Always use "memory" for variables created within functions. Bad practice?

Question

Is it bad practice to systematically use the memory keyword for variables declared within a function (other than constructor)?

Answer 1

It's good practice to explicitly use memory or storage to clarify whether a variable contains a pointer to memory or storage.

You should not always use the memory keyword, because it could make your code really inefficient: When you're assigning an array from storage to a memory pointer variable, the entire array will be read from storage and copied into memory.

Here's an example of a situation where you should not use memory:

contract Test
{
    uint256[1000] lotsOfNumbersInStorage;
    uint256[1000] moreNumbersInStorage;
    function calculateSomething(uint256 a) public view returns(uint256)
    {
        uint256[1000] memory theChosenArray;
        if (a > 0) theChosenArray = lotsOfNumbersInStorage;
        else theChosenArray = moreNumbersInStorage;
        return theChosenArray[3] * theChosenArray[a];
    }
}

Those assignments to theChosenArray look innocent, but they will copy the entire array from storage into memory, which will cost at least 200000 gas (200 gas per storage read * 1000 elements). In this case, you should declare theChosenArray as a storage pointer, not a memory pointer.