For loop (rarely used in Ruby) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple for loop

What is the output of this Ruby code using a for loop?

Ruby

for i in 1..3
  puts i * 2
end

A

2
4
6

B

1
2
3

C

1
4
9

D Error: SyntaxError

💡 Hint

Remember the for loop iterates over the range 1 to 3 and multiplies each by 2.

The for loop goes through numbers 1, 2, and 3. Each number is multiplied by 2 and printed, so the output is 2, 4, and 6 each on a new line.

❓ Predict Output

intermediate

For loop with array iteration

What will this Ruby code print?

Ruby

arr = ['a', 'b', 'c']
for letter in arr
  print letter
end

A ['a', 'b', 'c']

B Error: undefined method 'print'

C a b c

D abc

💡 Hint

The print method outputs without newlines or spaces.

The for loop goes through each element in the array and prints it without spaces or newlines, so the output is 'abc'.

❓ Predict Output

advanced

For loop with break statement

What is the output of this Ruby code?

Ruby

for i in 1..5
  break if i > 3
  puts i
end

A

1
2
3
4
5

B

1
2

C

1
2
3

D Error: break outside loop

💡 Hint

The break stops the loop when i is greater than 3.

The loop iterates i from 1 to 5. For i=1,2,3: i > 3 is false, so puts i. For i=4: i > 3 is true, break before puts i. Output is 1 2 3 .

❓ Predict Output

advanced

For loop variable scope

What is the value of variable x after this Ruby code runs?

Ruby

x = 10
for x in 1..3
  # loop body empty
end
puts x

A Error: variable scope conflict

B 3

C 1

D 10

💡 Hint

In Ruby, the for loop variable reassigns the outer variable.

The for loop assigns x to 1, then 2, then 3. After the loop, x keeps the last value 3.

🧠 Conceptual

expert

Why is for loop rarely used in Ruby?

Which reason best explains why the for loop is rarely used in Ruby?

A Because Ruby prefers using iterators like each which have better variable scope handling

B Because for loops do not support ranges in Ruby

C Because for loops are slower than while loops in Ruby

D Because for loops cause syntax errors in modern Ruby versions

💡 Hint

Think about how Ruby handles variables inside loops.

Ruby prefers iterator methods like each because they create a new variable scope, avoiding side effects on outer variables, unlike for loops which reuse the outer variable.

Practice

1. What does the following Ruby for loop do?

for i in 1..3
puts i
end

easy

A. Prints numbers 1, 2, and 3 each on a new line

B. Prints numbers 1 to 2 only

C. Prints numbers 0 to 3

D. Causes a syntax error

Solution

1. Step 1: Understand the range 1..3

   The range 1..3 includes the numbers 1, 2, and 3.

2. Step 2: Loop through each number and print

   The loop prints each number on its own line using puts.

3. Final Answer:

   Prints numbers 1, 2, and 3 each on a new line -> Option A

4. Quick Check:

   for loop with 1..3 prints 1, 2, 3 [OK]

Hint: Remember 1..3 includes 3, so loop prints 1, 2, 3 [OK]

Common Mistakes:

• Thinking 1..3 excludes 3
• Confusing puts with print
• Expecting syntax error

2. Which of the following is the correct syntax for a for loop in Ruby to iterate over an array ["a", "b", "c"]?

easy

A. for x to ["a", "b", "c"] puts x end

B. for (x : ["a", "b", "c"]) puts x end

C. for x in ["a", "b", "c"] do puts x end

D. for x in ["a", "b", "c"] puts x

Solution

1. Step 1: Check the correct Ruby for loop syntax

   Ruby uses for variable in collection do ... end or without do.

2. Step 2: Validate each option

   for x in ["a", "b", "c"] do puts x end uses correct syntax with for x in array do ... end. for (x : ["a", "b", "c"]) puts x end and C use invalid syntax. for x in ["a", "b", "c"] puts x misses end to close the loop.

3. Final Answer:

   for x in ["a", "b", "c"] do puts x end -> Option C

4. Quick Check:

   Correct Ruby for loop uses 'for x in collection do ... end' [OK]

Hint: Remember: for x in collection do ... end is correct Ruby syntax [OK]

Common Mistakes:

• Using colon or 'to' instead of 'in'
• Omitting the 'end' keyword
• Adding parentheses incorrectly

3. What is the output of this Ruby code?

sum = 0
for i in 1..4
sum += i
end
puts sum

medium

A. Error

B. 9

C. 0

D. 10

Solution

1. Step 1: Understand the loop range and sum initialization

   The loop runs from 1 to 4, and sum starts at 0.

2. Step 2: Calculate sum after each iteration

   sum = 0+1=1, then 1+2=3, then 3+3=6, then 6+4=10.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   Sum of 1 to 4 is 10 [OK]

Hint: Add numbers 1 through 4 to get 10 quickly [OK]

Common Mistakes:

• Forgetting to include last number 4
• Starting sum at 1 instead of 0
• Syntax errors in loop

4. Identify the error in this Ruby for loop:

for i in 1..3
puts i


medium

A. Wrong range syntax

B. Missing end to close the loop

C. Variable i not defined

D. Using puts inside loop is invalid

Solution

1. Step 1: Check loop syntax completeness

   The loop starts with for but does not have a matching end.

2. Step 2: Confirm other parts are correct

   The range 1..3 and variable i are valid, and puts is allowed inside loops.

3. Final Answer:

   Missing end to close the loop -> Option B

4. Quick Check:

   Every for loop needs an 'end' [OK]

Hint: Always close for loops with 'end' [OK]

Common Mistakes:

• Forgetting 'end' keyword
• Thinking variable must be declared first
• Believing puts is invalid inside loops

5. You want to create a Ruby for loop that prints only even numbers from 2 to 10 inclusive. Which code snippet achieves this correctly?

hard

A. for i in 2..10 do
puts i if i % 2 == 0
end

B. for i in 2..10 do
puts i if i % 2 == 1
end

C. for i in 1..10 do
puts i if i % 2 == 1
end

D. for i in 2..10
puts i
end

Solution

1. Step 1: Identify the range and condition for even numbers

   The range 2..10 includes numbers 2 through 10. Even numbers satisfy i % 2 == 0.

2. Step 2: Check each option's condition

   for i in 2..10 do puts i if i % 2 == 0 end prints numbers in 2..10 only if even. for i in 2..10 do puts i if i % 2 == 1 end prints odd numbers. for i in 1..10 do puts i if i % 2 == 1 end prints odd numbers 1,3,5,7,9. for i in 2..10 puts i end prints all numbers without filtering.

3. Final Answer:

   for i in 2..10 do puts i if i % 2 == 0 end -> Option A

4. Quick Check:

   Filter with 'if i % 2 == 0' inside for loop [OK]

Hint: Use 'if i % 2 == 0' inside for loop to print evens [OK]

Common Mistakes:

• Using wrong modulo condition for even numbers
• Not filtering numbers inside loop
• Starting range at 1 instead of 2