Short-circuiting in conditions in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the balance is positive before sending funds.

Blockchain / Solidity

if (balance [1] 0) {
    sendFunds();
}

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

A<

B!=

C>

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using equality operator instead of greater than

Checking if balance is less than zero

The condition should check if balance is greater than zero to proceed with sending funds.

2fill in blank

medium

Complete the code to short-circuit and avoid division by zero.

Blockchain / Solidity

if (denominator != 0 && numerator [1] denominator) {
    processResult();
}

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

A==

B>

C<

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using greater than or less than instead of equality

Not checking denominator first

The code checks if numerator equals denominator only if denominator is not zero, avoiding division by zero errors.

3fill in blank

hard

Fix the error in the condition to short-circuit correctly when checking user authorization.

Blockchain / Solidity

if (isAdmin() [1] isOwner()) {
    grantAccess();
}

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

A&

B||

C&&

D|

Attempts:

3 left

💡 Hint

Common Mistakes

Using logical AND instead of OR

Using bitwise operators

The logical OR operator || short-circuits and grants access if either isAdmin() or isOwner() returns true.

4fill in blank

hard

Fill both blanks to create a short-circuit condition that checks if the contract is active and the caller is authorized.

Blockchain / Solidity

if (contract[1]active && caller[2]authorized) {
    executeTransaction();
}

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

A.

B==

C!=

D&&

Attempts:

3 left

💡 Hint

Common Mistakes

Using comparison operators instead of dot

Using logical operators inside property access

Accessing properties uses the dot operator .. Both contract.active and caller.authorized check the status.

5fill in blank

hard

Fill all three blanks to create a short-circuit condition that checks if the user is verified, has enough tokens, and the transaction amount is positive.

Blockchain / Solidity

if (user[1]verified && tokens [2] amount && amount [3] 0) {
    processPayment();
}

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

A.

B>=

C>

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using equality instead of comparison

Using dot operator incorrectly

The dot operator . accesses user.verified. The tokens must be greater or equal to the amount, and the amount must be greater than zero.

Practice

1. What does short-circuiting mean in blockchain condition checks using && and ||?

easy

A. It stops checking further conditions once the result is decided.

B. It always checks all conditions regardless of the first result.

C. It reverses the order of condition checks.

D. It only works with numeric conditions.

Solution

1. Step 1: Understand short-circuiting concept

   Short-circuiting means the program stops checking conditions as soon as the final result is known.

2. Step 2: Apply to && and ||

   For &&, if the first condition is false, no need to check further. For ||, if the first condition is true, no need to check further.

3. Final Answer:

   It stops checking further conditions once the result is decided. -> Option A

4. Quick Check:

   Short-circuiting = Stops early [OK]

Hint: Stop checking when outcome is certain [OK]

Common Mistakes:

• Thinking all conditions always run
• Confusing && and || behavior
• Assuming short-circuiting only applies to numbers

2. Which of the following is the correct syntax for short-circuit AND in a blockchain smart contract condition?

easy

A. if (condition1 | condition2) { execute(); }

B. if (condition1 & condition2) { execute(); }

C. if (condition1 || condition2) { execute(); }

D. if (condition1 && condition2) { execute(); }

Solution

1. Step 1: Identify correct AND operator syntax

   The short-circuit AND operator is &&, which checks the second condition only if the first is true.

2. Step 2: Compare options

   if (condition1 && condition2) { execute(); } uses &&, correct for short-circuit AND. Options B and D use single & or | which are bitwise operators, not short-circuit. if (condition1 || condition2) { execute(); } uses OR operator.

3. Final Answer:

   if (condition1 && condition2) { execute(); } -> Option D

4. Quick Check:

   Short-circuit AND = && [OK]

Hint: Use double ampersand for AND short-circuit [OK]

Common Mistakes:

• Using single & instead of &&
• Confusing AND (&&) with OR (||)
• Using bitwise operators by mistake

3. Consider this blockchain code snippet:

bool a = false;
bool b = true;
if (a && (b = false)) { }
return b;

What is the returned value?

medium

A. false

B. Syntax error

C. true

D. Runtime error

Solution

1. Step 1: Analyze short-circuit AND behavior

   Since a is false, the b = false part is not executed due to short-circuiting.

2. Step 2: Determine final value of b

   b remains true because the assignment did not happen. So, return b; returns true.

3. Final Answer:

   true -> Option C

4. Quick Check:

   False && skips second = b stays true [OK]

Hint: False && skips right side, no assignment [OK]

Common Mistakes:

• Assuming b becomes false anyway
• Ignoring short-circuit effect
• Thinking code throws error

4. Find the error in this blockchain condition:

if (isValid || checkStatus()) {
  execute();
}

where checkStatus() is a function that must run always.

medium

A. Function checkStatus() may not run due to short-circuiting.

B. No error, code is correct.

C. Syntax error: missing semicolon.

D. Logical AND should be used instead of OR.

Solution

1. Step 1: Understand short-circuit OR behavior

   If isValid is true, checkStatus() will not run because of short-circuiting.

2. Step 2: Identify problem with function call

   Since checkStatus() must always run, using || causes it to sometimes skip, which is an error.

3. Final Answer:

   Function checkStatus() may not run due to short-circuiting. -> Option A

4. Quick Check:

   OR short-circuit skips right if left true [OK]

Hint: OR skips right if left true, function may not run [OK]

Common Mistakes:

• Assuming function always runs
• Confusing syntax error with logic error
• Using AND instead of OR without reason

5. You want to check two blockchain conditions: isOwner and hasBalance. You must run logAccess() always, but only execute transfer() if both conditions are true. Which code correctly uses short-circuiting?

hard

A. if (isOwner && hasBalance) { logAccess(); transfer(); }

B. logAccess(); if (isOwner && hasBalance) { transfer(); }

C. if (isOwner & hasBalance) { logAccess(); transfer(); }

D. if (isOwner && hasBalance && logAccess()) { transfer(); }

Solution

1. Step 1: Identify when logAccess() must run

   logAccess() must run always, so it should be called outside the condition.

2. Step 2: Check condition for transfer()

   transfer() runs only if both isOwner and hasBalance are true, so use && inside the if.

3. Step 3: Analyze options

   if (isOwner && hasBalance) { logAccess(); transfer(); } puts logAccess() inside the if, so it only runs if conditions true. if (isOwner && hasBalance && logAccess()) { transfer(); } calls logAccess() inside condition, so it may not run due to short-circuiting. if (isOwner & hasBalance) { logAccess(); transfer(); } uses single & which is bitwise, not short-circuit. logAccess(); if (isOwner && hasBalance) { transfer(); } calls logAccess() first always, then conditionally transfer().

4. Final Answer:

   logAccess(); if (isOwner && hasBalance) { transfer(); } -> Option B

5. Quick Check:

   Call always outside, conditionally inside [OK]

Hint: Call always-run functions outside conditions [OK]

Common Mistakes:

• Putting always-run function inside condition
• Using bitwise & instead of &&
• Calling functions in wrong order