Units conversion example in C

example.c

#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>

struct conversion {
    const char *from, *to;
    double scale, offset;
} conversions[] = {
    { "Celsius", "Fahrenheit", 9./5, 32 },
    { "Fahrenheit", "Celsius", 5./9, -32 * 5./9 },
    { "Kilometers", "Miles", 1/1.609344, 0 },
    { "Miles", "Kilometers", 1.609344, 0 },
    { "Kilograms", "Pounds", 1/0.45359237, 0 },
    { "Pounds", "Kilograms", 0.45359237, 0 }
};

void checked_scanf(const char *fmt, ...)
{
    va_list varargs;
    va_start(varargs, fmt);
    
    for(char line[256]; fgets(line, sizeof(line), stdin); ) {
        if(vsscanf(line, fmt, varargs) == 1) {
            return;
        }
        puts("Invalid input, try again.");
    }
    puts("EOF encountered without valid input, giving up.");
    exit(1);
}

int main(void)
{
    const int num_conversions = sizeof(conversions) / sizeof(conversions[0]);

    for(;;) {
        puts("What would you like to convert?");
        for(int i = 0; i < num_conversions; i++) {
            printf("%d. %s to %s\n", i + 1, conversions[i].from, conversions[i].to);
        }

        int choice;
        for(;;) {
            checked_scanf("%d", &choice);       
            if(choice >= 1 && choice <= num_conversions) {
                --choice;
                break;
            }
            puts("Invalid choice, try again.");
        }
        
        double low, high, step;
        printf("Please enter a lower limit: ");
        checked_scanf("%lf", &low);
        printf("Please enter an upper limit: ");
        checked_scanf("%lf", &high);
        printf("Please enter the increment value: ");
        checked_scanf("%lf", &step);

        printf("\n%10s %10s\n---------- ----------\n", conversions[choice].from, conversions[choice].to);
        for(double val = low; val <= high; val += step) {
            double converted = val * conversions[choice].scale + conversions[choice].offset;
            printf("%10.2f %10.2f\n", val, converted);
        }
        
        puts("\nWould you like to convert something else? y/n");
        char quit[2];
        checked_scanf("%1[yYnN]", &quit);
        if(quit[0] == 'n' || quit[0] == 'N') break;
    }
    return 0;
}