box-o-sand/digressions/temperatures.c

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


double c2f(double celsius) {
    return ((9.0 / 5.0) * celsius) + 32.0;
}


double f2c(double fahrenheit) {
    return (5.0 / 9.0) * (fahrenheit - 32.0);
}


int die_usage(const char *msg) {
    printf("Usage: temperatures <temp>[fFcC]\n");
    printf("%s\n", msg);
    exit(1);
}


int main(int argc, char * argv[]) {
    if (argc < 2) {
        die_usage("");
    }

    char msg[255];
    double temp;
    char scale = '\0';
    const char * temp_string = argv[1];

    sscanf(temp_string, "%lf%c", &temp, &scale);
    switch (scale) {
        case 'c':
        case 'C':
            printf("%.2lf\n", c2f(temp));
            break;
        case 'f':
        case 'F':
            printf("%.2lf\n", f2c(temp));
            break;
        default:
            sprintf(msg, "Unknown scale '%c'", scale);
            die_usage(msg);
            break;
    }

    return 0;
}