Storage vs memory usage in Blockchain / Solidity - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a storage variable in Solidity.

Blockchain / Solidity

uint256 public [1];

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

AstoredValue

BmemoryValue

CtempValue

DlocalValue

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a variable name that implies temporary or memory usage.

Confusing storage with memory keyword.

In Solidity, variables declared at contract level like storedValue are stored in storage by default.

2fill in blank

medium

Complete the function to declare a memory array parameter in Solidity.

Blockchain / Solidity

function processData(uint256[] [1] data) public {}

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

Apersistent

Bmemory

Ccalldata

Dstorage

Attempts:

3 left

💡 Hint

Common Mistakes

Using storage for function parameters which causes errors.

Confusing calldata with memory.

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

3fill in blank

hard

Fix the error in the function by choosing the correct data location for the string parameter.

Blockchain / Solidity

function setName(string [1] name) public {}

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

Astorage

Bpersistent

Cmemory

Dcalldata

Attempts:

3 left

💡 Hint

Common Mistakes

Using memory unnecessarily which increases gas cost.

Using storage which is invalid for function parameters.

For external function parameters, calldata is the recommended data location for strings to save gas and avoid copying.

4fill in blank

hard

Fill the blank to create a temporary memory array for the function return. Mappings are always stored in storage.

Blockchain / Solidity

mapping(address => uint) balances;
function getTopBalances() public view returns (uint[] {{BLANK_2}}) {}

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

Astorage

Bcalldata

Cmemory

Dpersistent

Attempts:

3 left

💡 Hint

Common Mistakes

Declaring mapping as memory which is invalid.

Returning arrays without specifying memory location.

Mappings are always stored in storage (no keyword needed). Function return arrays should be declared as memory to hold temporary data.

5fill in blank

hard

Fill both blanks to declare a memory struct variable and assign a value.

Blockchain / Solidity

struct User {
    uint id;
    string name;
}
User [2] userMemory;
userMemory.id = {{BLANK_3}};

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

Astorage

Bmemory

C10

Dcalldata

Attempts:

3 left

💡 Hint

Common Mistakes

Declaring struct with memory keyword which is invalid.

Assigning a string instead of a number to id.

The struct is declared at the contract level. The variable userMemory is declared in memory for temporary use. Assigning 10 to id sets the value.

Practice

1. Which of the following best describes storage in blockchain smart contracts?

easy

A. Permanent data saved on the blockchain that persists between function calls.

B. Temporary data used only during a function execution and discarded afterward.

C. Data stored off-chain for faster access.

D. Encrypted data that cannot be accessed by the contract.

Solution

1. Step 1: Understand storage purpose

   Storage holds data permanently on the blockchain, so it remains after function execution.

2. Step 2: Compare with memory

   Memory is temporary and only lasts during function execution, unlike storage.

3. Final Answer:

   Permanent data saved on the blockchain that persists between function calls. -> Option A

4. Quick Check:

   Storage = permanent data [OK]

Hint: Storage keeps data after functions finish, memory does not [OK]

Common Mistakes:

• Confusing memory with storage permanence
• Thinking storage is temporary
• Assuming storage is off-chain
• Believing storage is encrypted by default

2. Which of the following is the correct way to declare a variable in Solidity that uses memory?

easy

A. uint memory[] numbers;

B. uint[] storage numbers;

C. memory uint[] numbers;

D. uint[] memory numbers;

Solution

1. Step 1: Recall Solidity syntax for memory arrays

   In Solidity, the correct syntax is type[] memory variableName; for memory arrays.

2. Step 2: Check each option

   uint[] memory numbers; matches the correct syntax: uint[] memory numbers;. Others have wrong order or keywords.

3. Final Answer:

   uint[] memory numbers; -> Option D

4. Quick Check:

   Memory arrays use 'type[] memory name' syntax [OK]

Hint: Memory keyword comes after type and brackets in Solidity [OK]

Common Mistakes:

• Placing 'memory' before type
• Using 'storage' instead of 'memory'
• Incorrect array syntax
• Missing brackets for array type

3. What will be the output of the following Solidity function?

pragma solidity ^0.8.0;
contract Test {
    uint[] storageArray;
    function addAndReturn() public returns (uint) {
        uint[] memory tempArray = new uint[](1);
        tempArray[0] = 5;
        storageArray.push(10);
        return tempArray[0];
    }
}

medium

A. 0

B. 10

C. 5

D. Compilation error

Solution

1. Step 1: Analyze tempArray usage

   The function creates a memory array with one element set to 5, then returns that element.

2. Step 2: Understand storageArray effect

   storageArray.push(10) adds 10 to storage but does not affect the returned value.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   Return value from memory array = 5 [OK]

Hint: Return value from memory array, storage change doesn't affect output [OK]

Common Mistakes:

• Confusing storage push with return value
• Expecting storageArray value as output
• Thinking memory array persists after function
• Assuming compilation error due to storage usage

4. Identify the error in this Solidity function that tries to copy a storage array to memory:

pragma solidity ^0.8.0;
contract Example {
    uint[] storageArray;
    function copyArray() public view returns (uint[] memory) {
        uint[] memory tempArray = storageArray;
        return tempArray;
    }
}

medium

A. Cannot assign storage array directly to memory array.

B. Function must be non-view to modify storage.

C. Missing visibility specifier for storageArray.

D. Memory arrays cannot be returned from functions.

Solution

1. Step 1: Understand storage to memory assignment

   In Solidity, you cannot directly assign a storage array to a memory array variable.

2. Step 2: Identify correct copying method

   You must copy elements manually or use a loop to transfer data from storage to memory.

3. Final Answer:

   Cannot assign storage array directly to memory array. -> Option A

4. Quick Check:

   Direct storage to memory assignment is invalid [OK]

Hint: Storage arrays need manual copying to memory arrays [OK]

Common Mistakes:

• Trying direct assignment from storage to memory
• Ignoring need for loops to copy arrays
• Assuming view functions can modify storage
• Believing memory arrays can't be returned

5. You want to write a Solidity function that temporarily modifies an array for calculations without changing the stored data. Which is the best approach?

hard

A. Declare the array as memory in the contract state.

B. Copy the storage array to a memory array, modify the memory array, then discard it.

C. Use a storage pointer to the array and modify it temporarily.

D. Modify the storage array directly and revert changes after calculations.

Solution

1. Step 1: Understand temporary modification needs

   To avoid changing stored data, use a memory copy for temporary changes.

2. Step 2: Evaluate options

   Copy the storage array to a memory array, modify the memory array, then discard it. copies storage to memory, modifies memory, and discards changes, preserving storage.

3. Step 3: Reject incorrect options

   A declares memory in state (invalid syntax); B risks permanent changes if revert fails; C modifies storage directly.

4. Final Answer:

   Copy the storage array to a memory array, modify the memory array, then discard it. -> Option B

5. Quick Check:

   Use memory copy for temporary changes [OK]

Hint: Use memory copy to avoid changing stored data permanently [OK]

Common Mistakes:

• Modifying storage directly for temporary needs
• Trying to declare storage variables as memory in contract
• Assuming revert undoes all changes safely
• Using storage pointers for temporary changes