Memory allocation in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to allocate memory for a new variable in a smart contract.

Blockchain / Solidity

uint256 public value = [1];

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Anone

Bnull

Cundefined

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'null' or 'undefined' which are not valid in Solidity for uint256 initialization.

In blockchain smart contracts, especially Solidity, uninitialized uint256 variables default to 0. Explicitly setting it to 0 allocates memory with a known initial value.

2fill in blank

medium

Complete the code to allocate a dynamic array in memory inside a function.

Blockchain / Solidity

function allocateArray() public pure returns (uint256[] memory) {
    uint256[] memory arr = new uint256[]([1]);
    return arr;
}

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A5

Bsize

Clength

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using variable names like 'size' or 'length' without defining them.

When allocating a dynamic array in memory in Solidity, you must specify its fixed size as a number inside the new expression.

3fill in blank

hard

Fix the error in the code to correctly allocate memory for a struct array.

Blockchain / Solidity

struct Data {
    uint256 id;
    string name;
}

function createData() public pure returns (Data[] memory) {
    Data[] memory dataArray = new Data[]([1]);
    return dataArray;
}

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Asize

BData

C0

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using the struct name or undefined variables as size.

When allocating memory for an array of structs, you must specify the size as a number. Using '0' creates an empty array without errors.

4fill in blank

hard

Fill both blanks to allocate memory for a bytes array and set its length.

Blockchain / Solidity

function allocateBytes() public pure returns (bytes memory) {
    bytes memory b = new bytes([1]);
    b.length = [2];
    return b;
}

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A10

B20

D15

Attempts:

3 left

💡 Hint

Common Mistakes

Setting length to a different value than allocated size.

Using invalid sizes like 20 or 15 when allocation is 10.

In Solidity, you allocate a bytes array with a fixed size using new bytes(size). The length property is read-only for bytes, so setting it to the same size as allocated is consistent.

5fill in blank

hard

Fill all three blanks to allocate memory for a fixed-size array, initialize it, and return the first element.

Blockchain / Solidity

function fixedArray() public pure returns (uint256) {
    uint256[[1]] memory arr;
    arr[[2]] = [3];
    return arr[0];
}

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A3

B0

C42

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 1 or size 1 which is too small.

Assigning a value other than 42.

The fixed-size array length is 3. The first element index is 0, which is set to 42. Returning arr[0] returns 42.