int whitelist[], size_t len) {
    int ret;
    while (ret = input_int(), !is_in_array(ret, whitelist, len)) {
        puts("Valore non ammesso");
    }
    return ret;
}

void calculate_change(int total, int *count_5, int *count_1) {
    *count_5 = total / 5;
    total -= 5 * *count_change;

    *count_1 = total;
}

int main(void) {
    assert(LEN(products_str) == LEN(products_price));
    assert(LEN(products_str) == LAST_PRODUCT);

    int selected_product;
    int credit = 0;
    int inserted_coin;
    int count_change_5 = 0, count_change_1 = 0;

    puts("Seleziona il prodotto");
    print_numbered_strings(products_str, LEN(products_str));
    selected_product = input_int_range(0, LAST_PRODUCT);

    printf("Costo: %d centesimi\n", products_price[selected_product]);

    puts("Inserisci monete");
    while (credit < products_price[selected_product]) {
        inserted_coin = input_int_filter(coins, LEN(coins));
        credit += inserted_coin;
    }

    calculate_change(
            credit - products_price[selected_product],
            &count_change_5, &count_change_1
    );

    printf("Resto: %d monete da 5, %d monete da 1\n", count_change_5, count_change_1);

    return 0;
}