If-else statements in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of an if-else statement?

An if-else statement lets a program choose between two paths: one if a condition is true, and another if it is false.

beginner

How do you write a simple if-else statement in Solidity?

You write it like this:

if (condition) {
  // code if true
} else {
  // code if false
}

beginner

What happens if the if condition is false and there is no else block?

The program skips the if block and continues with the code after it, doing nothing inside the if.

intermediate

Can you have multiple conditions checked in an if-else structure?

Yes, by using else if blocks between if and else, you can check many conditions in order.

intermediate

Why are if-else statements important in blockchain smart contracts?

They let contracts make decisions, like checking if a user has enough balance before sending tokens.

What does an if-else statement do in a program?

A Chooses between two actions based on a condition

B Repeats code multiple times

C Stores data permanently

D Defines a new function

An if-else statement lets the program pick one action if a condition is true, and another if false.

In Solidity, what keyword starts the condition check?

A if

B for

C while

D switch

The keyword 'if' starts the condition check in Solidity and many other languages.

What happens if the if condition is false and there is no else block?

A The program crashes

B The if block runs anyway

C The program skips the if block and continues

D The else block runs

Without an else block, if the condition is false, the program just skips the if block.

How can you check multiple conditions in an if-else structure?

A Using multiple if statements only

B Using else if blocks

C Using switch-case only

D Using loops

You use else if blocks to check multiple conditions in order.

Why are if-else statements useful in smart contracts?

A To store user data

B To connect to the internet

C To create new tokens

D To make decisions based on conditions

If-else statements let smart contracts decide what to do based on conditions like balances or permissions.

Explain how an if-else statement controls the flow of a program.

Think about choosing between two roads based on a sign.

Describe a real-life example where an if-else statement would be useful in a blockchain smart contract.

Imagine sending money only if you have enough.

Practice

1.

What does an if-else statement do in a blockchain smart contract?

easy

A. It chooses between two actions based on a condition.

B. It repeats a set of instructions multiple times.

C. It stores data permanently on the blockchain.

D. It creates a new blockchain network.

Solution

1. Step 1: Understand the role of if-else

   An if-else statement checks a condition and decides which code to run next.

2. Step 2: Apply to blockchain context

   In smart contracts, it helps decide actions like approving transactions or rejecting them.

3. Final Answer:

   It chooses between two actions based on a condition. -> Option A

4. Quick Check:

   If-else = choose action [OK]

Hint: If-else picks one path out of two based on condition [OK]

Common Mistakes:

• Confusing if-else with loops
• Thinking if-else stores data
• Believing if-else creates blockchains

2.

Which of the following is the correct syntax for an if-else statement in Solidity?

uint balance = 100;

easy

A. if (balance > 50) then { /* code */ } else { /* code */ }

B. if balance > 50 then { /* code */ } else { /* code */ }

C. if balance > 50: { /* code */ } else: { /* code */ }

D. if (balance > 50) { /* code */ } else { /* code */ }

Solution

1. Step 1: Recall Solidity if-else syntax

   Solidity uses parentheses around conditions and braces for code blocks without 'then'.

2. Step 2: Compare options

   Only if (balance > 50) { /* code */ } else { /* code */ } matches Solidity syntax: if (condition) { } else { }.

3. Final Answer:

   if (balance > 50) { /* code */ } else { /* code */ } -> Option D

4. Quick Check:

   Solidity if-else = parentheses + braces [OK]

Hint: Solidity if-else uses parentheses and braces, no 'then' [OK]

Common Mistakes:

• Using 'then' keyword
• Missing parentheses around condition
• Using colons instead of braces

3.

What will be the output of this Solidity function when amount = 150?

function checkAmount(uint amount) public pure returns (string memory) {
  if (amount > 100) {
    return "Large amount";
  } else {
    return "Small amount";
  }
}

medium

A. Compilation error

B. "Small amount"

C. "Large amount"

D. No output

Solution

1. Step 1: Check the condition with amount = 150

   Since 150 > 100, the if condition is true.

2. Step 2: Determine which return runs

   The function returns "Large amount" because the if branch runs.

3. Final Answer:

   "Large amount" -> Option C

4. Quick Check:

   150 > 100 -> "Large amount" [OK]

Hint: Check condition value to pick correct return [OK]

Common Mistakes:

• Confusing greater than with less than
• Assuming else runs when if is true
• Expecting syntax errors

4.

Find the error in this Solidity snippet:

function checkBalance(uint balance) public pure returns (string memory) {
  if balance > 0 {
    return "Positive";
  } else {
    return "Zero or Negative";
  }
}

medium

A. Missing parentheses around the if condition

B. Missing semicolon after return statements

C. Incorrect return type

D. Missing else keyword

Solution

1. Step 1: Check if statement syntax

   In Solidity, the condition must be inside parentheses: if (condition).

2. Step 2: Identify the error

   The code has if balance > 0 without parentheses, causing a syntax error.

3. Final Answer:

   Missing parentheses around the if condition -> Option A

4. Quick Check:

   if condition needs parentheses [OK]

Hint: Always put condition inside parentheses in if [OK]

Common Mistakes:

• Omitting parentheses in if condition
• Thinking semicolons are missing after return
• Confusing return type errors

5.

You want to write a Solidity function that returns "Access granted" only if userRole is exactly "admin", otherwise returns "Access denied". Which code snippet correctly uses if-else to do this?

hard

A. if (userRole = "admin") { return "Access granted"; } else { return "Access denied"; }

B. if (userRole == "admin") { return "Access granted"; } else { return "Access denied"; }

C. if userRole == "admin" { return "Access granted"; } else { return "Access denied"; }

D. if (userRole.equals("admin")) { return "Access granted"; } else { return "Access denied"; }

Solution

1. Step 1: Understand string comparison in Solidity

   Solidity uses == to compare strings for equality, not = or methods like equals().

2. Step 2: Check syntax correctness

   if (userRole == "admin") { return "Access granted"; } else { return "Access denied"; } uses == with parentheses and braces correctly.

3. Final Answer:

   if (userRole == "admin") { return "Access granted"; } else { return "Access denied"; } -> Option B

4. Quick Check:

   Use == for string comparison in if [OK]

Hint: Use == to compare strings in Solidity if conditions [OK]

Common Mistakes:

• Using single = instead of ==
• Omitting parentheses in if
• Using Java-style equals() method