I have a contract and need to check, if the String-Array inside a Struct is of a mapping is set. I found this answer: What is the zero value for a string? I convert my String to Bytes and check if it's unequal 0.

I tried the same. Here is my code.

contract MyContract{

   struct artist {
      string name;
      string[] songNames;
   mapping(address => artist) artists;

   function checkDefaultValue() public view returns (uint256) {
      return bytes(artists[msg.sender].name).length;


I compiled and migrated it with truffle and got the following:

BN {
  negative: 0,
  words: [
    25535600,       45482263,
    41138111,       21965320,
    15007926,       53592304,
    65526573,       9237065,
    33654519,       3241105,
    <1 empty item>
  length: 10,
  red: null

The problem is, the length is 10. Then I set the string with the following function:

   function Upload(string memory _songName) public {

But only the words changed, after I run the function: checkDefaultValue(). The length is still 10.

What am I doing wrong or how to I check, if the String is set from a specific address? And I want to do it with required. Not an if.

2 Answers 2


how about a different approach to the test, and what's returned, which might be easier.

check for length and return true/false

here I'm just passing in a string or "" as a logic tester

function isDefault(string memory _str) public pure returns (bool) {
    if(bytes(_str).length == 0) return true;
    return false;

The contract, as written, is returning zero (try it in Remix), so the focus should be on the testing methodology. Consider passing the BN to .toString() to see a lossless representation of the decimal value.

You can make a nice little pure function for this concern:

function isNull(string memory val) internal pure returns (bool) {
  return bytes(val).length == 0;

and then use it to validate input:

require(!isNull(inputStr), "input cannot be empty");

Hope it helps.

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