Expected Primary Expression (Solidity) - mapping

Question

I am creating a simple smart contract, however, I am getting an error on my last function ("ViewNotes") stating that the compiler was "Expected Primary Expression"? Can I not check the value at a mapping (of address => string) against the value 0 ?
My code:
pragma solidity ^0.4.4;
contract Logistics{
address public owner;
mapping(address => string) notes;
modifier onlyOwner() {
require(msg.sender == owner);
_;
}
constructor(address genesis) public {
owner = genesis;
}
function sign(string signedNote) public onlyOwner{
notes[owner] = signedNote; //gaurenteed that msg.sender == owner
}
function transferOwnership(address nuOwner) onlyOwner {
owner = nuOwner;
}
function viewNotes(address participant) public returns(string){ // signed note on success nothing on fail
if(notes[participant] !== 0){
return (notes(participant));
}
}
}

Answer 1

There are a couple issues. The primary issue is that you misspelled !=. (You have an extra equals sign. !== is an operator in JavaScript, but not in Solidity.)
Once you fix that, you'll find that you can't compare a string to the number 0. You probably want to check the string's length? You'll need to cast to bytes to do that:
function viewNotes(address participant) public returns (string) {
if (bytes(notes[participant]).length != 0) {
return notes[participant];
}
}
That said, I believe this is probably equivalent to just:
function viewNotes(address participant) public returns (string) {
return notes[participant];
}
And you could instead just make notes public:
mapping(address => string) public notes;
That way, Solidity will generate a getter function for you, and people can just call notes(addr), making viewNotes redundant.
Fixing up a couple other warnings, getting rid of the modifier in favor of a direct ownership check, and assigning initial ownership to the deployer, here's my take on the contract:
pragma solidity ^0.4.24;
contract Logistics{
address public owner = msg.sender;
mapping(address => string) public notes;
function sign(string note) public {
require(msg.sender == owner);
notes[owner] = note;
}
function transferOwnership(address newOwner) public {
require(msg.sender == owner);
owner = newOwner;
}
}