Using for directive in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the for directive in blockchain programming?

The for directive is used to repeat a block of code multiple times, often to process lists or arrays of data such as transactions or blocks.

beginner

How does a for directive help when working with blockchain data?

It helps by automating repetitive tasks like iterating over transactions in a block or accounts in a ledger, making code simpler and less error-prone.

intermediate

Example of a simple for loop in blockchain smart contract pseudocode?

for (int i = 0; i < transactions.length; i++) { processTransaction(transactions[i]); } This loops through all transactions to process each one.

intermediate

What happens if the for directive's loop condition is never false?

The loop will run forever, causing an infinite loop which can freeze or crash the blockchain program or smart contract.

intermediate

Why is it important to limit iterations in blockchain for loops?

Because blockchain operations cost gas or fees, limiting iterations saves resources and prevents running out of gas during execution.

What does the for directive do in blockchain programming?

A Encrypts transactions

B Stores data permanently

C Creates new blocks

D Repeats code multiple times

The for directive repeats a block of code multiple times, useful for processing lists or arrays.

What is a risk of a for loop without a proper end condition?

A Infinite loop

B Faster execution

C Data loss

D Automatic error fixing

Without a proper end condition, the loop runs forever causing an infinite loop.

Why should blockchain for loops limit the number of iterations?

A To create more transactions

B To increase block size

C To save gas fees

D To speed up mining

Limiting iterations saves gas fees, which are costs for running blockchain operations.

Which of these is a typical use of a for directive in blockchain code?

A Iterate over transactions

B Generate cryptographic keys

C Validate block hashes

D Broadcast network messages

For loops commonly iterate over transactions or accounts to process them one by one.

In blockchain smart contracts, what does the for directive help avoid?

A Block creation

B Manual repetition of code

C Data encryption

D Network latency

For loops avoid writing the same code repeatedly by automating repetition.

Explain how the for directive works in blockchain programming and why it is useful.

Think about how you would handle many transactions one by one.

Describe the risks of using a for directive incorrectly in blockchain smart contracts.

What happens if the loop never stops?

Practice

1. What does the for directive do in blockchain programming?

easy

A. Creates a new blockchain transaction

B. Encrypts blockchain data

C. Deletes data from the blockchain

D. Repeats a block of code for each item in a list

Solution

1. Step 1: Understand the purpose of the for directive

   The for directive is used to repeat code multiple times, once for each item in a list or collection.

2. Step 2: Relate to blockchain data processing

   In blockchain, it helps process multiple data items like transactions or blocks easily by looping through them.

3. Final Answer:

   Repeats a block of code for each item in a list -> Option D

4. Quick Check:

   for directive = repeat code per item [OK]

Hint: For loops repeat code for each item in a list [OK]

Common Mistakes:

• Confusing for directive with data encryption
• Thinking for creates or deletes blockchain data
• Mixing for directive with transaction creation

2. Which of the following is the correct syntax to loop over an array blocks using the for directive?

easy

A. for (let block of blocks) { /* code */ }

B. for (let block in blocks) { /* code */ }

C. for blocks (let block) { /* code */ }

D. loop (block in blocks) { /* code */ }

Solution

1. Step 1: Identify correct for loop syntax for arrays

   In modern blockchain programming (JavaScript style), for...of loops over array elements directly.

2. Step 2: Compare options

   for (let block of blocks) { /* code */ } uses for (let block of blocks), which is correct. for (let block in blocks) { /* code */ } uses in which loops keys, not values. Options C and D are invalid syntax.

3. Final Answer:

   for (let block of blocks) { /* code */ } -> Option A

4. Quick Check:

   Use for...of to loop array values [OK]

Hint: Use for...of to loop over array items directly [OK]

Common Mistakes:

• Using for...in to loop arrays (loops keys, not values)
• Writing invalid loop syntax
• Confusing loop keywords like loop or for blocks

3. What will be the output of this code snippet?

const blocks = [101, 102, 103];
let result = '';
for (const block of blocks) {
  result += block + '-';
}
console.log(result);

medium

A. 101-102-103

B. 101-102-103-

C. 101102103

D. SyntaxError

Solution

1. Step 1: Understand the loop operation

   The loop goes through each number in blocks: 101, 102, 103. Each number is added to result with a '-' after it.

2. Step 2: Trace the string concatenation

   After loop: result = '101-' + '102-' + '103-' = '101-102-103-'

3. Final Answer:

   101-102-103- -> Option B

4. Quick Check:

   Concatenate each block with '-' = 101-102-103- [OK]

Hint: Add '-' after each item in loop to get trailing dash [OK]

Common Mistakes:

• Expecting no trailing dash at the end
• Confusing for...of with for...in output
• Thinking output is concatenated without dashes

4. Identify the error in this for loop that processes blockchain transactions:

const transactions = [10, 20, 30];
for (transaction of transactions) {
  console.log(transaction);
}

medium

A. for loop syntax is invalid

B. Incorrect array name

C. Missing declaration keyword for loop variable

D. console.log cannot print transaction

Solution

1. Step 1: Check loop variable declaration

   The loop variable transaction is used without let or const, which causes a ReferenceError in strict mode.

2. Step 2: Validate other parts

   Array name and syntax are correct. console.log can print numbers. So only missing declaration is the error.

3. Final Answer:

   Missing declaration keyword for loop variable -> Option C

4. Quick Check:

   Declare loop variable with let or const [OK]

Hint: Always declare loop variable with let or const [OK]

Common Mistakes:

• Forgetting let or const before loop variable
• Assuming console.log can't print numbers
• Thinking array name is wrong

5. You have a list of blockchain block heights: [5, 10, 15, 20]. Using a for directive, how do you create a new array with each height doubled?

hard

A. for (let i = 0; i < heights.length; i++) { doubled[i] = heights[i] * 2; }

B. for (heights of doubled) { doubled.push(heights * 2); }

C. for (let h in heights) { doubled.push(h * 2); }

D. for (h of heights) { doubled.push(h * 2); }

Solution

1. Step 1: Understand the goal and data

   We want to double each number in heights and store in doubled array. heights = [5,10,15,20]

2. Step 2: Analyze each option

   for (h of heights) { doubled.push(h * 2); } uses for...of but missing let or const declaration for loop variable h, so ReferenceError. for (let i = 0; i < heights.length; i++) { doubled[i] = heights[i] * 2; } uses classic for loop with index to assign doubled[i] = heights[i]*2, which works correctly. for (let h in heights) { doubled.push(h * 2); } uses for...in which loops keys (indexes as strings), so multiplying keys is wrong. for (heights of doubled) { doubled.push(heights * 2); } has invalid syntax (for (heights of doubled)) and reversed arrays.

3. Final Answer:

   for (let i = 0; i < heights.length; i++) { doubled[i] = heights[i] * 2; } -> Option A

4. Quick Check:

   Use index loop to assign doubled values [OK]

Hint: Use index loop to fill new array with doubled values [OK]

Common Mistakes:

• Using for...in to loop array values
• Not initializing doubled array before pushing
• Writing invalid loop syntax