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Vedant Chainani
Vedant Chainani

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Day 4 - Variables and Scopes

This is Day 4 of 30 in Solidity Series
Today I Learned About Variables and Scopes in Solidity.

Variables

Solidity supports three types of variables.

  • State Variables โˆ’ Variables whose values are permanently stored in a contract storage.

  • Local Variables โˆ’ Variables whose values are present till function is executing.

  • Global Variables โˆ’ Special variables exists in the global namespace used to get information about the blockchain.


While naming your variables in Solidity, keep the following rules in mind.

  • You should not use any of the Solidity reserved keywords as a variable name. For example, break or boolean variable names are not valid.
  • Solidity variable names should not start with a numeral (0-9). They must begin with a letter or an underscore character. For example, 123test is an invalid variable name but _123test is a valid one.
  • Solidity variable names are case-sensitive. For example, Name and name are two different variables.

State Variable

Variables whose values are permanently stored in a contract storage.

pragma solidity ^0.8.6;

contract SolidityTest {
   uint storedData;      // State variable
   constructor() public {
      storedData = 10;   // Using State variable
   }
}
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Local Variable

Variables whose values are available only within a function where it is defined. Function parameters are always local to that function.

pragma solidity ^0.8.6;
contract SolidityTest {
   uint storedData; // State variable
   constructor() public {
      StoredData = 10;
   }
   function getResult() public view returns(uint){
      uint a = 1; // local variable
      uint b = 2;
      uint Result = a + b;
      return Result; //access the local variable
   }
}
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Global Variables

These are special variables which exist in global workspace and provide information about the blockchain and transaction properties.

Name Returns
blockhash(uint blockNumber) returns (bytes32) Hash of the given block - only works for 256 most recent, excluding current, blocks
block.coinbase (address payable) Current block miner's address
block.difficulty (uint) Current block difficulty
block.gaslimit (uint) Current block gaslimit

More on Global Variables here


Variable Scopes

Scope of local variables is limited to function in which they are defined but State variables can have three types of scopes.

  • Public โˆ’ Public state variables can be accessed internally as well as via messages. For a public state variable, an automatic getter function is generated.
  • Internal โˆ’ Internal state variables can be accessed only internally from the current contract or contract deriving from it without using this.
  • Private โˆ’ Private state variables can be accessed only internally from the current contract they are defined not in the derived contract from it.

eg-

pragma solidity ^0.8.6;
contract C {
   uint public data = 30;
   uint internal iData= 10;

   function x() public returns (uint) {
      data = 3; // internal access
      return data;
   }
}
contract Caller {
   C c = new C();
   function f() public view returns (uint) {
      return c.data(); //external access
   }
}
contract D is C {
   function y() public returns (uint) {
      iData = 3; // internal access
      return iData;
   }
   function getResult() public view returns(uint){
      uint a = 1; // local variable
      uint b = 2;
      uint result = a + b;
      return storedData; //access the state variable
   }
}
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GitHub logo Envoy-VC / 30-Days-of-Solidity

30 Days of Solidity step-by-step guide to learn Smart Contract Development.

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