Data Types Part II

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Objectives

So far we have looked at local variable now we switch our attention to
other types of variables supported by the C programming language:

1. Global Variables



2. Constant Data Types

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Global variables

Variables can be declared as either local variables which can be used
inside the function it has been declared in (more on this in further sections)
and global variables which are known throughout the entire program. Global
variables are created by declaring them outside any function. For example:

int max;

 main()
  {
    .....
  }
 f1()
  {
    .....
  }

The int max can be used in both main and function f1 and any
changes made to it will remain consistent for both functions. The understanding
of this will become clearer when you have studied the section on functions but I
felt I couldn't complete a section on data types without mentioning global
and local variables.

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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 achieved 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 individual 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.