Solidity – Understanding the Default Value for an Uninitialized Nested Mapping

You check that a value is defined in the mapping by checking it is not zero.

If an explicit setting of zero has meaning for your application, you need auxiliary data (or structure) to track when a value of zero has been explicitly set.

A lightweight approach would be to add a bool property to the struct (say named initialized), and to set it to true when the zero is explicitly set. As all bools are false (0) by default, you can check against the value true.

Alternatively, check that each member of the struct is zero. If a member is a string, cast it to bytes and then check its length is zero.

For an example see How to test if a struct state variable is set

Another data structure or mapping may be needed depending on the application.

Here is a related example on using a pair to check the meaning of zero:

contract C {
    uint[] counters;
    function getCounter(uint index)
        returns (uint counter, bool error) {
            if (index >= counters.length) return (0, true);
            else return (counters[index], false);
        }
    function checkCounter(uint index) {
        var (counter, error) = getCounter(index);
        if (error) { ... }
        else { ... }
    }
}

The second error you get tells you that external functions cannot have internal types as arguments (i.e. structs and mappings). You can solve that one by specifying internal or private for your function.

For the first error, im not sure if it is a bug or not, but you cannot pass mappings as arguments. What you could do is wrap them in a struct and pass the struct as a parameter.

One final notice is that if you declare your mapping as a state variable then you have access to it directly in your function. So in you example table could just be a state variable...especially since you only use it for storage purposes.