Variables in C

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Variables in C : As we saw earlier, an entity that may vary during program execution is called a variable. Variable names are the names given to locations in memory. These locations can contain integer, real or character constants or a digit. In any language, the types of variable that it can support depend on the types of constants that it can handle. This is because a particular type of variable can hold only the same type of constant.
 For example : an integer variable can hold only an integer constant a real variable can hold only a real constant and a character variable can hold only a character constant.

Upper and lowercase letters are distinct because C is case-sensitive.

Example:

int a;  
float b;  
char c;  

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Here, a, b, c are variables. The int, float, char are the data types.
We can also provide values while declaring the variables as given below:

int a=10,b=20;//declaring 2 variable of integer type  
float f=20.8;  
char c='A';  

Rules for defining Variables in C

1. A variable name is any combination of 1 to 31 alphabets, digits or underscores.Do not create unnecessarily long variable names as it adds to your typing effort.
2. The first character in the variable name must be an alphabet or underscores
3. No commas or blanks are allowed within a variable name.
4. No special symbol other than an underscore (as in gross_sal) can be used in a variable name.

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Valid variable names:

int a;  
int _ab;   

Invalid variable names:

int 2;  
int long;  

Types of Variables in C

There are many types of variables in c:

1. local variable
2. global variable
3. static variable
4. automatic variable
5. external variable

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Local Variable

A variable that is declared inside the function or block is called a local variable.

Example:

void function1(){  
int x=10;//local variable  
}  

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Global Variable

A variable that is declared outside the function or block is called a global variable. Any function can change the value of the global variable. It is available to all the functions.

Example:

int value=20;//global variable  
void function1(){  
int x=10;//local variable  
}  

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Static Variable

A variable that is declared with the static keyword is called static variable.

Example:

void function1(){  
int x=10;//local variable  
static int y=10;//static variable  
x=x+1;  
y=y+1;  
printf("%d,%d",x,y);  
}  

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Automatic Variable

All variables in C that are declared inside the block, are automatic variables by default. We can explicitly declare an automatic variable using auto keyword.

Example:

void main(){  
int x=10;//local variable (also automatic)  
auto int y=20;//automatic variable  
}  

External Variables in C

We can share a variable in multiple C source files by using an external variable. To declare an external variable, you need to use extern keyword.

myfile.h

Example:

#include "myfile.h"  
#include <stdio.h>  
void printValue(){  
    printf("Global variable: %d", global_variable);  
}  

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