C programming language (2nd Edition) -- Reading Notes -- 1.4

Look at symbolic constants . Used in the program 300、20 And so on “ Magic number ” Not a good habit , They can hardly provide any information to people who will read the program later , And make it difficult to modify the program . One way to deal with these numbers is to give them meaningful names .#define The instruction can put the symbol name （ Symbolic constant ） Defined as a specific string ：

#define name replace text

After the definition , All that appear in the program are #define Name defined in （ It is not locked with quotation marks , Nor is it part of any other name ） Will be replaced with the corresponding replacement text . among , The name has the same form as the ordinary variable name ： A sequence of letters and numbers beginning with letters ; The replacement text can be any sequence of characters , Not limited to numbers .

#include <stdio.h>
#define LOWER 0 /* lower limit of table */
#define UPPER 300 /* upper limit of table */
#define STEP 20 /* step  size */
 /*print Fahrenheit-Celsius table*/
main()
{
	int fahr ;
	
	for (fahr = LOWER ;fahr <= UPPER; fahr = fahr + STEP )
		printf ("%3d %6.1d\n",fahr , (5.0 /9.0)*(fahr - 32));
}

among ,LOWER、UPPER And STEP Are all symbolic vectors , Not variables , So it doesn't need to appear in the declaration .

Symbolic Constant names are usually spelled in uppercase letters , It is easy to distinguish variable names spelled in lowercase letters . Be careful #define There is no semicolon at the end of the instruction line .