Converting between different units of temperature

Question

The task is to convert a temperature in a list of Fahrenheit, Celsius, or Kelvin to the two other temperatures in the list.

Here is my code:

/*
 * #define LANG C
 */
#include <stdio.h> /** standard io **/
#include <stdlib.h> /** exit(), atof and the lot **/
#include <unistd.h> /** needed for getopt() **/
/*** SYMBOIC CONSTANTS ***/
#define VERSION "1.0" /** version string array **/
#define HELP help(argv[0]) /** will insert this instead of help(argv[0]) every time **/
#define FLOATARG atof(optarg) /** atof is kind of vague, so a #define is better **/
#define TOVERH "FAHR\tCELSIUS\tKELVIN\n" /** just so I don't have to type it out ever time **/
#define OPTS "vk:f:c:h" /** I prefer a #define to this -> b:ull:s::hi:t: **/
/** For reference, an option followed by a double colon **/
/** signifies that it takes no arguments **/
/** while an option followed by one colon signifies **/
/** that it takes one argument. **/
static const char *ver = VERSION;
void help(char *s);
int check(float N) { /** check if kelvin temperature is celsius 0 **/
 if(N > 273.1499 && N < 273.15001) { /** I had to set this limit. There is no other way. **/
 return 1;
 }
 else {
 return 0;
 }
}
/*** Fahrenheit ***/
float FK(float F) {
 return(((5.0 / 9.0) * (F - 32.0)) + 273.15);
}
float FC(float F) {
 return((5.0 / 9.0) * (F - 32.0));
}
void printfahr(float F) {
 printf(TOVERH);
 printf("%4.3f\t%5.3f\t%5.3f\n", F, FC(F), FK(F));
}
/** Celsius **/
float CK(float C) {
 return((C + 273.15));
}
float CF(float C) {
 return((C * (9.0 / 5.0) - 32.0));
}
void printc(float C) {
 printf(TOVERH);
 printf("%4.3f\t%5.3f\t%5.3f\n", CF(C), C, CK(C));
}
/*** Kelvin ***/
float KC(float K) {
 return(K - 273.15);
}
float KF(float K) {
 return((K - 273.15) * (9.0 / 5.0) - 32.0);
}
void printk(float K) {
 printf(TOVERH);
 printf("%4.3f\t%5.3f\t%5.3f\n", KF(K), KC(K), K);
}
int main(int argc, char *argv[]) {
 if(argc == 1) {
 HELP;
 exit(argc);
 }
 else {
 char arg = '0円';
 while((arg = getopt(argc, argv, OPTS)) != -1)
 {
 switch(arg) {
 case 'c':
 if(optarg == NULL)
 {
 HELP;
 exit(1);
 }
 else {
 printc(FLOATARG);
 }
 break;
 case 'f':
 if(optarg == NULL)
 {
 HELP;
 exit(1);
 }
 else {
 printfahr(FLOATARG);
 }
 break;
 case 'h':
 HELP;
 break;
 case 'k':
 if(optarg == NULL)
 {
 HELP;
 exit(1);
 }
 else {
 if(check(FLOATARG) != 0)
 {
 printf(TOVERH);
 printf("32.0\t0\t273.15\n");
 }
 else {
 printk(FLOATARG);
 }
 }
 break;
 case 'v':
 printf("%s\n", ver);
 break;
 default:
 HELP;
 break;
 }
 }
 }
 return 0;
}
void help(char *s) {
 printf("Usage:\t%s [-c Celsius] [-f Fahrenheit] [-h] [-k Kelvin] [-v]\n", s);
}

Answer 1

• Numerical consts are double, either add them eg as 5.0f or use double.
• Define a constants for 0°C = 273.15K and also for Fahrenheit to Celsius conversation (avoid repeated magic constants 273.15, 9.0 / 5.0, 32.0).
• In check the distance from 273.15 is not symmetric, probably a typo.
• The name check doesn't imply what is checked, meaningful names not limited to this function would be a recommended.

A modified version could include:

#include <math.h>
const double Celsius2KelvinShift = 273.15;
const double Fahrenheit2KCelsiusShift = 32.0;
const double Fahrenheit2CelsiusFactor = 5.0 / 9.0;
/** check if kelvin temperature is celsius 0 **/
int checkIf0Celsius(double N)
{
 const double tol = 0.001; /** I had to set this limit. There is no other way. **/
 return fabs(N - Celsius2KelvinShift) < tol;
}
/*** Fahrenheit ***/
double Fahrenheit2Kelvin(double F) 
{
 return Fahrenheit2CelsiusFactor * (F - Fahrenheit2KCelsiusShift) + Celsius2KelvinShift;
}
double Fahrenheit2Celsius(double F) 
{
 return Fahrenheit2CelsiusFactor * (F - Fahrenheit2KCelsiusShift);
}
/** Celsius **/
double Celsius2Kelvin(double C) 
{
return C + Celsius2KelvinShift;
}
double Celsius2Fahrenheit(double C) 
{
 return C / Fahrenheit2CelsiusFactor - Fahrenheit2KCelsiusShift;
}
/*** Kelvin ***/
double Kelvin2Celsius(double K) 
{
 return K - Celsius2KelvinShift;
}
double Kelvin2Fahrenheit(double K) {
 return (K - Celsius2KelvinShift) / Fahrenheit2CelsiusFactor - Fahrenheit2KCelsiusShift;
}

• Distant Graphics: Good answer. As for the asymmetry, I set it because I wasn't sure how far to go; I saw that 273.15 as a float was in fact being stored as 273.1994 or something like that; I originally intended to ask on SO why this was the case, but... I got a 6-Month ban because too many people downvoted my questions there :/