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Variables in C

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C Variables
  • C variable is a named location in a memory where a program can manipulate the data. This location is used to hold the value of the variable.
  • The value of the C variable may get change in the program.
  • C variable might be belonging to any of the data type like int, float, char etc.

Rules for naming C variable:
  • Variable name must begin with letter or underscore.
  • Variables are case sensitive.
  • They can be constructed with digits, letters.
  • No special symbols are allowed other than underscore.
  • sum, height, _value are some examples for variable name.
  • Declaring & initializing C variable:
  • Variables should be declared in the C program before to use.
  • Memory space is not allocated for a variable while declaration. It happens only on variable definition.
  • Variable initialization means assigning a value to the variable.

S.No  Type  Syntax Example
1 Variable declaration data_type variable_name; int x, y, z; char flat, ch;
2 Variable initialization data_type variable_name = value; int x = 50, y = 30; char flag = ‘x’, ch=’l’;

There are three types of variables in C program They are
  • Local variable
  • Global variable
  • Environment variable

1. Example program for local variable in C:
  • The scope of local variables will be within the function only.
  • These variables are declared within the function and can’t be accessed outside the function.
  • In the below example, m and n variables are having scope within the main function only. These are not visible to test function.
  • Like wise, a and b variables are having scope within the test function only. These are not visible to main function.

Code: 
#include<stdio.h>
void test();
int main()
{
int m = 22, n = 44;
// m, n are local variables of main function
/*m and n variables are having scope
within this main function only.
These are not visible to test funtion.*/
/* If you try to access a and b in this function,
you will get 'a' undeclared and 'b' undeclared error */
printf("\nvalues : m = %d and n = %d", m, n);
test();
}
void test()
{
int a = 50, b = 80;
// a, b are local variables of test function
/*a and b variables are having scope
within this test function only.
These are not visible to main function.*/
/* If you try to access m and n in this function,
you will get 'm' undeclared and 'n' undeclared
error */printf("\nvalues : a = %d and b = %d", a, b);
}
Output: 
values : m = 22 and n = 44
values : a = 50 and b = 80

2. Example program for global variable in C:
  • The scope of global variables will be throughout the program. These variables can be accessed from anywhere in the program.
  • This variable is defined outside the main function. So that, this variable is visible to main function and all other sub functions.

Code: 
#include<stdio.h>
void test();int m = 22, n = 44;
int a = 50, b = 80;
int main()
{
printf("All variables are accessed from main function");
printf("\nvalues: m=%d:n=%d:a=%d:b=%d", m,n,a,b);
test();
}
void test()
{
printf("\n\nAll variables are accessed from" \
" test function");
printf("\nvalues: m=%d:n=%d:a=%d:b=%d", m,n,a,b);
}
Output: 
All variables are accessed from main function
values : m = 22 : n = 44 : a = 50 : b = 80

All variables are accessed from test function
values : m = 22 : n = 44 : a = 50 : b = 80

3. Environment variables in C:
  • Environment variable is a variable that will be available for all C applications and C programs.
  • We can access these variables from anywhere in a C program without declaring and initializing in an application or C program.
  • The inbuilt functions which are used to access, modify and set these environment variables are called environment functions.
  • There are 3 functions which are used to access, modify and assign an environment variable in C.
    • 1. setenv()
    2. getenv()
    3. putenv()
Example program for getenv() function in C:
This function gets the current value of the environment variable. Let us assume that environment variable DIR is assigned to “/usr/bin/test/”.

Code: 
#include <stdio.h>
#include <stdlib.h>
int main()
{
printf("Directory = %s\n",getenv("DIR"));
return 0;
}
Output: 
/usr/bin/test/

Example program for setenv() function in C:
This function sets the value for environment variable. Let us assume that environment variable “FILE” is to be assigned “/usr/bin/example.c”

Code: 
#include <stdio.h>
#include <stdlib.h>
int main()
{
setenv("FILE", "/usr/bin/example.c",50);
printf("File = %s\n", getenv("FILE"));
return 0;
}
Output 
File = /usr/bin/example.c

Example program for putenv() function in C:
This function modifies the value for environment variable. Below example program shows that how to modify an existing environment variable value.

Code: 
#include <stdio.h>
#include <stdlib.h>
int main()
{
setenv("DIR", "/usr/bin/example/",50);
printf("Directory name before modifying = " \
"%s\n", getenv("DIR"));
putenv("DIR=/usr/home/");
printf("Directory name after modifying = " \
"%s\n", getenv("DIR"));
return 0;
}
Output 
Directory name before modifying = /usr/bin/example/
Directory name after modifying = /usr/home/

Difference between variable declaration & definition in C:

 S.no   Variable declaration Variable definition
1 Declaration tells the compiler about data type and size of the variable. Definition allocates memory for the variable.
2 Variable can be declared many times in a program. It can happen only one time for a variable in a program.
3 The assignment of properties and identification to a variable. Assignments of storage space to a variable.
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