Balance checking in Blockchain / Solidity - Time & Space Complexity

Time Complexity: Balance checking

O(n)

Understanding Time Complexity

When checking a balance on a blockchain, it is important to know how the time to get that balance changes as the number of accounts grows.

We want to understand how long it takes to find the balance for one account when many accounts exist.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

function getBalance(user) {
  for (let i = 0; i < accounts.length; i++) {
    if (accounts[i].address === user) {
      return accounts[i].balance;
    }
  }
  return 0;
}

This code looks through a list of accounts to find the balance of the given user address.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: A loop that checks each account's address one by one.
• How many times: Up to the total number of accounts in the list.

How Execution Grows With Input

As the number of accounts grows, the time to find a balance grows roughly the same way.

Input Size (n)	Approx. Operations
10	Up to 10 checks
100	Up to 100 checks
1000	Up to 1000 checks

Pattern observation: The number of checks grows directly with the number of accounts.

Final Time Complexity

Time Complexity: O(n)

This means the time to find a balance grows in a straight line with the number of accounts.

Common Mistake

[X] Wrong: "Checking a balance always takes the same time no matter how many accounts there are."

[OK] Correct: Because this code looks through each account one by one, more accounts mean more checks and more time.

Interview Connect

Understanding how balance checking scales helps you explain how blockchain systems handle many users efficiently.

Self-Check

"What if we stored accounts in a map (key-value store) instead of a list? How would the time complexity change?"

Practice

1.

What does balance checking in blockchain primarily tell you?

easy

A. The private key of a wallet

B. The transaction history of an address

C. How much cryptocurrency an address currently holds

D. The mining difficulty of the blockchain

Solution

1. Step 1: Understand the purpose of balance checking

   Balance checking is used to find out the amount of cryptocurrency held by a blockchain address.

2. Step 2: Differentiate from other blockchain data

   Transaction history, private keys, and mining difficulty are different concepts unrelated to balance checking.

3. Final Answer:

   How much cryptocurrency an address currently holds -> Option C

4. Quick Check:

   Balance checking = cryptocurrency amount [OK]

Hint: Balance checking shows funds held by an address [OK]

Common Mistakes:

• Confusing balance with transaction history
• Thinking balance reveals private keys
• Mixing balance with blockchain mining info

2.

Which of the following is the correct way to call a function getBalance for an address addr in a blockchain library?

?

easy

A. getBalance(addr)

B. getBalance = addr

C. getBalance.addr()

D. getBalance->addr()

Solution

1. Step 1: Identify correct function call syntax

   In most programming languages, calling a function with an argument uses parentheses: functionName(argument).

2. Step 2: Check each option

   getBalance(addr) uses correct syntax: getBalance(addr). Options A, C, and D use invalid or incorrect syntax.

3. Final Answer:

   getBalance(addr) -> Option A

4. Quick Check:

   Function call syntax = getBalance(addr) [OK]

Hint: Function calls use parentheses with arguments [OK]

Common Mistakes:

• Using assignment instead of function call
• Using dot notation incorrectly for function calls
• Using arrow notation which is not valid here

3.

Given this JavaScript code snippet using a blockchain API:

async function check() {
  const balance = await blockchainAPI.getBalance('0x123');
  console.log(balance);
}
check();

What will be printed if the address has 500 tokens?

medium

A. undefined

B. 500

C. Error: getBalance is not a function

D. null

Solution

1. Step 1: Understand async/await usage

   The function uses await to get the balance asynchronously, so balance will hold the returned value.

2. Step 2: Check what is logged

   console.log(balance) will print the balance value, which is 500 tokens for the given address.

3. Final Answer:

   500 -> Option B

4. Quick Check:

   Async balance fetch = 500 [OK]

Hint: Await returns the actual balance value [OK]

Common Mistakes:

• Ignoring async and expecting immediate value
• Assuming undefined if await is missing
• Confusing error messages with correct output

4.

Find the error in this Python code that tries to get a balance:

def get_balance(address):
    balance = blockchain.getBalance(address)
    return balance

print(get_balance)

medium

A. Using print instead of return

B. Incorrect function name getBalance

C. Missing import statement for blockchain

D. Missing parentheses when calling get_balance in print

Solution

1. Step 1: Check how get_balance is used in print

   The code prints get_balance without parentheses, so it prints the function object, not the result.

2. Step 2: Correct usage to call the function

   Adding parentheses and an argument like print(get_balance('0xabc')) calls the function and prints the balance.

3. Final Answer:

   Missing parentheses when calling get_balance in print -> Option D

4. Quick Check:

   Function call needs () [OK]

Hint: Call functions with () to get results [OK]

Common Mistakes:

• Printing function name instead of calling it
• Assuming function runs without parentheses
• Ignoring required function arguments

5.

You want to check balances of multiple addresses and store them in a dictionary in Python. Which code correctly does this?

addresses = ['0x1', '0x2', '0x3']
balances = {}
for addr in addresses:
    balances[addr] = blockchain.getBalance(addr)
print(balances)

hard

A. This code correctly stores balances for each address in a dictionary

B. This code will cause a syntax error due to missing colon

C. This code overwrites balances instead of storing all

D. This code does not call getBalance correctly

Solution

1. Step 1: Analyze the loop and dictionary usage

   The for loop iterates over each address and assigns the balance to balances[addr], storing all balances.

2. Step 2: Check syntax and function call

   Syntax is correct with colon and indentation; getBalance is called properly with each address.

3. Final Answer:

   This code correctly stores balances for each address in a dictionary -> Option A

4. Quick Check:

   Loop + dict assignment = correct balances [OK]

Hint: Use a loop to assign balances to dictionary keys [OK]

Common Mistakes:

• Forgetting colon after for statement
• Overwriting dictionary instead of adding keys
• Calling getBalance without argument