Storage vs memory usage in Blockchain / Solidity - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Understanding storage and memory in Solidity

What will be the output of this Solidity function when called?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Test {
    uint[] public numbers;

    function addNumbers() public returns (uint) {
        uint[] memory temp = new uint[](3);
        temp[0] = 1;
        temp[1] = 2;
        temp[2] = 3;

        numbers = temp;
        return numbers.length;
    }
}

A 0

B Runtime error

C Compilation error

D 3

💡 Hint

Remember that assigning a memory array to a storage array copies the data.

The function creates a memory array of length 3, assigns values, then copies it to the storage array 'numbers'. The storage array length becomes 3, so the function returns 3.

❓ Predict Output

intermediate

Effect of modifying memory vs storage variables

What will be the value of 'data[0]' after calling 'modify()'?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Example {
    uint[] public data;

    constructor() {
        data.push(10);
        data.push(20);
    }

    function modify() public {
        uint[] memory temp = data;
        temp[0] = 99;
    }
}

A 10

B 99

C Compilation error

D Runtime error

💡 Hint

Modifying a memory copy does not affect storage.

The 'temp' array is a memory copy of 'data'. Changing 'temp[0]' does not change 'data[0]' in storage, so 'data[0]' remains 10.

🧠 Conceptual

advanced

Gas cost difference between storage and memory

Which statement about gas costs in Solidity is correct?

A Reading from storage is cheaper than reading from memory.

B Writing to storage is more expensive than writing to memory.

C Memory usage permanently increases gas cost after function execution.

D Storage variables are temporary and cheaper to use than memory variables.

💡 Hint

Consider the permanence of storage vs memory.

Writing to storage is expensive because it changes the blockchain state permanently. Memory is temporary and cheaper to write to during function execution.

❓ Predict Output

advanced

Storage pointer vs memory copy behavior

What will be the output of the function 'getFirst()' after calling 'update()'?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract PointerTest {
    uint[] public arr;

    constructor() {
        arr.push(5);
        arr.push(10);
    }

    function update() public {
        uint[] storage ref = arr;
        ref[0] = 100;
    }

    function getFirst() public view returns (uint) {
        return arr[0];
    }
}

A 100

B 0

C 5

D Compilation error

💡 Hint

Storage pointers modify the original storage data.

The 'ref' variable is a storage pointer to 'arr'. Changing 'ref[0]' changes 'arr[0]' in storage, so 'getFirst()' returns 100.

🧠 Conceptual

expert

Choosing storage vs memory for function parameters

Which is the best reason to use 'memory' instead of 'storage' for a function parameter in Solidity?

A To allow the function to modify the original storage variable directly.

B To permanently save changes to the blockchain state.

C To reduce gas costs by avoiding expensive storage writes.

D To make the variable accessible outside the function.

💡 Hint

Think about temporary data and gas costs.

Using 'memory' creates a temporary copy that does not write to storage, saving gas. 'Storage' parameters refer to persistent blockchain data and are more expensive to modify.