Using for directive in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Using for directive

Start

↓

Initialize loop variable

↓

Check loop condition

| Yes↓

Execute loop body

↓

Update loop variable

↓

Check loop condition

No↓

End loop

The for directive starts by setting a variable, checks a condition, runs the loop body if true, updates the variable, and repeats until the condition is false.

Execution Sample

Blockchain / Solidity

for (let i = 0; i < 3; i++) {
  console.log(i);
}

This code prints numbers 0, 1, and 2 using a for loop.

Execution Table

Iteration	i	Condition i < 3	Action	Output
1	0	true	Print i=0	0
2	1	true	Print i=1	1
3	2	true	Print i=2	2
4	3	false	Exit loop

💡 i reaches 3, condition i < 3 is false, loop ends

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
i	0	1	2	3	3

Key Moments - 2 Insights

Why does the loop stop when i equals 3?

Because the condition i < 3 becomes false at iteration 4 (see execution_table row 4), so the loop exits.

When is the loop variable i updated?

After executing the loop body in each iteration, i is increased by 1 before the next condition check (see variable_tracker changes).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of i during the third iteration?

A 2

B 3

C 1

D 0

💡 Hint

Check the 'i' column in the execution_table at iteration 3.

At which iteration does the loop condition become false?

A Iteration 3

B Iteration 2

C Iteration 4

D Iteration 1

💡 Hint

Look at the 'Condition i < 3' column in the execution_table.

If the loop condition changed to i <= 3, how many times would the loop run?

A 3 times

B 4 times

C 2 times

D 5 times

💡 Hint

Think about when i <= 3 is true based on variable_tracker values.

Concept Snapshot

for directive syntax:
for (initialization; condition; update) {
  // code to repeat
}

- Initialize variable once
- Check condition before each loop
- Run code if true
- Update variable after each loop
- Repeat until condition false

Full Transcript

The for directive runs a loop by first setting a variable, then checking if it meets a condition. If yes, it runs the loop body, then updates the variable. This repeats until the condition is false, then the loop stops. For example, a loop from 0 to 2 prints 0, 1, 2. The variable i starts at 0 and increases by 1 each time. When i reaches 3, the condition fails and the loop ends.

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