Storage layout in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a storage variable in Solidity.

Blockchain / Solidity

contract Storage {
    uint256 public [1];
}

Drag options to blanks, or click blank then click option'

AstorageValue

BmemoryVar

Ctemp

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'memoryVar' which suggests memory instead of storage.

Using temporary names like 'temp' which are unclear.

The variable storageValue is declared as a storage variable of type uint256.

2fill in blank

medium

Complete the code to read a storage variable inside a function.

Blockchain / Solidity

contract Storage {
    uint256 public storedData;
    function getData() public view returns (uint256) {
        return [1];
    }
}

Drag options to blanks, or click blank then click option'

AstoredData()

Bthis.storedData

CstoredData

DgetData

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'storedData()' which is incorrect syntax for variables.

Using 'getData' which is the function name, not the variable.

To return the value of a storage variable, just use its name storedData.

3fill in blank

hard

Fix the error in the code to correctly update a storage variable.

Blockchain / Solidity

contract Storage {
    uint256 public count;
    function increment() public {
        count [1] 1;
    }
}

Drag options to blanks, or click blank then click option'

A=

B+=

C-=

D*=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' which replaces the value instead of adding.

Using '-=' which decreases the value.

The operator += adds 1 to the current value of count.

4fill in blank

hard

Fill both blanks to declare and initialize a mapping in storage.

Blockchain / Solidity

contract Storage {
    mapping(address => [1]) public balances;
    function setBalance(address user, uint256 amount) public {
        balances[user] [2] amount;
    }
}

Drag options to blanks, or click blank then click option'

Auint256

B=

C+=

Daddress

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+=' which adds to the existing value instead of setting it.

Using 'address' as the value type instead of 'uint256'.

The mapping stores uint256 values for each address. To set a balance, use the assignment operator =.

5fill in blank

hard

Fill both blanks to create a struct, declare a storage variable, and update its field.

Blockchain / Solidity

contract Storage {
    struct User  { {
        uint256 id;
        string name;
    }
    User public {BLANK_2}};
    function setUser(uint256 _id, string memory _name) public {
        [2].id = _id;
        {{BLANK_3}}.name = _name;
    }
}

Drag options to blanks, or click blank then click option'

A {

Buser

DUser

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the opening brace '{' after struct name.

Using 'User' instead of the variable name when updating fields.

The struct body starts with a brace '{'. The storage variable is named user. Inside the function, we update user's fields.

Practice

1. What does storage layout in a blockchain smart contract primarily describe?

easy

A. The order of function execution

B. The gas cost of transactions

C. How contract variables are stored in storage slots

D. The network consensus mechanism

Solution

1. Step 1: Understand storage layout concept

   Storage layout defines how contract variables are arranged and saved in storage slots on the blockchain.

2. Step 2: Differentiate from other concepts

   It is not about function order, gas cost, or consensus, but specifically about variable storage.

3. Final Answer:

   How contract variables are stored in storage slots -> Option C

4. Quick Check:

   Storage layout = variable storage arrangement [OK]

Hint: Storage layout = variable storage slots arrangement [OK]

Common Mistakes:

• Confusing storage layout with function execution order
• Thinking storage layout relates to gas costs
• Mixing storage layout with consensus mechanisms

2. Which of the following is the correct Solidity syntax to declare a uint256 variable named count stored in contract storage?

easy

A. uint256 count;

B. uint256 count = storage;

C. storage uint256 count;

D. var count uint256;

Solution

1. Step 1: Recall Solidity variable declaration

   In Solidity, storage variables are declared simply by specifying the type and name, e.g., uint256 count;.

2. Step 2: Identify incorrect syntax

   Options A, B, and D use invalid or unsupported syntax for storage variable declaration.

3. Final Answer:

   uint256 count; -> Option A

4. Quick Check:

   Correct Solidity declaration = uint256 count; [OK]

Hint: Declare storage vars as 'type name;' in Solidity [OK]

Common Mistakes:

• Adding 'storage' keyword in variable declaration
• Using 'var' keyword incorrectly
• Assigning 'storage' as a value

3. Given the Solidity contract snippet:

contract Example {
    uint128 a = 1;
    uint128 b = 2;
    uint256 c = 3;
}

How are the variables a, b, and c stored in storage slots?

medium

A. a and b share slot 0; c is in slot 1

B. Each variable occupies its own slot: a slot 0, b slot 1, c slot 2

C. a and c share slot 0; b is in slot 1

D. All variables share slot 0

Solution

1. Step 1: Understand slot packing rules

   Two uint128 variables fit into one 256-bit slot because 128 + 128 = 256 bits.

2. Step 2: Assign slots based on order and size

   a and b share slot 0; c is a uint256 and occupies slot 1 alone.

3. Final Answer:

   a and b share slot 0; c is in slot 1 -> Option A

4. Quick Check:

   Slot packing = smaller vars share slots [OK]

Hint: Smaller vars pack into one slot if total ≤ 256 bits [OK]

Common Mistakes:

• Assuming each variable always gets its own slot
• Mixing variable order in slots
• Ignoring bit size when packing variables

4. Consider this Solidity contract snippet:

contract Test {
    uint256 x;
    bool y;
    uint256 z;
}

What is the main storage layout issue here?

medium

A. The contract uses too many storage slots causing runtime error

B. The uint256 variables cannot be stored in the same slot

C. The variables are declared in the wrong order causing a syntax error

D. The bool y wastes storage slot space between x and z

Solution

1. Step 1: Analyze variable sizes and order

   uint256 uses full 256-bit slot; bool uses 8 bits but placed between two uint256 variables.

2. Step 2: Identify wasted storage

   Because bool y is between x and z, it cannot share slots efficiently, wasting space.

3. Final Answer:

   The bool y wastes storage slot space between x and z -> Option D

4. Quick Check:

   Storage gaps caused by small vars between large vars [OK]

Hint: Place small vars together to avoid wasted slots [OK]

Common Mistakes:

• Thinking variable order causes syntax errors
• Assuming all vars always share slots
• Believing too many slots cause runtime errors

5. You want to optimize storage in this Solidity contract:

contract Optimize {
    uint128 a;
    uint256 b;
    uint128 c;
    bool d;
}

Which variable order best minimizes storage slots used?

hard

A. uint256 b; uint128 a; uint128 c; bool d;

B. uint128 a; uint128 c; bool d; uint256 b;

C. bool d; uint128 a; uint256 b; uint128 c;

D. uint128 a; bool d; uint256 b; uint128 c;

Solution

1. Step 1: Group smaller variables to pack slots

   Placing uint128 a, uint128 c, and bool d together allows packing into fewer slots.

2. Step 2: Place large uint256 b separately

   uint256 b requires a full slot, so placing it last avoids gaps.

3. Step 3: Evaluate options

   uint128 a; uint128 c; bool d; uint256 b; groups small vars first, then large var last, minimizing slots used.

4. Final Answer:

   uint128 a; uint128 c; bool d; uint256 b; -> Option B

5. Quick Check:

   Group small vars first, large vars last [OK]

Hint: Put small vars together before large vars to save slots [OK]

Common Mistakes:

• Scattering small vars causing wasted slots
• Placing large vars before small vars
• Ignoring bool size when packing