temperature.c

/// temperature
///
/// a CLI tool to convert between celsius and fahrenheit.
/// concepts: data types, getopt

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

float convert(char *str) {
    char *e;
    float f;

    errno = 0;

    f = strtof(str, &e);

    if (errno != 0 || str == e) {
        return 0.0;
    } else {
        return f;
    }
}

int main(int argc, char **argv) {
    enum { UNSET, FAHRENHEIT, CELSIUS } unit = UNSET;
    float temperature;

    short option;
    int option_index = 0;

    static struct option long_options[] = {
        {"celsius", required_argument, 0, 'c'},
        {"fahrenheit", required_argument, 0, 'f'},
    };

    while ((option = getopt_long(argc, argv, "c:f:", long_options,
                                 &option_index)) > -1) {
        switch (option) {
            case 'f':
                temperature = convert(optarg);
                unit = CELSIUS;
                break;
            case 'c':
                temperature = convert(optarg);
                unit = FAHRENHEIT;
                break;
            case '?':
                fprintf(stderr, "invalid option: %c.\n", optopt);
                return 1;
            default:
                fprintf(stderr, "unknown option: %c.\n", option);
                return 1;
        }
    }

    if (unit == UNSET) {
        fprintf(stderr, "at least one flag required.\n");
        return 1;
    } else if (unit == FAHRENHEIT) {
        printf("%.1f°F\n", temperature * 9 / 5 + 32);
    } else {
        printf("%.1f°C\n", (temperature - 32) * 5 / 9);
    }

    return 0;
}