How to do it…

  1. To check the balance of tokens in each address, the usual balanceOf function from ERC-20 is present in the standard. It accepts an address as input and returns the number of non-fungible tokens owned by the address. Create the same function to store the balances:

function balanceOf(address _owner) external view returns (uint256);
  1. Create a mapping with the address as the key and count as the value to store the number of tokens:
mapping (address => uint256) internal ownedTokensCount;
  1. Modify the balanceOf function to read the value from the mapping:
function balanceOf(address _owner) public view returns (uint256) {    require(_owner != address(0));    return ownedTokensCount[_owner];}
  1. Since the ownership of each token can be ...

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