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I am trying to create a smart contract using Solidity 0.4.4.
I was wondering if there is a way to set a mapping with some values already entered to an empty one?
For example:
This initailises a new mappping
mapping (uint => uint) map;
Here I add some values
map[0] = 1;
map[1] = 2;
How can I set the map back to empty without iterating through all the keys?
I have tried delete but my contract does not compile
634
You cannot clear all mapping values without specificy the key. Thus, Solidity doesn't know the keys of the mapping. Since the keys are arbitrary and not stored along, the only way to delete values is to know the key for each stored value. Your remove() function is correct to clear the values of a specific mapping key.
You can rewrite your code like this: tests[0] = Test({ id: 0 }); Solidity will make votes an empty mapping by default. Btw, there's also a typo in your code: should be tests[0] instead of test[0] .
Can you iterate over a mapping Solidity? In Solidity, you can define a mapping. The mapping holds the key-value pairs. However, there is no way to iterate over the mapping entries in Solidity.
Unfortunately, you can't. See the Solidity documentation for details on the reasons why. Your only option is to iterate through the keys.
If you don't know your set of keys ahead of time, you'll have to keep the keys in a separate array inside your contract.
110
I believe there is another way to handle this problem.
If you define your mapping with a second key, you can increment that key to essentially reset your mapping.
For example, if you wanted to your mapping to reset every year, you could define it like this:
uint256 private _year = 2021;
mapping(uint256 => mapping(address => uint256)) private _yearlyBalances;
Adding and retrieving values works just like normal, with an extra key:
_yearlyBalances[_year][0x9101910191019101919] = 1;
_yearlyBalances[_year][0x8101810181018101818] = 2;
When it's time to reset everything, you just call
_year += 1
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