[Ethereum] Gas cost call vs send for pure/view functions

gaspuresolidityview

I'm aware of the fact that pure/view functions cost no gas with a simple "call".
But what happens if i have a function that changes the contract state based on the pure/view function result. Do i have to pay gas for the pure/view functions looping if i do a "send"?

In my example I want to find a number in an array and return the index.

function indexOfUint(uint256[] _array, uint _value) public pure returns (uint256) {
    bool exist;
    for (uint i=0; i<_array.length;i++){
        if(_array[i] == _value){
            exist = true;
            return i;
        }
    }
    assert(exist == true);
}

Best Answer

There are two statements in https://github.com/ethereum/solidity/issues/992

the keyword view is introduced for functions (it replaces constant). Calling a view cannot alter the behavior of future interactions with any contract. This means such functions cannot use SSTORE, cannot send or receive ether and can only call other view or pure functions.

the keyword pure is introduced for functions, they are view functions with the additional restriction that their value only depends on the function arguments. This means they cannot use SSTORE, SLOAD, cannot send or receive ether, cannot use msg or block and can only call other pure functions.

It means view and pure can't change state of contract. If you need to change state of contract you need to remove pure, view keywords.

function indexOfUint(uint256[] _array, uint _value) public pure returns (uint256) {
    bool exist;
    ...
}

The function does not change state of contract. exist variable stores in stack

Related Solutions

Solidity – Handling Unimplemented Functions in Contracts

The problem is that totalSupply() implementation is missing. Seems like autogenerated getters do not count. So you need to explicitly create the function.

contract StandardToken is Token {
    uint256 _totalSupply;

    function totalSupply() constant returns (uint256 totalSupply) {
        totalSupply = _totalSupply;
    }
}

solidity – How to Optimize Gas Usage with View and Pure Functions?

As I understand,

Pure and View functions don't cost any gas to call if they're called externally from outside

In this type of case, there won't be any transaction initiated because this will be like just querying the blockchain for its current state and nothing will be changed.

But they do cost gas if called internally by another function.

This means there's already a transaction to change the state of the blockchain, and that process of changing state need to use that pure function, say for calculations. Gas cost for a transaction depends on the number of EVM opcodes executed while completing it, so executing that pure function is also within that set of opcodes. That's why it's said that it costs gas.

Consider the following contract:

pragma solidity ^0.4.24;


contract PureFunctionTest {

   uint state;

    function addNumbers(uint a, uint b) public pure returns (uint) {
       return a +b ;
   }

   function updateState(uint a, uint b) public {
       
     uint c = addNumbers(a,b);
     state = c;
   }

   function addThreeNumbers(uint a, uint b, uint c) public pure returns (uint) {
       
       uint temp = addNumbers(a,b);
       uint num  = addNumbers(temp,c);
       return num;
   }
}

Just calling addNumbers won't cost anything. But calling updateState will cost including cost to addNumbers(a,b) as well. Calling addThreeNumbers won't cost gas even if it called addNumbers internally since no transaction is needed throughout the function call.

Best Answer

Related Solutions

Solidity – Handling Unimplemented Functions in Contracts

solidity – How to Optimize Gas Usage with View and Pure Functions?

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