For and while loops in Blockchain / Solidity - Mini Project: Build & Apply

Counting Blocks with For and While Loops

📖 Scenario: Imagine you are working with a simple blockchain that stores blocks as numbers. You want to count and display blocks using loops.

🎯 Goal: Build a program that uses both for and while loops to count blocks in a blockchain.

📋 What You'll Learn

Create a list called blocks with the numbers 1 to 5

Create a variable called count and set it to 0

Use a for loop with variable block to iterate over blocks and increase count by 1 each time

Use a while loop to print numbers from 1 up to count

💡 Why This Matters

🌍 Real World

Counting blocks or transactions is common in blockchain programs to track progress or verify data.

💼 Career

Understanding loops is essential for blockchain developers to process data efficiently and automate tasks.

1

Create the blocks list

Create a list called blocks with the numbers 1, 2, 3, 4, and 5.

Blockchain / Solidity

# Create the list blocks with numbers 1 to 5
# Your code here

Hint

Use square brackets [] to create a list with the numbers inside separated by commas.

2

Create the count variable

Create a variable called count and set it to 0.

Blockchain / Solidity

blocks = [1, 2, 3, 4, 5]
# Create a variable count and set it to 0
# Your code here

Hint

Just write count = 0 on a new line.

3

Use a for loop to count blocks

Use a for loop with variable block to iterate over blocks and increase count by 1 inside the loop.

Blockchain / Solidity

blocks = [1, 2, 3, 4, 5]
count = 0
# Use a for loop to increase count by 1 for each block
# Your code here

Hint

Use for block in blocks: and inside the loop write count += 1.

4

Use a while loop to print numbers

Create a variable called i and set it to 1. Use a while loop to print i while it is less than or equal to count. Increase i by 1 each time inside the loop.

Blockchain / Solidity

blocks = [1, 2, 3, 4, 5]
count = 0
for block in blocks:
    count += 1
# Create i = 1 and use a while loop to print i while i <= count
# Increase i by 1 inside the loop
# Your code here

Hint

Start with i = 1. Use while i <= count: and inside print i and then increase i by 1.

Practice

1. Which statement best describes a for loop in blockchain programming?

easy

A. It repeats code while a condition is true.

B. It runs only once and then stops.

C. It repeats code a fixed number of times.

D. It stops the program immediately.

Solution

1. Step 1: Understand the for loop purpose

   A for loop is designed to repeat code a set number of times, often counting through a range or list.

2. Step 2: Compare with other loop types

   A while loop repeats while a condition is true, not a fixed count. The other options do not describe for loops correctly.

3. Final Answer:

   It repeats code a fixed number of times. -> Option C

4. Quick Check:

   for loop = fixed repeats [OK]

Hint: For loops count repeats; while loops check conditions [OK]

Common Mistakes:

• Confusing for loops with while loops
• Thinking for loops run only once
• Believing for loops stop the program

2. Which of the following is the correct syntax for a while loop in blockchain programming?

easy

A. while (condition) { // code }

B. while condition: // code

C. while condition { // code }

D. while (condition): // code

Solution

1. Step 1: Identify correct syntax style

   In blockchain programming languages like Solidity, the while loop uses parentheses for the condition and curly braces for the code block.

2. Step 2: Check each option

   while (condition) { // code } matches the correct syntax: while (condition) { // code }. Others miss parentheses or use incorrect punctuation.

3. Final Answer:

   while (condition) { // code } -> Option A

4. Quick Check:

   Correct while syntax uses parentheses and braces [OK]

Hint: Remember: while(condition) { code } uses parentheses and braces [OK]

Common Mistakes:

• Omitting parentheses around condition
• Using colon instead of braces
• Mixing syntax from other languages

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

uint sum = 0;
for (uint i = 1; i <= 3; i++) {
  sum += i;
}
return sum;

medium

A. 6

B. 3

C. 0

D. Error

Solution

1. Step 1: Analyze the loop iterations

   The loop runs with i = 1, 2, 3. Each time, sum adds i: sum = 0+1, then 1+2, then 3+3.

2. Step 2: Calculate the final sum

   Sum after loop: 0+1=1, 1+2=3, 3+3=6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   1+2+3 = 6 [OK]

Hint: Add numbers from 1 to 3 to get sum quickly [OK]

Common Mistakes:

• Stopping loop early
• Adding wrong values
• Confusing sum initialization

4. Identify the error in this code snippet:

uint i = 0;
while (i < 3)
  i++;

medium

A. Condition should be i <= 3

B. Missing braces {} for while loop body

C. Variable i should start at 1

D. Incorrect increment operator

Solution

1. Step 1: Check loop body syntax

   The while loop lacks braces {}, so only the next statement is inside the loop.

2. Step 2: Understand impact of missing braces

   Without braces, the loop runs only the single statement i++;. This is valid but can cause confusion or errors if more statements are added.

3. Final Answer:

   Missing braces {} for while loop body -> Option B

4. Quick Check:

   Always use braces for clarity in loops [OK]

Hint: Use braces {} to clearly define loop body [OK]

Common Mistakes:

• Assuming multiple statements run without braces
• Changing variable start unnecessarily
• Altering loop condition wrongly

5. You want to create a loop that counts down from 5 to 1 and stores each number in an array. Which code snippet correctly uses a for loop to do this in blockchain programming?

A) for (uint i = 5; i > 0; i--) { arr.push(i); }
B) for (uint i = 1; i <= 5; i++) { arr.push(i); }
C) for (uint i = 5; i >= 1; i++) { arr.push(i); }
D) for (uint i = 5; i > 0; i++) { arr.push(i); }

hard

A. Infinite loop due to i++ with i > 0

B. Counts up from 1 to 5

C. Infinite loop due to i++ with i >= 1

D. Counts down from 5 to 1 correctly

Solution

1. Step 1: Analyze each loop's counting direction

   Counts down from 5 to 1 correctly counts down from 5 to 1 using i-- and condition i > 0. Counts up from 1 to 5 counts up. Options C and D increase i but have conditions that cause infinite loops.

2. Step 2: Confirm correct loop behavior

   Counts down from 5 to 1 correctly correctly pushes 5,4,3,2,1 into the array and stops when i reaches 0.

3. Final Answer:

   Counts down from 5 to 1 correctly -> Option D

4. Quick Check:

   Decrement i with i-- and condition i > 0 for countdown [OK]

Hint: Use i-- and i > 0 to count down safely [OK]

Common Mistakes:

• Using i++ when counting down
• Wrong loop condition causing infinite loops
• Confusing counting up vs down