Variables in C

• A variable is a name of memory location. 
• It is used to store data. Its value can be changed and it can be reused many times.
• It is a way to represent memory location through symbol so that it can be easily identified.

Let's see the syntax to declare a variable:

1. type variable_list;  

The example of declaring variable is given below:

1. int a;  
2. float b;  
3. char c;  

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

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

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 local variable.
• It must be declared at the start of the block.

1. void function1(){  
2. int x=10;//local variable  
3. }  

• You must have to initialize the local variable before it is used.

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Global Variable

• A variable declared outside of the function or block is known as global variable. Any function can change the global variable's value. It's available to all of the positions.
• It must be declared at the start of the block.

1. int value=20;//global variable  
2. void function1(){  
3. int x=10;//local variable  
4. }  

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Static Variable

• A variable that is declared with static keyword is called static variable.
• It retains its value between multiple function calls.

1. void function1(){  
2. int x=10;//local variable  
3. static int y=10;//static variable  
4. x=x+1;  
5. y=y+1;  
6. printf("%d,%d",x,y);  
7. }  

• If you call this function many times, local variable will print the same value for each function call e.g, 11,11,11 and so on. 
• But static variable will print the incremented value in each function call e.g. 11, 12, 13 and so on.

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Automatic Variable

• All variables in C that is declared inside the block, are automatic variables by default. By we can explicitly declare automatic variable using auto keyword.

1. void main(){  
2. int x=10;//local variable (also automatic)  
3. auto int y=20;//automatic variable  
4. }  

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External Variable

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

myfile.h

1. extern int x=10;//external variable (also global)  

program1.c

1. #include "myfile.h"  
2. #include <stdio.h>  
3. void printValue(){  
4.     printf("Global variable: %d", global_variable);  
5. }