View and pure functions in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a view function call

Consider this Solidity contract snippet with a view function. What will be the output when calling getBalance() after deployment?

Blockchain / Solidity

contract Wallet {
    uint private balance = 100;
    
    function getBalance() public view returns (uint) {
        return balance;
    }
}

A Transaction rejected

B 0

C Compilation error

D 100

💡 Hint

View functions read state without changing it.

The getBalance() function is a view function that returns the stored balance which is initialized to 100. Calling it does not change state and returns 100.

❓ Predict Output

intermediate

Pure function output with parameters

What is the output of calling add(5, 3) in this Solidity contract?

Blockchain / Solidity

contract Calculator {
    function add(uint a, uint b) public pure returns (uint) {
        return a + b;
    }
}

A Compilation error

B 8

C 0

D Transaction reverted

💡 Hint

Pure functions return values based only on inputs.

The add function is pure and returns the sum of a and b. Calling add(5, 3) returns 8.

❓ Predict Output

advanced

Effect of modifying state in a view function

What happens when this contract is compiled and deployed?

Blockchain / Solidity

contract Test {
    uint x = 10;
    
    function wrongView() public view returns (uint) {
        x = 20;
        return x;
    }
}

A Returns 10

B Returns 20

C Compilation error due to state modification in view function

D Runtime error when calling wrongView

💡 Hint

View functions cannot modify state variables.

The wrongView function tries to modify the state variable x inside a view function, which is not allowed and causes a compilation error.

❓ Predict Output

advanced

Pure function calling a view function

What error occurs when compiling this contract?

Blockchain / Solidity

contract Mixed {
    uint y = 5;
    
    function getY() public view returns (uint) {
        return y;
    }
    
    function pureCaller() public pure returns (uint) {
        return getY();
    }
}

A Compilation error: pure function cannot call view function

B Runtime error when calling pureCaller

C Returns 0

D Returns 5

💡 Hint

Pure functions cannot read or modify state or call view functions.

The pureCaller function is declared pure but calls getY, a view function that reads state. This is disallowed and causes a compilation error.

🧠 Conceptual

expert

Gas cost difference between view and pure functions

Which statement about gas costs when calling view and pure functions externally (via call) is correct?

A Both view and pure functions called externally cost no gas

B View functions cost gas, pure functions do not

C Both view and pure functions always cost gas

D Pure functions cost gas, view functions do not

💡 Hint

External calls to view or pure functions do not change blockchain state.

When called externally (not in a transaction), both view and pure functions do not consume gas because they do not modify state and run locally on the node.

Practice

1. Which statement best describes a view function in a blockchain smart contract?

easy

A. It reads blockchain data but does not modify it.

B. It modifies blockchain data and stores new values.

C. It neither reads nor writes blockchain data.

D. It only accepts inputs and returns no output.

Solution

1. Step 1: Understand the role of view functions

   View functions are designed to read data from the blockchain without changing it.

2. Step 2: Compare options with definition

   It reads blockchain data but does not modify it. matches this definition exactly, while others describe modifying or ignoring blockchain data.

3. Final Answer:

   It reads blockchain data but does not modify it. -> Option A

4. Quick Check:

   View function = reads only [OK]

Hint: View functions read data but never change it [OK]

Common Mistakes:

• Confusing view with functions that modify data
• Thinking view functions can write data
• Assuming view functions do not read blockchain data

2. Which of the following is the correct syntax to declare a pure function in Solidity?

easy

A. function calculate() public pure returns (uint) {}

B. function calculate() public view returns (uint) {}

C. function calculate() public returns (uint) {}

D. function calculate() pure returns (uint) {}

Solution

1. Step 1: Recall Solidity function declaration syntax

   Functions must specify visibility (e.g., public) and state mutability (pure/view) before returns.

2. Step 2: Identify correct pure function syntax

   function calculate() public pure returns (uint) {} correctly includes visibility and pure keyword before returns. function calculate() pure returns (uint) {} misses visibility, which is required.

3. Final Answer:

   function calculate() public pure returns (uint) {} -> Option A

4. Quick Check:

   Pure function syntax = public pure returns [OK]

Hint: Pure functions need visibility and 'pure' keyword before returns [OK]

Common Mistakes:

• Omitting visibility keyword
• Using view instead of pure
• Placing pure after returns

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

function multiply(uint a, uint b) public pure returns (uint) {
    return a * b;
}

// Called with multiply(3, 4)

medium

A. Error: function cannot modify state

B. 7

C. 0

D. 12

Solution

1. Step 1: Understand the function logic

   The function multiplies two unsigned integers and returns the result.

2. Step 2: Calculate the output for inputs 3 and 4

   3 multiplied by 4 equals 12, so the function returns 12.

3. Final Answer:

   12 -> Option D

4. Quick Check:

   3 * 4 = 12 [OK]

Hint: Multiply inputs directly in pure function for output [OK]

Common Mistakes:

• Adding instead of multiplying
• Expecting state change error
• Confusing pure with view function behavior

4. Identify the error in this Solidity function declared as view:

uint count = 0;
function increment() public view returns (uint) {
    count += 1;
    return count;
}

medium

A. Missing returns keyword.

B. Cannot modify state variable in a view function.

C. Function should be pure, not view.

D. No error, code is correct.

Solution

1. Step 1: Understand view function restrictions

   View functions cannot modify state variables; they only read data.

2. Step 2: Analyze the code behavior

   The function increments a state variable, which is a write operation, causing an error.

3. Final Answer:

   Cannot modify state variable in a view function. -> Option B

4. Quick Check:

   View functions = no state changes [OK]

Hint: View functions cannot change state variables [OK]

Common Mistakes:

• Thinking view allows state changes
• Confusing pure and view keywords
• Ignoring state variable modification

5. You want to create a function that calculates the sum of two numbers without reading or modifying any blockchain data. Which function type should you use and how should it be declared in Solidity?

hard

A. Use a view function declared as function sum(uint a, uint b) public view returns (uint).

B. Use a regular function declared as function sum(uint a, uint b) public returns (uint).

C. Use a pure function declared as function sum(uint a, uint b) public pure returns (uint).

D. Use a payable function declared as function sum(uint a, uint b) public payable returns (uint).

Solution

1. Step 1: Identify function requirements

   The function must not read or modify blockchain data, only use inputs to calculate sum.

2. Step 2: Choose correct function type and declaration

   Pure functions do not read or write blockchain data, so declaring it as public pure with inputs and returns is correct.

3. Final Answer:

   Use a pure function declared as function sum(uint a, uint b) public pure returns (uint). -> Option C

4. Quick Check:

   Pure = no blockchain data access [OK]

Hint: Pure functions only use inputs, no blockchain data access [OK]

Common Mistakes:

• Using view instead of pure
• Using payable unnecessarily
• Not specifying pure keyword