Solidity - Functions

A function is a group of reusable code which can be called anywhere in your program. This eliminates the need of writing the same code again and again. It helps programmers in writing modular codes. Functions allow a programmer to divide a big program into a number of small and manageable functions.
Like any other advanced programming language, Solidity also supports all the features necessary to write modular code using functions. This section explains how to write your own functions in Solidity.
Before we use a function, we need to define it. The most common way to define a function in Solidity is by using the function keyword, followed by a unique function name, a list of parameters (that might be empty), and a statement block surrounded by curly braces.
The basic syntax is shown here.
function function-name(parameter-list) scope returns() {
   //statements
}

Try the following example. It defines a function called getResult that takes no parameters −
pragma solidity ^0.5.0;

contract Test {
   function getResult() public view returns(uint){
      uint a = 1; // local variable
      uint b = 2;
      uint result = a + b;
      return result;
   }
}
To invoke a function somewhere later in the Contract, you would simply need to write the name of that function as shown in the following code.
Try the following code to understand how the string works in Solidity.
pragma solidity ^0.5.0;

contract SolidityTest {   
   constructor() public{       
   }
   function getResult() public view returns(string memory){
      uint a = 1; 
      uint b = 2;
      uint result = a + b;
      return integerToString(result); 
   }
   function integerToString(uint _i) internal pure 
      returns (string memory) {
      
      if (_i == 0) {
         return "0";
      }
      uint j = _i;
      uint len;
      
      while (j != 0) {
         len++;
         j /= 10;
      }
      bytes memory bstr = new bytes(len);
      uint k = len - 1;
      
      while (_i != 0) {
         bstr[k--] = byte(uint8(48 + _i % 10));
         _i /= 10;
      }
      return string(bstr);//access local variable
   }
}
Run the above program using steps provided in Solidity First Application chapter.
0: string: 3

Till now, we have seen functions without parameters. But there is a facility to pass different parameters while calling a function. These passed parameters can be captured inside the function and any manipulation can be done over those parameters. A function can take multiple parameters separated by comma.
Try the following example. We have used a uint2str function here. It takes one parameter.
pragma solidity ^0.5.0;

contract SolidityTest {   
   constructor() public{       
   }
   function getResult() public view returns(string memory){
      uint a = 1; 
      uint b = 2;
      uint result = a + b;
      return integerToString(result); 
   }
   function integerToString(uint _i) internal pure 
      returns (string memory) {
      
      if (_i == 0) {
         return "0";
      }
      uint j = _i;
      uint len;
      
      while (j != 0) {
         len++;
         j /= 10;
      }
      bytes memory bstr = new bytes(len);
      uint k = len - 1;
      
      while (_i != 0) {
         bstr[k--] = byte(uint8(48 + _i % 10));
         _i /= 10;
      }
      return string(bstr);//access local variable
   }
}
Run the above program using steps provided in Solidity First Application chapter.
0: string: 3

A Solidity function can have an optional return statement. This is required if you want to return a value from a function. This statement should be the last statement in a function.
As in above example, we are using uint2str function to return a string.
In Solidity, a function can return multiple values as well. See the example below −
pragma solidity ^0.5.0;

contract Test {
   function getResult() public view returns(uint product, uint sum){
      uint a = 1; // local variable
      uint b = 2;
      product = a * b;
      sum = a + b;
  
      //alternative return statement to return 
      //multiple values
      //return(a*b, a+b);
   }
}
Run the above program using steps provided in Solidity First Application chapter.
0: uint256: product 2
1: uint256: sum 3


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