Memory allocation in Blockchain / Solidity - Mini Project: Build & Apply

Memory Allocation in Blockchain Smart Contracts

📖 Scenario: You are building a simple smart contract that stores user balances on a blockchain. Efficient memory allocation is important to keep the contract lightweight and fast.

🎯 Goal: Create a smart contract that initializes user balances, sets a maximum allowed balance, updates balances using a loop, and finally outputs the updated balances.

📋 What You'll Learn

Create a dictionary called balances with exact entries for three users and their initial balances

Create a variable called max_balance and set it to 1000

Use a for loop with variables user and balance to iterate over balances.items() and increase each balance by 100 without exceeding max_balance

Print the final balances dictionary

💡 Why This Matters

🌍 Real World

Smart contracts on blockchains often manage user balances and must allocate memory efficiently to avoid high costs and slow execution.

💼 Career

Understanding memory allocation and safe updates in smart contracts is essential for blockchain developers to write secure and efficient code.

1

DATA SETUP: Create initial user balances

Create a dictionary called balances with these exact entries: 'Alice': 500, 'Bob': 750, 'Charlie': 300

Blockchain / Solidity

# Create the balances dictionary with initial values
# Your code here

Hint

Use curly braces {} to create a dictionary with keys as user names and values as their balances.

2

CONFIGURATION: Set maximum allowed balance

Create a variable called max_balance and set it to 1000

Blockchain / Solidity

balances = {'Alice': 500, 'Bob': 750, 'Charlie': 300}
# Set the maximum allowed balance
# Your code here

Hint

Just assign the number 1000 to the variable max_balance.

3

CORE LOGIC: Increase balances safely

Use a for loop with variables user and balance to iterate over balances.items(). Increase each balance by 100 but do not let it exceed max_balance. Update the balances dictionary accordingly.

Blockchain / Solidity

balances = {'Alice': 500, 'Bob': 750, 'Charlie': 300}
max_balance = 1000
# Increase each balance by 100 without exceeding max_balance
# Your code here

Hint

Use a loop to go through each user and balance. Add 100, then check if it goes over max_balance. If yes, set it to max_balance.

4

OUTPUT: Display updated balances

Write print(balances) to display the updated balances dictionary

Blockchain / Solidity

balances = {'Alice': 500, 'Bob': 750, 'Charlie': 300}
max_balance = 1000
for user in balances:
    new_balance = balances[user] + 100
    if new_balance > max_balance:
        new_balance = max_balance
    balances[user] = new_balance
# Print the balances dictionary
# Your code here

Hint

Use the print() function to show the final dictionary.

Practice

1. What is the main purpose of memory in blockchain smart contracts?

easy

A. To temporarily store data during function execution

B. To permanently save user data on the blockchain

C. To allocate gas for contract deployment

D. To encrypt data before storing

Solution

1. Step 1: Understand memory role in smart contracts

   Memory is used to hold data temporarily while a function runs.

2. Step 2: Differentiate memory from storage

   Storage saves data permanently on the blockchain, memory does not.

3. Final Answer:

   To temporarily store data during function execution -> Option A

4. Quick Check:

   Memory = Temporary data space [OK]

Hint: Memory is temporary, storage is permanent [OK]

Common Mistakes:

• Confusing memory with storage
• Thinking memory saves data permanently
• Assuming memory costs more gas

2. Which of the following is the correct syntax to declare a dynamic array in memory in Solidity?

easy

A. uint[] myArray = memory new uint[](5);

B. uint[] storage myArray = new uint[](5);

C. memory uint[] myArray = new uint[](5);

D. uint[] memory myArray = new uint[](5);

Solution

1. Step 1: Recall correct memory array declaration syntax

   In Solidity, dynamic memory arrays are declared as type[] memory var = new type[](size);

2. Step 2: Check each option

   uint[] memory myArray = new uint[](5); matches the correct syntax exactly.

3. Final Answer:

   uint[] memory myArray = new uint[](5); -> Option D

4. Quick Check:

   Memory array syntax = uint[] memory myArray = new uint[](5); [OK]

Hint: Use 'memory' keyword before variable name for memory arrays [OK]

Common Mistakes:

• Using 'storage' instead of 'memory' for temporary arrays
• Placing 'memory' after variable name incorrectly
• Omitting 'new' keyword for dynamic arrays

3. What will be the output of this Solidity function snippet?

function test() public pure returns (uint) {
    uint[] memory arr = new uint[](3);
    arr[0] = 10;
    arr[1] = 20;
    arr[2] = 30;
    return arr[1];
}

medium

A. 10

B. 20

C. 30

D. 0

Solution

1. Step 1: Analyze array assignments

   Array elements are set as arr[0]=10, arr[1]=20, arr[2]=30.

2. Step 2: Identify returned value

   The function returns arr[1], which is 20.

3. Final Answer:

   20 -> Option B

4. Quick Check:

   arr[1] = 20 [OK]

Hint: Remember array indexing starts at 0 [OK]

Common Mistakes:

• Confusing index 1 with index 0
• Assuming default zero value is returned
• Misreading array initialization

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

uint[] public data;

function copyArray() public {
    uint[] memory temp = data;
}

medium

A. Cannot assign storage array directly to memory array

B. Missing visibility keyword for storage array

C. Memory arrays cannot be declared inside functions

D. Function must be marked as pure

Solution

1. Step 1: Understand storage to memory assignment rules

   Direct assignment from storage array to memory array is not allowed in Solidity.

2. Step 2: Identify correct error

   The code tries to assign data (storage) directly to temp (memory), causing a compilation error.

3. Final Answer:

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

4. Quick Check:

   Storage to memory direct assignment = Error [OK]

Hint: Copy elements manually from storage to memory [OK]

Common Mistakes:

• Assuming direct assignment copies array
• Ignoring visibility requirements
• Marking function pure incorrectly

5. You want to create a function that returns a new memory array containing only the even numbers from a given storage array. Which approach correctly uses memory allocation?

hard

A. Declare a memory array with length zero, push evens dynamically, then return it

B. Declare a storage array, copy evens, then return it

C. Declare a memory array with max length, copy evens manually, then return a sliced array

D. Declare a memory array with the same length as storage, copy evens, then resize the array

Solution

1. Step 1: Understand memory array limitations

   Memory arrays in Solidity have fixed size; cannot push dynamically.

2. Step 2: Choose correct approach for filtering

   Declare a memory array with max length, copy even numbers manually, then return a sliced array with correct size.

3. Final Answer:

   Declare a memory array with max length, copy evens manually, then return a sliced array -> Option C

4. Quick Check:

   Memory arrays fixed size = Declare a memory array with max length, copy evens manually, then return a sliced array [OK]

Hint: Memory arrays fixed size; slice after copying [OK]

Common Mistakes:

• Trying to push to memory arrays
• Using storage arrays for temporary filtering
• Not resizing array before returning