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Blockchain / Solidityprogramming~30 mins

Reading contract state in Blockchain / Solidity - Mini Project: Build & Apply

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Reading contract state
📖 Scenario: You are working with a simple blockchain smart contract that stores a greeting message. You want to read the current greeting from the contract.
🎯 Goal: Build a small program that connects to the contract and reads the greeting message stored in its state.
📋 What You'll Learn
Create a variable with the contract address
Create a variable with the contract ABI (Application Binary Interface)
Connect to the blockchain provider
Create a contract instance using the address and ABI
Call the contract's greeting function to read the message
Print the greeting message
💡 Why This Matters
🌍 Real World
Reading contract state is essential for decentralized apps to display current data stored on the blockchain, like balances, messages, or settings.
💼 Career
Blockchain developers often need to read contract state to build user interfaces and verify contract data without changing the blockchain.
Progress0 / 4 steps
1
Set up contract address and ABI
Create a variable called contract_address and set it to the string "0x1234567890abcdef1234567890abcdef12345678". Then create a variable called contract_abi and set it to the list [{"constant":true,"inputs":[],"name":"greet","outputs":[{"name":"","type":"string"}],"payable":false,"stateMutability":"view","type":"function"}].
Blockchain / Solidity
Hint

Use a string for the contract address and a list with a dictionary for the ABI.

2
Connect to blockchain provider
Create a variable called provider and set it to a new instance of Web3.HTTPProvider with the URL "https://mainnet.infura.io/v3/YOUR-PROJECT-ID". Then create a variable called w3 and set it to a new Web3 instance using provider.
Blockchain / Solidity
Hint

Use Web3.HTTPProvider with the Infura URL and then create a Web3 instance.

3
Create contract instance and read greeting
Create a variable called contract by calling w3.eth.contract with address=contract_address and abi=contract_abi. Then create a variable called greeting by calling the greet function on contract.functions and using .call() to read the value.
Blockchain / Solidity
Hint

Use w3.eth.contract with address and abi, then call the greet function.

4
Print the greeting message
Write a print statement to display the value of the variable greeting.
Blockchain / Solidity
Hint

Use print(greeting) to show the message.

Practice

(1/5)
1. What is the main purpose of using .call() when interacting with a blockchain smart contract?
easy
A. To send tokens to another address
B. To read data from the contract without changing its state
C. To deploy a new smart contract
D. To mine a new block on the blockchain

Solution

  1. Step 1: Understand what .call() does

    .call() is used to read data from a smart contract without creating a transaction or changing the blockchain state.

  2. Step 2: Compare with other blockchain actions

    Sending tokens or deploying contracts changes state and requires transactions, unlike .call().

  3. Final Answer:

    To read data from the contract without changing its state -> Option B

  4. Quick Check:

    .call() reads state without transactions [OK]
Hint: Use .call() only to read data, not to write [OK]
Common Mistakes:
  • Thinking .call() sends transactions
  • Confusing .call() with contract deployment
  • Assuming .call() changes contract state
2. Which of the following is the correct syntax to read a contract's public variable balance using .call() in JavaScript?
easy
A. const bal = contract.methods.balance().call();
B. const bal = contract.call.methods.balance();
C. const bal = contract.methods.balance();
D. const bal = contract.balance.call();

Solution

  1. Step 1: Recall the correct method call pattern

    To read a contract variable, use contract.methods.variableName().call() in JavaScript.

  2. Step 2: Check each option's syntax

    const bal = contract.methods.balance().call(); matches the correct pattern. Options B, C, and D have incorrect method chaining or missing .call().

  3. Final Answer:

    const bal = contract.methods.balance().call(); -> Option A

  4. Quick Check:

    Correct syntax = const bal = contract.methods.balance().call(); [OK]
Hint: Remember: contract.methods.<name>().call() reads state [OK]
Common Mistakes:
  • Placing .call() before .methods
  • Omitting .call() when reading
  • Using contract.balance.call() directly
3. Given the following Solidity contract snippet:
contract Wallet {
    uint public balance = 100;
    function getBalance() public view returns (uint) {
        return balance;
    }
}

What will be the output of this JavaScript code?
const bal = await contract.methods.getBalance().call();
console.log(bal);
medium
A. 100
B. undefined
C. Error: getBalance is not a function
D. 0

Solution

  1. Step 1: Understand the Solidity function

    The getBalance() function returns the current balance value, which is 100.

  2. Step 2: Analyze the JavaScript call

    The JavaScript code calls getBalance() using .call(), which reads the value without changing state, returning 100.

  3. Final Answer:

    100 -> Option A

  4. Quick Check:

    Calling view function returns stored value [OK]
Hint: View functions return stored values via .call() [OK]
Common Mistakes:
  • Expecting a transaction receipt instead of value
  • Confusing .call() with sending a transaction
  • Assuming default value is zero
4. You try to read a contract's state variable using contract.methods.value.call (without parentheses). What error will you most likely encounter?
medium
A. TypeError: contract.methods.value.call is not a function, missing parentheses
B. SyntaxError: Unexpected token
C. No error, returns the value directly
D. TypeError: contract.methods.value.call is not a function

Solution

  1. Step 1: Identify the missing parentheses issue

    .call is a function and must be invoked with parentheses ().

  2. Step 2: Understand the error message

    Without parentheses, JavaScript treats .call as a property, causing a TypeError indicating it's not a function call.

  3. Final Answer:

    TypeError: contract.methods.value.call is not a function -> Option D

  4. Quick Check:

    Missing () on .call() causes TypeError [OK]
Hint: Always add () after .call to execute it [OK]
Common Mistakes:
  • Forgetting parentheses on .call()
  • Assuming .call is a property, not a function
  • Ignoring JavaScript function call syntax
5. You want to read multiple state variables owner (address) and totalSupply (uint) from a deployed contract efficiently. Which approach is best?
hard
A. Call contract.methods.owner().call() and contract.methods.totalSupply().call() separately
B. Use contract.methods.owner.call and contract.methods.totalSupply.call without parentheses
C. Create a new contract function that returns both variables in a tuple and call it once
D. Read owner with .call() and read totalSupply from the blockchain directly

Solution

  1. Step 1: Understand multiple calls cost

    Calling each variable separately sends multiple requests, which is less efficient.

  2. Step 2: Use a combined function

    Creating a contract function that returns both variables together reduces calls and improves efficiency.

  3. Final Answer:

    Create a new contract function that returns both variables in a tuple and call it once -> Option C

  4. Quick Check:

    Batch reading state reduces calls and improves performance [OK]
Hint: Batch reads in one call for efficiency [OK]
Common Mistakes:
  • Making multiple separate calls unnecessarily
  • Trying to read blockchain data outside contract calls
  • Forgetting parentheses on .call()
Recommended

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10 questions~5 mineasy hard