Data Location Assignment [OLD]

badtoken · December 25, 2020, 2:58pm

function updateBalance(uint id, uint balance) public {
//User memory user = users[id];
users[id].balance = balance;
}

kHarsh · December 25, 2020, 7:48pm

pragma solidity 0.7.5;
contract MemoryAndStorage {

mapping(uint => User) users;

struct User{
    uint id;
    uint balance;
}

function addUser(uint id, uint balance) public {
    users[id] = User(id, balance);
}

function updateBalance(uint id, uint balance) public {
     //User memory user = users[id];
     //user.balance = balance;
     users[id].balance = balance;
}

function getBalance(uint id) view public returns (uint) {
    return users[id].balance;
}

}

The reason the balance did not update initially is because, a new User variable which is a struct created in memory each time the update is called so the balance to state variable users mapping never got called.

Javier_Flores · December 29, 2020, 4:03pm

Change memory to storage.

jon_m · December 30, 2020, 12:51am

Hi @Caspian1234,
Welcome to the forum! It’s great to see you here I hope you’ve been enjoying this course.

Apologies for the delay in giving you feedback on this assignment.

Your solution is correct but be careful not to mix up the names:

user is the name of the local variable. You’ve also used this to reference the mapping, but the mapping is users with an s. These details are important, because your code as it is will not execute correctly.

RCV · January 1, 2021, 9:20pm

pragma solidity 0.7.5;
contract MemoryAndStorage {

    mapping(uint => User) users;

    struct User{
        uint id;
        uint balance;
    }

    function addUser(uint id, uint balance) public {
        users[id] = User(id, balance);
    }

    function updateBalance(uint id, uint balance) public {
         User storage user = users[id];
         user.balance = balance;
    }

    function getBalance(uint id) view public returns (uint) {
        return users[id].balance;
    }

}

madingo · January 3, 2021, 2:21pm

Data Location Assignment.

One possible solution to The Update:

constructor(){
User = (uint id, uint balance);
}
function addUser (uint_ id, string memory uint_balance) public{
User [id] = User (id, balance);

function updateBalance (uint_ id, string memory unit_balance) public{
User memory user = user [id]
User. balance = balance;
assert (balance [User id] == Previous User balance - amount);

function getBalance (uint id) view public return (uint) {
return balance [id];
}
}

rallen221 · January 7, 2021, 11:09pm

pragma solidity 0.5.1;
contract MemoryAndStorage {

mapping(User => uint) users;

}

struct User{
    uint id;
    uint balance;
}

function addUser(uint id, uint balance) public {
    users[id] = User(id, balance);
}
/*Memory is local storage, also we are dealing with "uint and we do not need to use "memory"since it's not a string varible in the argument "storage replace "memory"*/
function updateBalance(uint id, uint balance) public {
     User storage user = new users[_id];
     user.balance = balance;
}

function getBalance(uint id) view public returns (uint) {
    return users[id].balance;
}

}

simoleo · January 12, 2021, 12:14am

I fix that using similar function for both AddUser and Update balance : P
Not specifying the data allocation will save me some bites agains the other who chose the ‘storage’ fix ?

function addUser(uint id, uint balance) public {
    users[id] = User(id, balance);
}

function updateBalance(uint id, uint balance)  public {
     users[id].balance = balance;
}

simoleo · January 12, 2021, 12:18am

I think I’m using the same bites as the other longer solution :
User storage user = new users[_id];
user.balance = balance;

Since this direct mapping :
users[id].balance = balance;
works with ‘uint’ and they are stored in the storage.

jon_m · January 12, 2021, 6:01pm

Hi @simoleo,

Both alternative solutions will consume the same amount of gas with optimization turned on. This is because they are effectively performing exactly the same operations.

Maybe it’s just a slip, but adding the new keyword to the following line is incorrect:

It should be:

 function updateBalance(uint id, uint balance) public {
        User storage user = users[id];
        user.balance = balance;
    }

simoleo · January 12, 2021, 11:38pm

Hi @jon_m thank you for pointing these out! appreciate it

lloyda1 · January 14, 2021, 1:38pm

All that needed changing was the update function THe problem with it was that it created a new local variable User object within the function. Thus any changes made to this object would be separate from the original global user. Just allowed changes to be made to the original user!

pragma solidity 0.7.5;
contract MemoryAndStorage {

mapping(uint => User) users;

struct User{
    uint id;
    uint balance;
}

function addUser(uint id, uint balance) public {
    users[id] = User(id, balance);
}

function updateBalance(uint id, uint balance) public {
     //User memory user = users[id];
     //user.balance = balance;
     users[id].balance = balance;
}

function getBalance(uint id) view public returns (uint) {
    return users[id].balance;
}

}

Maya · January 16, 2021, 3:15am

Thought of in a simple way, the user id information is lost each time the function is run because it goes to memory. That information needs to be put into storage.

pragma solidity 0.5.12;

contract MemoryAndStorage{
    mapping(uint => User) users;
    
    struct User {
        uint id;
        uint balance;
    }
    
    function addUser(uint id, uint balance)public {
        users[id] = User(id, balance);
    }
    
    function updateBalance(uint id, uint balance)public {
        User storage user = users[id];
        user.balance = balance;
       // users[id] = user;
       //users[id].balance = balance;
    }
    
    function getBalance (uint id) view public returns(uint){
        return users[id].balance;
    }
    
}

Martin_Offermann · January 20, 2021, 4:52pm

I did it the lazy way let’s say Changed memory to storage in function updateBalance. That did the trick.

abuga · January 22, 2021, 1:32pm

@Martin_Offermann

that is 100% correct. but i would like to see your code

Tumbleur · January 26, 2021, 9:26am

Well after thinking about it for a while and trying plenty of stuff, I decided to switch memory to storage in the updateBalance function !!! Worked just fine haha.

jon_m · January 27, 2021, 4:27am

Hi @Tumbleur,

Correct
But as @abuga says above, we would like to see your code

Charlene_Nicer · January 28, 2021, 6:33am

pragma solidity 0.7.5;
contract MemoryAndStorage {

    mapping(uint => User) users;

    struct User{
        uint id;
        uint balance;
    }

    function addUser(uint id, uint balance) public {
        users[id] = User(id, balance);
    }

    function updateBalance(uint id, uint balance) public {
         users[id].balance = balance;
    }

    function getBalance(uint id) view public returns (uint) {
        return users[id].balance;
    }

}

jonp · January 28, 2021, 1:30pm

When you create a new local variable such as below - doesn’t this incur extra gas charges?

The local variable is referencing the storage address but the local variable itself (pointer to the storage address of this particular user) will incur a gas fee.

function updateBalance(uint id, uint balance)public {
User storage user = users[id];
user.balance = balance;
}

Therefore, isn’t it more efficient to implement the solution as follows as you’re not creating any extra local variables?:

function updateBalance(uint id, uint balance)public {
users[id].balance = balance
}

paddyc1 · January 31, 2021, 12:23pm

pragma solidity 0.5.1;
contract MemoryAndStorage {

    mapping(uint => User) users;

    struct User{
        uint id;
        uint balance;
    }

    function addUser(uint id, uint balance) public {
        users[id] = User(id, balance);
    }

    function updateBalance(uint id, uint balance) public {
        User storage user = users[id];
        user.balance = balance;
    }

    function getBalance(uint id) view public returns (uint) {
        return users[id].balance;
    }

}