Mapping is a great structure in solidity, however a big drawback is to make an iteration on that. Basically it is not really possible, what you can do are the followings:
1. If the key-space is small like working with uint8, you can iterate the whole key-space.
2. You can use a pattern that adds the used key into an array as well, like bellow. Certainly this pattern increases the gas consumption.
contract Iterrativemapping {
mapping(keyType = valueType) myMapping;
keyType[] possibleKeys;
function insert (keyType key, valuType value) public {
possibleKeys.push(key);
myMapping[key] = value;
}
function remove ....
function setValue ...
}
3. Last but not least, you can create an event that is triggered at setting the value. Certainly this scenario does not provide a way to iterate on the mapping from a solidity contract, but there will be an off-chain log available about possible keys, values and about the whole history that can be evaluated off-chain.
contract Mappingwithevent {
mapping(keyType = valueType) myMapping;
event ValueSetEvent(keyType, valueType);
function setValue (keyType key, valuType value) public {
myMapping[key] = value;
ValueSetEvent(key, value);
}
}
