Storage vs memory vs calldata in Blockchain / Solidity - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a function parameter stored in memory.

Blockchain / Solidity

function greet(string [1] name) public pure returns (string memory) {
    return name;
}

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

Astorage

Bcalldata

Cvar

Dmemory

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'storage' for function parameters causes errors.

Using 'var' is not valid in Solidity.

Function parameters that are strings or arrays usually use memory to store temporary data during function execution.

2fill in blank

medium

Complete the code to declare a function parameter that is read-only and cheaper to use.

Blockchain / Solidity

function readData(string [1] data) external view returns (string memory) {
    return data;
}

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

Astorage

Bcalldata

Cmemory

Dconstant

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'memory' instead of 'calldata' increases gas cost.

Trying to modify calldata variables causes errors.

calldata is used for function parameters that are read-only and passed from external calls, saving gas.

3fill in blank

hard

Fix the error in the code by choosing the correct data location for the state variable.

Blockchain / Solidity

contract Example {
    string [1] name;

    function setName(string memory newName) public {
        name = newName;
    }
}

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

Acalldata

Bmemory

Cstorage

Dconstant

Attempts:

3 left

💡 Hint

Common Mistakes

Declaring state variables as 'memory' causes compilation errors.

Using 'calldata' for state variables is invalid.

State variables must be stored in storage because they persist on the blockchain.

4fill in blank

hard

Complete the code to create a function that accepts a read-only array and stores it in state.

Blockchain / Solidity

contract DataStore {
    uint[] public data;

    function storeData(uint[] [1] input) public {
        data = input;
    }
}

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

Acalldata

Bmemory

C.slice()

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to call .slice() on calldata arrays causes errors.

Using memory instead of calldata increases gas cost.

The function parameter uses calldata for a read-only array. The assignment copies the array directly, so no extra method is needed.

5fill in blank

hard

Fill both blanks to declare a function that modifies a storage array using a memory copy.

Blockchain / Solidity

contract Modify {
    uint[] public numbers;

    function updateNumbers(uint[] [1] newNumbers) public {
        uint[] storage nums = numbers;
        nums = newNumbers;
        nums.push([2]);
    }
}

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

Amemory

C100

Dcalldata

Attempts:

3 left

💡 Hint

Common Mistakes

Using calldata prevents modification.

Trying to assign calldata array directly to storage without copying.

The function parameter uses memory to hold the new array. The assignment copies the array. Then, 100 is pushed to the storage array.

Practice

1. Which of the following correctly describes storage in Solidity?

easy

A. It stores data permanently on the blockchain.

B. It holds temporary data only during function execution.

C. It is used for read-only input data in external functions.

D. It is a type of event logging mechanism.

Solution

1. Step 1: Understand storage role

   storage variables are saved permanently on the blockchain and persist between function calls.

2. Step 2: Compare with memory and calldata

   memory is temporary during function execution, and calldata is read-only input data for external functions.

3. Final Answer:

   It stores data permanently on the blockchain. -> Option A

4. Quick Check:

   Storage = permanent data [OK]

Hint: Storage is permanent, memory is temporary, calldata is input [OK]

Common Mistakes:

• Confusing storage with memory as temporary
• Thinking calldata can be modified
• Assuming storage is read-only

2. Which keyword is used to declare a function parameter that is read-only and passed from an external caller?

easy

A. calldata

B. storage

C. memory

D. event

Solution

1. Step 1: Identify parameter data location for external input

   Function parameters from external callers that should not be modified use calldata.

2. Step 2: Recall other data locations

   memory is for temporary data inside functions, storage is persistent, and event is unrelated to data location.

3. Final Answer:

   calldata -> Option A

4. Quick Check:

   External input = calldata [OK]

Hint: External read-only params use calldata [OK]

Common Mistakes:

• Using memory for external parameters
• Confusing event with data location
• Using storage for function parameters

3. What will be the output of this Solidity function when called externally with _data = [1, 2, 3]?

function process(uint[] calldata _data) external pure returns (uint) {
    uint sum = 0;
    for (uint i = 0; i < _data.length; i++) {
        sum += _data[i];
    }
    return sum;
}

medium

A. Runtime error

B. 6

C. Compilation error

D. 0

Solution

1. Step 1: Understand calldata usage

   The function receives an array _data as calldata, which is read-only but accessible for reading elements.

2. Step 2: Calculate sum of array elements

   Sum of [1, 2, 3] is 1 + 2 + 3 = 6, which is returned.

3. Final Answer:

   6 -> Option B

4. Quick Check:

   Sum of input array = 6 [OK]

Hint: Sum array elements from calldata works like memory [OK]

Common Mistakes:

• Thinking calldata cannot be read
• Expecting compilation error due to calldata
• Confusing sum calculation logic

4. Identify the error in this Solidity function:

function updateData(uint[] calldata _input) external {
    _input[0] = 10;
}

medium

A. Calldata arrays cannot be passed as parameters.

B. Function must be marked pure.

C. Missing return statement.

D. Cannot assign to calldata array elements.

Solution

1. Step 1: Check mutability of calldata

   calldata is read-only, so assigning to _input[0] causes an error.

2. Step 2: Validate other options

   The function can be non-pure, no return needed, and calldata arrays are allowed as parameters.

3. Final Answer:

   Cannot assign to calldata array elements. -> Option D

4. Quick Check:

   Calldata is read-only [OK]

Hint: Calldata arrays are read-only, no assignment allowed [OK]

Common Mistakes:

• Trying to modify calldata elements
• Thinking pure is required here
• Assuming return is mandatory

5. You want to write a Solidity function that accepts a large array from an external caller, processes it temporarily, and then stores the result permanently. Which combination of data locations should you use for the parameter and the storage variable?

hard

A. Parameter as memory, storage variable as calldata

B. Parameter as storage, storage variable as memory

C. Parameter as calldata, storage variable as storage

D. Parameter as calldata, storage variable as memory

Solution

1. Step 1: Choose parameter data location for large external input

   calldata is efficient and read-only for large external arrays, avoiding unnecessary copying.

2. Step 2: Choose storage variable for permanent data

   To keep data permanently on blockchain, use storage for the variable that stores the processed result.

3. Final Answer:

   Parameter as calldata, storage variable as storage -> Option C

4. Quick Check:

   External input = calldata, permanent store = storage [OK]

Hint: Use calldata for input, storage for permanent save [OK]

Common Mistakes:

• Using memory for large external input (inefficient)
• Trying to store permanent data in memory or calldata
• Confusing storage and memory roles