[2016-06-27] Challenge #273 [Easy] Getting a degree (with bonus)

degree.c

/**
 * degree.c
 *
 * Reddit Daily Programmer Challenge #273 [EASY]
 * Getting a degree
 *
 * https://www.reddit.com/r/dailyprogrammer/comments/4q35ip/20160627_challenge_273_easy_getting_a_degree/
 * 
 */
 
#include <cs50.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>

#define pi 3.141592

char units[2];
float num;
float conversion;

float convert(float* numbers, char* conversion_units);

int main(void)
{
    while (scanf("%f%s", &num, units) == 2)
    {
        float answer = convert(&num, units);
        if (answer != -1)
            printf("%2g%c\n", answer, units[1]);
    }
    return 0;
}

float convert(float* numbers, char* conversion_units)
{
    if (!strcmp(units,"dr")) // degrees to radians
    {
        return *numbers * (pi / 180.0);
    }
    else if (!strcmp(units,"rd")) // radians to degrees
    {
        return *numbers * (180.0 / pi);
    }
    else if (!strcmp(units,"cf")) // celsius to farenheit
    {
        return *numbers * 1.8 + 32;
    }
    else if (!strcmp(units,"fc")) // farenheit to celsius
    {
        return (*numbers - 32) * 0.5556;
    }
    else if (!strcmp(units,"ck")) // celsius to kelvin
    {
        return *numbers + 273.15;
    }
    else if (!strcmp(units,"kc")) // kelvin to celsius
    {
        return *numbers - 273.15;
    }
    else if (!strcmp(units,"fk")) // farenheit to kelvin
    {
        return ((convert(numbers, "fc")) + 273.15);
    }
    else if (!strcmp(units,"kf")) // kelvin to farenheit
    {
        return *numbers * 1.8 - 459.67;
    }
    else
        printf("No candidate for conversion\n");
        return -1;
        
    return 0;
}

"fk" conversion recursive call doesn't work. Infinite loop.