While loop in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a while loop in Ruby?

A while loop repeats a block of code as long as a given condition is true. It checks the condition before each repetition.

beginner

How do you write a simple while loop in Ruby that counts from 1 to 5?

You can write:

i = 1
while i <= 5
  puts i
  i += 1
end

This prints numbers 1 to 5.

intermediate

What happens if the condition in a while loop is never false?

The loop runs forever, causing an infinite loop. This can freeze your program unless you stop it manually.

beginner

Can you use a while loop with a condition that changes inside the loop? Give an example.

Yes. For example:

count = 3
while count > 0
  puts count
  count -= 1
end

The condition changes as count decreases.

intermediate

What is the difference between a while loop and an until loop in Ruby?

A while loop runs while the condition is true.
An until loop runs until the condition becomes true (runs while condition is false).

What does a while loop do in Ruby?

A Repeats code while a condition is true

B Repeats code once regardless of condition

C Runs code only if condition is false

D Stops code execution immediately

A while loop repeats the code block as long as the condition remains true.

What will this code print?

i = 1
while i < 3
  puts i
  i += 1
end

A 1 2

B 1 2 3

C 2 3

D 3

The loop runs while i is less than 3, so it prints 1 and 2.

What happens if the condition in a while loop is false at the start?

A The loop runs forever

B The loop does not run at all

C The loop runs once

D The loop runs twice

If the condition is false initially, the while loop does not run even once.

Which of these can cause an infinite while loop?

A Incrementing a variable inside the loop

B Condition is false initially

C Condition never becomes false

D Using break inside the loop

If the condition never becomes false, the loop never stops and runs forever.

How do you stop a while loop early in Ruby?

A Use the exit keyword

B Use the stop keyword

C Use the continue keyword

D Use the break keyword

The break keyword immediately exits the loop.

Explain how a while loop works in Ruby and give a simple example.

Think about counting numbers using a variable.

What are the risks of using a while loop and how can you avoid them?

Consider what happens if the condition never changes.

Practice

1. What does a while loop do in Ruby?

easy

A. Stops the program immediately

B. Runs code only once regardless of condition

C. Repeats code as long as a condition is true

D. Executes code after the loop ends

Solution

1. Step 1: Understand the purpose of a while loop

   A while loop runs the code inside it repeatedly as long as the condition is true.

2. Step 2: Compare options with the definition

   Only Repeats code as long as a condition is true correctly describes this behavior.

3. Final Answer:

   Repeats code as long as a condition is true -> Option C

4. Quick Check:

   while loop = repeats while true [OK]

Hint: Remember: while loops run only if condition is true [OK]

Common Mistakes:

• Thinking while loop runs code once
• Confusing while with if statement
• Assuming it stops program immediately

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

easy

A. while condition
# code
loop

B. while (condition) {
# code
}

C. while condition:
# code

D. while condition do
# code
end

Solution

1. Step 1: Recall Ruby while loop syntax

   Ruby uses while condition do ... end or while condition ... end without braces.

2. Step 2: Check each option

   while condition do # code end matches Ruby syntax. Options B and C use other languages' syntax. while condition # code loop is invalid.

3. Final Answer:

   while condition do\n # code\nend -> Option D

4. Quick Check:

   Ruby while loop uses do/end [OK]

Hint: Ruby uses do/end, not braces or colons for while [OK]

Common Mistakes:

• Using braces {} like in JavaScript
• Using colon : like in Python
• Ending with 'loop' keyword

3. What is the output of this Ruby code?

i = 1
while i <= 3 do
  puts i
  i += 1
end

medium

A. 1 2 3

B. 1 2 3 4

C. 0 1 2 3

D. No output

Solution

1. Step 1: Trace the loop iterations

   i starts at 1. Loop runs while i <= 3. It prints i then adds 1 each time.

2. Step 2: List printed values

   Prints 1, then 2, then 3. When i becomes 4, condition fails and loop stops.

3. Final Answer:

   1 2 3 -> Option A

4. Quick Check:

   Prints 1 to 3 inclusive [OK]

Hint: Count from start to condition limit inclusive [OK]

Common Mistakes:

• Including 4 in output
• Starting from 0 instead of 1
• No output due to syntax errors

4. Identify the error in this Ruby code:

count = 5
while count > 0
  puts count
end

medium

A. Wrong variable name used in condition

B. Missing increment or decrement causing infinite loop

C. Syntax error: missing 'do' keyword

D. puts should be outside the loop

Solution

1. Step 1: Analyze loop condition and body

   count starts at 5 and condition is count > 0. Inside loop, count is never changed.

2. Step 2: Understand loop behavior

   Since count never decreases, condition stays true forever, causing infinite loop.

3. Final Answer:

   Missing increment or decrement causing infinite loop -> Option B

4. Quick Check:

   Loop needs variable change to stop [OK]

Hint: Always update loop variable inside while [OK]

Common Mistakes:

• Forgetting to change loop variable
• Assuming 'do' is mandatory
• Thinking puts outside loop fixes error

5. How can you use a while loop to sum all even numbers from 2 to 10 inclusive in Ruby?

hard

A. i = 2
sum = 0
while i <= 10 do
sum += i
i += 2
end
puts sum

B. i = 1
sum = 0
while i <= 10 do
if i % 2 == 0
sum += i
end
i += 1
end
puts sum

C. sum = 0
while i <= 10 do
sum += i
i += 1
end
puts sum

D. i = 2
sum = 0
while i < 10 do
sum += i
i += 2
end
puts sum

Solution

1. Step 1: Understand the goal

   We want to add even numbers from 2 to 10 inclusive.

2. Step 2: Check each option's logic

   i = 2 sum = 0 while i <= 10 do sum += i i += 2 end puts sum starts at 2 and increments by 2 up to 10 inclusive, summing even numbers correctly. i = 1 sum = 0 while i <= 10 do if i % 2 == 0 sum += i end i += 1 end puts sum checks each number from 1 to 10 and adds if even, also correct but less efficient. sum = 0 while i <= 10 do sum += i i += 1 end puts sum sums all numbers, not just even. i = 2 sum = 0 while i < 10 do sum += i i += 2 end puts sum stops before 10, missing 10.

3. Step 3: Choose best option

   i = 2 sum = 0 while i <= 10 do sum += i i += 2 end puts sum is more direct and efficient for summing even numbers from 2 to 10 inclusive.

4. Final Answer:

   Option A code correctly sums even numbers 2 to 10 inclusive -> Option A

5. Quick Check:

   Increment by 2 starting at 2, sum all [OK]

Hint: Increment by 2 starting at 2 to sum even numbers [OK]

Common Mistakes:

• Forgetting to include 10 in sum
• Adding all numbers, not just even
• Using wrong loop condition