Why If-else statements in Blockchain / Solidity? - Purpose & Use Cases

The Big Idea

What if your blockchain could instantly decide what to do next without you lifting a finger?

The Scenario

Imagine you are manually checking each transaction on a blockchain to decide if it should be approved or rejected based on certain rules.

You write down every condition on paper and then try to apply them one by one for each transaction.

The Problem

This manual checking is slow and tiring.

It’s easy to forget a rule or make mistakes when handling many transactions.

Also, you cannot quickly change the rules or handle new cases without rewriting everything.

The Solution

If-else statements let you write clear rules in code that automatically check conditions and decide what to do next.

This makes your blockchain program smart and fast, handling many cases without errors.

Before vs After

✗ Before

if transaction_amount > 1000:
approve
else:
reject

✓ After

if transaction_amount > 1000:
    approve()
else:
    reject()

What It Enables

If-else statements enable your blockchain code to make decisions automatically, adapting to different situations instantly.

Real Life Example

In a blockchain voting system, if a user’s vote is valid and within the deadline, the vote is counted; otherwise, it is ignored.

Key Takeaways

If-else statements automate decision-making in code.

They reduce errors and speed up processing.

They make blockchain programs flexible and reliable.

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