box-o-sand/digressions/temperatures.c

#include <stdio.h>
#include <stdlib.h>
#include <string.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 usage(const char *msg) {
    printf("Usage: temperatures <temp>[fFcC]\n");
    if (msg != NULL) {
        printf("%s\n", msg);
    }
    return 1;
}


int main(int argc, char * argv[]) {
    if (argc < 2) {
        usage(NULL);
        return 2;
    }

    int ret;
    char msg[255];
    double temp;
    char scale = '\0';
    const char *in_fmt = "%lf%c";
    const char *out_fmt = "%.2lf%c\n";

    if (((int)strlen(argv[1]) == 1) && (argv[1][0] == '-')) {
        ret = fscanf(stdin, in_fmt, &temp, &scale);
    } else {
        const char * temp_string = argv[1];
        ret = sscanf(temp_string, in_fmt, &temp, &scale);
    }

    if ((ret == 0) || (ret == EOF)) {
        usage("No temperature provided!");
        return 1;
    }

    if (ret == 1) {
        usage("No scale provided!");
        return 1;
    }

    switch (scale) {
        case 'c':
        case 'C':
            printf(out_fmt, c2f(temp), 'F');
            break;
        case 'f':
        case 'F':
            printf(out_fmt, f2c(temp), 'C');
            break;
        default:
            sprintf(msg, "Unknown scale: '%c'", scale);
            usage(msg);
            break;
    }

    return 0;
}