Constant Data type

1.Constant Data Types:
                   Constants refer to fixed values that may not be altered by the program. All the data types we have previously covered can be defined as constant data types if we so wish to do so. The
constant data types must be defined before the main function. The format is as follows:
#define CONSTANTNAME value
for example:
#define SALESTAX 0.05
The constant name is normally written in capitals and does not have a semi-colon at the end. The use of constants is mainly for making your programs easier to be understood and modified
by others and yourself in the future. An example program now follows:

#define SALESTAX 0.05
#include <stdio.h>
main()
 {
   float amount, taxes, total;
   printf("Enter the amount purchased : ");
   scanf("%f",&amount);
   taxes = SALESTAX*amount;
   printf("The sales tax is £%4.2f",taxes);
   printf("\n The total bill is £%5.2f",total);
 }

The float constant SALESTAX is defined with value 0.05. Three float variables are declared amount, taxes and total. Display message to the screen is archieved using printf and user
input handled by scanf. Calculation is then performed and results sent to the screen. If the value of SALESTAX alters in the future it is very easy to change the value where it is defined rather
than go through the whole program changing the indivual values separately, which would be very time consuming in a large program with several references. The program is also improved
when using constants rather than values as it improves the clarity.