can someone help out with the stated error, i am trying to run a code to check the status of a String variable. while compiling using the online solidity compiler i am getting two errors on customcheck and portcheck. see the code below:

contract tradefin {

    uint public prodcode;
    address public seller;
    address public buyer;
    uint public price;
    uint public units;
    address public port;
    address public customs;
    address public delivery;
    string public portcheck;
    string public customscheck;
    string public deliverycheck;
    string public msg1;

    function initiate(uint code, address vendor, address applicant, 
        uint amount, uint quantity, address p, address c, address d) {
        prodcode = code;
        seller = vendor;
        buyer = applicant;
        price = amount;
        units = quantity;
        port = p;
        customs = c;
        delivery = d;

    function review() constant returns (uint retval) {
        return uint(prodcode);

    function finall() constant returns (string retval) {
        return string(deliverycheck);

    function approve() constant returns (string retval) {
        if (msg.sender == port) {
            portcheck = "signed";

        if (msg.sender == customs) {
            if (portcheck == "signed")             
                customscheck = "signed";
                msg1 = "Port yet to sign";
            return msg1;

        if (msg.sender == delivery) {
            if (customscheck == "signed")
                deliverycheck = "signed";
                msg1 = "Custom yet to sign";
            return msg1;            

As in Java, the == operator does not compare the literals of two strings. You should use the StringUtils contract instead.

if (StringUtils.equal(portcheck,"signed")) {...}
  • 1
    thanks Roland that helped. i ended up trying something else (used address to validate), can you help with another question around the same code (changed a bit), you can view it at 4631 -> in your search bar. – srbamb Jun 3 '16 at 17:03

If you don't want to load the StringUtils contract, I found it's also possible to simply: keccak256(portcheck) == keccak256("signed").

Can anyone confirm that this would also cost less gas than a function which evaluates each char of both strings?

(Sorry if adding an answer to a Q with a "checked" response is frowned upon... I just noticed that this method hadn't been mentioned and thought it might be helpful!)

EDIT: Just created a quick gas mileage harness and it looks like my suspicions were right. keccak256 comparison uses dramatically less gas than character comparison.

EDIT: Originally called sha3, but the use of this function has now been deprecated in favour of keccak256 (sha3 is an alias to keccak256)

  • 2
    wow would of never thought of this. simple and it works. – Miguel Mota May 26 '17 at 1:12
  • You can also pre-compute keccak256("signed") and hardcode the value into the contract for further gas savings, right? – MathuSum Mut Apr 15 '19 at 11:58
  • 5
    keccak256(bytes(portcheck)) == keccak256(bytes("signed")) – moeabdol Jul 21 '19 at 13:10

When possible, strings should not be used to describe state.

For example, the state of a switch can be modeled with trueand false.

For cases where more possibilities exist, solidity has enums. As they are just unsigned integers under the hood, they are much easier (and cheaper) to work with.

Of course there are cases like Oraclize queries where StringUtils is necessary.

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