Symbolic Constants

A final observation before we leave temperature conversion forever. It's bad practice to bury "magic numbers" like 300 and 20 in a program; they convey little information to someone who might have to read the program later, and they are hard to change in a systematic way. One way to deal with magic numbers is to give them meaningful names. A "#define" line defines a symbolic name or symbolic constant to be a particular string of characters:

#define *name replacement list*

Thereafter, any occurrence of name (not in quotes and not part of another name) will be replaced by the corresponding replacement text. The name has the same form as a variable name: a sequence of letters and digits that begins with a letter. The replacement text can be any sequence of characters; it is not limited to numbers.

#include <stdio.h>

#define LOWER 0    /* lower limit of table */
#define UPPER 300  /* upper limit */
#define STEP 20    /* step size */

/* print Fahrenheit-Celsius table */
int main() {

    int fahr;
    for (fahr = LOWER; fahr <= UPPER; fahr = fahr + STEP)
        printf("%3d %6.1f\n", fahr, (5.0/9.0)*(fahr-32));
    return 0;
}

The quantities LOWER, UPPER and STEP are symbolic constants, not variables, so they do not appear in declarations. Symbolic constant names are conventionally written in upper case so they can be readily distinguished from lower case variable names. Notice that there is no semicolon at the end of a #define line.

Note
Though it is definitely valid to use this approach, by doing so you are obscuring the potential type of the value. LOWER could be an int in one place or a float in another. Because of this it's better to keep definitions for actual constants like Pi or Tau or the speed of light. In all other cases, a clear type with a value at the top of your code or main function might be a better idea.