Upto and downto methods in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of using upto method

What is the output of the following Ruby code?

Ruby

result = []
5.upto(7) { |i| result << i }
puts result.join(",")

A 5,6,7

B 7,6,5

C 5,7

D 6,7

💡 Hint

Remember, upto counts from the starting number up to the ending number, including both.

The upto method starts at 5 and counts up to 7, including both 5 and 7, so the array contains 5, 6, and 7.

❓ Predict Output

intermediate

Output of using downto method

What will this Ruby code print?

Ruby

result = []
3.downto(1) { |i| result << i }
puts result.join("-")

A 1-2-3

B 3-2-1

C 3-1

D 2-1

💡 Hint

Downto counts down from the starting number to the ending number, including both.

The downto method starts at 3 and counts down to 1, so the array contains 3, 2, and 1.

🧠 Conceptual

advanced

Understanding return values of upto and downto

What is the return value of the following Ruby code snippet? ```ruby result = 1.upto(3) { |i| puts i } puts result ```

A

1
2
3
nil

B

1
2
3
3

C

1
2
3
1

D

1
2
3
Enumerator

💡 Hint

The upto method returns the receiver after iteration.

The upto method returns the starting number (receiver) after executing the block, so result is 1. But the code prints result, which is 1, after printing 1, 2, 3 inside the block.

❓ Predict Output

advanced

Output of combined upto and downto

What is the output of this Ruby code?

Ruby

output = []
2.upto(3) { |i| output << i }
3.downto(2) { |i| output << i }
puts output.join(",")

A 2,3,3,2

B 2,3,2,3

C 3,2,2,3

D 3,2,3,2

💡 Hint

The first loop counts up from 2 to 3, the second counts down from 3 to 2.

The first loop adds 2 and 3, the second adds 3 and 2, so the array is [2,3,3,2].

❓ Predict Output

expert

Counting iterations with upto and downto

How many times will the block run in this Ruby code?

Ruby

count = 0
10.downto(7) { count += 1 }
puts count

A 3

B 10

C 7

D 4

💡 Hint

Count how many numbers are between 10 and 7, including both.

The downto method runs the block for 10, 9, 8, and 7, so 4 times.

Practice

1. What does the Ruby method 5.upto(8) do?

easy

A. Counts from 5 down to 8, excluding 8

B. Counts from 8 down to 5, including both 8 and 5

C. Counts from 5 up to 8, including both 5 and 8

D. Counts from 8 up to 5, excluding 5

Solution

1. Step 1: Understand the upto method

   The upto method counts upwards from the starting number to the ending number, including both.

2. Step 2: Apply to the example 5.upto(8)

   This means it counts 5, 6, 7, 8 in order.

3. Final Answer:

   Counts from 5 up to 8, including both 5 and 8 -> Option C

4. Quick Check:

   upto counts up = A [OK]

Hint: Remember: upto counts up, downto counts down [OK]

Common Mistakes:

• Confusing upto with downto
• Thinking it excludes the last number
• Assuming it counts down instead of up

2. Which of the following is the correct syntax to count down from 10 to 7 using downto in Ruby?

easy

A. 10.downto(7) { |i| puts i }

B. 10.upto(7) { |i| puts i }

C. 7.downto(10) { |i| puts i }

D. 7.upto(10) { |i| puts i }

Solution

1. Step 1: Identify correct method for counting down

   To count down from 10 to 7, use downto starting at 10 and ending at 7.

2. Step 2: Check syntax correctness

   10.downto(7) { |i| puts i } correctly counts down and prints each number.

3. Final Answer:

   10.downto(7) { |i| puts i } -> Option A

4. Quick Check:

   Count down syntax = C [OK]

Hint: downto starts at bigger number, ends at smaller [OK]

Common Mistakes:

• Using upto instead of downto for counting down
• Reversing start and end numbers
• Missing the block syntax

3. What is the output of this Ruby code?

sum = 0
3.upto(5) { |i| sum += i }
puts sum

medium

A. 12

B. 15

C. 9

D. Error

Solution

1. Step 1: Understand the loop range

   3.upto(5) iterates over 3, 4, 5.

2. Step 2: Calculate sum of values

   sum = 0 + 3 + 4 + 5 = 12.

3. Final Answer:

   12 -> Option A

4. Quick Check:

   3 + 4 + 5 = 12 [OK]

Hint: Add all numbers from start to end inclusive [OK]

Common Mistakes:

• Adding only start and end numbers
• Confusing upto with downto
• Forgetting to include the last number

4. Identify the error in this Ruby code:

5.upto(3) { |i| puts i }

medium

A. Syntax error due to wrong method usage

B. No output because start is greater than end

C. Infinite loop occurs

D. Outputs numbers from 5 down to 3

Solution

1. Step 1: Analyze the upto method behavior

   upto counts up from start to end. Here start=5, end=3, which is invalid for counting up.

2. Step 2: Determine output

   Since 5 is already greater than 3, the block does not run and no output is produced.

3. Final Answer:

   No output because start is greater than end -> Option B

4. Quick Check:

   upto with start > end = no output [OK]

Hint: upto does nothing if start > end, no error [OK]

Common Mistakes:

• Expecting output when start > end
• Thinking it counts down automatically
• Assuming syntax error occurs

5. Using downto, how can you create an array of even numbers counting down from 10 to 2?

hard

A. arr = []
2.upto(10) { |i| arr << i if i.even? }

B. arr = []
2.downto(10) { |i| arr << i if i.even? }

C. arr = []
10.upto(2) { |i| arr << i if i.even? }

D. arr = []
10.downto(2) { |i| arr << i if i.even? }

Solution

1. Step 1: Choose correct method for counting down

   To count down from 10 to 2, use downto starting at 10 and ending at 2.

2. Step 2: Filter even numbers inside the block

   Use if i.even? to add only even numbers to the array.

3. Final Answer:

   arr = [] 10.downto(2) { |i| arr << i if i.even? } -> Option D

4. Quick Check:

   downto + even check = B [OK]

Hint: Use downto with condition inside block to filter [OK]

Common Mistakes:

• Using upto instead of downto for counting down
• Reversing start and end numbers
• Not filtering even numbers correctly