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Is it possible to generate a non-local variable with a function?

Weird question but it's for an exercise my senior asked me to do, I've scoured the web for an answer to no avail.

Instead of declaring the variable:

contract MetaCoin{
 uint counter=0;
}

Maybe do something like:

contract MetaCoint{
function createCounterVariable(){
  //creates counter variable
 }
}

It will be functionally identical to the declaring the variable, and other functions can access the generated variable somehow. At least this is how I interpret what I'm supposed to do.

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You could do something like this:

contract MetaCoint {
    uint public index;
    mapping(uint => uint) public counters;

    function addCounter() external {
        index++;
    }

    function incCounter(uint _index) external {
        require(_index < index, "counter was not created");
        counters[_index]++;
    }
}
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  • This is okay, but I think an array is more natural. (It avoids having to separately track the length, and it does the bounds checking automatically.) – user19510 Jun 5 '19 at 5:32
  • @smarx: Yes, I agree. Since counters will be added sequentially, there is really no point in a mapping here. I now understand your solution, which is a better (more simplified) solution for the given problem as it is currently stated. – goodvibration Jun 5 '19 at 5:36
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This really depends on what you're trying to do. There's really no such thing as "creating" a state variable. But you might be able to accomplish what you want with a mapping or array. Something like this (untested):

contract Counters {
    uint256[] public counters;
    event NewCounter(uint256 index);

    function createCounter() external {
        counters.push(0); // "create" a new counter
        emit NewCounter(counters.length - 1); // so the caller can see the ID of the new counter
    }

    function increment(uint256 index) external {
        counters[index] += 1;
    }
}
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  • I think I've seen something like accessing the storage of the contract directly like an array, I could be wrong though. – ly6161 Jun 5 '19 at 5:16
  • A little confusing, because there is no mapping here. – goodvibration Jun 5 '19 at 5:20
  • @goodvibration I suggested using a mapping or an array. I only gave one example, and it uses an array. Does that clear up your confusion? – user19510 Jun 5 '19 at 5:32
  • Yes, I just figured that a mapping would address the issue more accurately, but as you initially stated, it really depends on what this dude is trying to achieve... – goodvibration Jun 5 '19 at 5:33
  • My interpretation is something like this ethereum.stackexchange.com/questions/69931/… but creating a new variable instead of accessing an existing variable, not sure if it can be done. – ly6161 Jun 5 '19 at 7:28

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