Solidity - Variables

A Variable is basically a placeholder for the data which can be manipulated at runtime. Variables allow users to retrieve and change the stored information. 

Rules For Naming Variables

1. A variable name should not match with reserved keywords.

2. Variable names must start with a letter or an underscore (_), and may contain letters from "a to z" or "A to Z" or digits from "0 to 9" and characters also.

Example:

Geeks123, geeks, _123geeks are valid variable names 
123geeks, $Geeks, 12_geeks are invalid variable names

3. The name of variables are case sensitive i.e.

Example:

Geeks123 and geeks123 are different variables

Declaration of Variables

In Solidity declaration of variables is a little bit different, to declare a variable the user has to specify the data type first followed by access modifier. 

Syntax:

<type> <access modifier> <variable name> ; 

Example:

int public int_var; 

Types of Variables

Solidity is a statically typed language i.e. each declared variable always has a default value based on its data type, which means there is no concept of 'null' or 'undefined'. Solidity supports three types of variables: 

1. State Variables: Values of these variables are permanently stored in the contract storage. Each function has its own scope, and state variables should always be defined outside of that scope.

Example: In the below example. the contract Solidity_var_Test initializes the values of an unsigned integer state variable using a constructor.
 

// Solidity program to 
// demonstrate state 
// variables
pragma solidity ^0.5.0;

// Creating a contract
contract Solidity_var_Test {
   
   // Declaring a state variable
   uint8 public state_var;      

   // Defining a constructor
   constructor() public {
      state_var = 16;   
   }
}

Output : 

2. Local Variable: Values of these variables are present till the function executes and it cannot be accessed outside that function. This type of variable is usually used to store temporary values.

Example: In the below example, the contract Solidity_var_Test defines a function to declare and initialize the local variables and return the sum of the two local variables.

// Solidity program to demonstrate
// local variables
pragma solidity ^0.5.0;

// Creating a contract
contract Solidity_var_Test {

   // Defining function to show the declaration and
   // scope of local variables
   function getResult() public view returns(uint){
     
      // Initializing local variables
      uint local_var1 = 1; 
      uint local_var2 = 2;
      uint result = local_var1 + local_var2;
     
      // Access the local variable
      return result; 
   }
}

Output : 
 

3. Global Variables: These are some special variables that can be used globally and give information about the transactions and blockChain properties. Some of the global variables are listed below :

Example: In the below example, the contact Test uses the msg.sender variable to access the address of the person deploying the contract.

// Solidity program to 
// show Global variables
pragma solidity ^0.5.0;

// Creating a contract
contract Test { 
  
  // Defining a variable
  address public admin;
  
  // Creating a constructor to
  // use Global variable
  constructor() public {    
    admin = msg.sender;  
  }
 }

Output: