Using for directive in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to loop through each block in the blockchain.

Blockchain / Solidity

for block in [1] {
    // process block
}

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

Aledger

Bchain

Cblocks

Dblockchain

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name that does not hold the blocks.

Forgetting to use the correct variable name.

The variable blockchain holds the list of blocks to loop through.

2fill in blank

medium

Complete the code to print the hash of each block in the blockchain.

Blockchain / Solidity

for block in blockchain {
    print([1]);
}

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

Ablock.hash

Bblock.previousHash

Cblock.data

Dblock.index

Attempts:

3 left

💡 Hint

Common Mistakes

Printing the previous block's hash instead of the current block's hash.

Printing block data instead of the hash.

The hash property of each block contains its unique hash value.

3fill in blank

hard

Fix the error in the for loop to correctly iterate over the blockchain array.

Blockchain / Solidity

for (let [1] = 0; [1] < blockchain.length; [1]++) {
    console.log(blockchain[[1]].hash);
}

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

Ai

Bblock

Cindex

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name that is not consistent throughout the loop.

Using a variable name that conflicts with block objects.

The variable i is the conventional index variable used in for loops.

4fill in blank

hard

Fill both blanks to create a dictionary of block hashes and their data for blocks with index greater than 0.

Blockchain / Solidity

const blockData = {
  [1]: blockchain[[2]].data
  for [2] in blockchain if blockchain[[2]].index > 0
};

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

Ablockchain[blockIndex].hash

BblockIndex

Dblock

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong variable name for the loop.

Not accessing blockchain elements by index.

Use blockIndex as the loop variable and access the hash and data by index.

5fill in blank

hard

Fill both blanks to filter blocks with data value greater than 100 and create a new list of their hashes.

Blockchain / Solidity

const filteredHashes = blockchain.filter(block => block.data [1] 100).map(block => block.[2]);

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

A>

Bhash

C<

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong comparison operator.

Mapping to the wrong property.

Use > to filter blocks with data greater than 100 and map to their hash.

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