Declare map into function throws Data location must be "storage", "memory" or "calldata" for variable, but none was given

Question

I have this function to check if an array has duplicates:

function hasDuplicates(uint256[] memory _array) internal pure returns (bool) {
    mapping(uint256 => bool) seen;

    for (uint256 i = 0; i < _array.length; i++) {
        if (seen[_array[i]]) {
            return true;
        } else {
            seen[_array[i]] = true;
        }
    }
    return false;
}

but I get Data location must be "storage", "memory" or "calldata" for variable, but none was given. when I try to define mapping(uint256 => bool) seen;

Answer 1

This is not allowed in solidity. You can't dynamically define mappings inside a function as it can be only defined as a storage variable.

From docs :

By storage reference it means something like this:

contract a {
    mapping(uint256 => bool)  aseen;

function hasDuplicates(uint256[] memory _array)
    internal
    
    returns (bool)
{
    mapping(uint256 => bool) storage seen = aseen;

    for (uint256 i = 0; i < _array.length; i++) {
        if (seen[_array[i]]) {
            return true;
        } else {
            seen[_array[i]] = true;
        }
    }
    return false;
}
}