View and pure functions in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a view function that returns a stored number.

Blockchain / Solidity

function getNumber() public [1] returns (uint) {
    return storedNumber;
}

Drag options to blanks, or click blank then click option'

Aexternal

Bpure

Cpayable

Dview

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pure' instead of 'view' when reading state variables.

Forgetting to specify 'view' or 'pure' for functions that don't modify state.

The view keyword indicates the function reads state but does not modify it.

2fill in blank

medium

Complete the code to declare a pure function that returns the sum of two numbers.

Blockchain / Solidity

function add(uint a, uint b) public [1] returns (uint) {
    return a + b;
}

Drag options to blanks, or click blank then click option'

Apayable

Bpure

Cview

Dexternal

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'view' instead of 'pure' when the function does not read state.

Omitting the keyword for functions that do not access state.

The pure keyword means the function neither reads nor modifies the blockchain state.

3fill in blank

hard

Fix the error in the function declaration to make it a view function.

Blockchain / Solidity

function getBalance() public [1] returns (uint) {
    return balance;
}

Drag options to blanks, or click blank then click option'

Aprivate

Bpayable

Cview

Dpure

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pure' which disallows reading state variables.

Using 'payable' which is unrelated to reading state.

The function reads the state variable balance, so it must be marked as view.

4fill in blank

hard

Fill both blanks to create a pure function that returns the product of two numbers.

Blockchain / Solidity

function multiply(uint x, uint y) public [1] returns (uint) {
    return x [2] y;
}

Drag options to blanks, or click blank then click option'

Apure

Bview

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'view' instead of 'pure' for functions that do not access state.

Using '+' instead of '*' for multiplication.

The function does not read or modify state, so it is pure. The multiplication operator is *.

5fill in blank

hard

Fill all three blanks to create a view function that returns the length of a stored string.

Blockchain / Solidity

function getStringLength() public [1] returns (uint) {
    return [2].[3];
}

Drag options to blanks, or click blank then click option'

Aview

BstoredString

Clength

Dpure

Attempts:

3 left

💡 Hint

Common Mistakes

Marking the function as 'pure' when it reads state.

Using incorrect variable name or property for length.

The function reads the stored string variable storedString and returns its length. It must be marked view.

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