[Ethereum] constant versus public in solidity functions


Using Mix, if I create a contract in the scenario with address A as a parameter, clicking on getConstant in the html below returns A as I would expected it, and I see a JS flagged transaction.

If now I click on getPublic which is pretty much the same function bare the constant / public keyword I got a pending transaction in the scenario and an address that is not the one I expect.

So I might NOT understand something fundamental about what public means or implies but I fail at seeing what it is.

I have set this basic contract / html for this question :

contract constantOrPublic {
    address creator;
    address otheraddress;

    function constantOrPublic(address _otheraddress) 
        creator = msg.sender;
        otheraddress = _otheraddress;

    function getConstant() constant returns(address) {
        return otheraddress;
    function getPublic() public returns(address) {
        return otheraddress;


and this html page:

<script type='text/javascript'>

function getConstant() {
    var param = document.getElementById('c').value;
    var res = contracts['constantOrPublic'].contract.getConstant();
    document.getElementById('c').innerText = res;

function getPublic() {
    var param = document.getElementById('p').value;
    var res = contracts['constantOrPublic'].contract.getPublic();
    document.getElementById('p').innerText = res;

<body >
    <div id='c'>none</div>
    <button onclick='getConstant()'>getConstant</button>
    <div id='p'>none</div>
    <button onclick='getPublic()'>getPublic</button>

Best Answer

I'm not sure which part of the system assumes this (web3.js?), but if you don't declare a a function as constant it's assumed it's going to change contract state and thus sets up a transaction.

The declaration should look like this (note, I didn't compile it):

function getPublic() public constant returns(address) {

I checked the solidity docs, I'm not sure public is a useful keyword for a function, because public is the default.

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