Upto and downto methods in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the upto method do in Ruby?

The upto method counts from a starting number up to an ending number, running a block of code for each number in that range.

beginner

How does the downto method work in Ruby?

The downto method counts down from a starting number to an ending number, running a block of code for each number in that range.

beginner

Example: What will this code print?

5.upto(7) { |n| puts n }

It will print:
5
6
7
Because upto counts from 5 up to 7, printing each number.

beginner

Example: What will this code print?

3.downto(1) { |n| puts n }

It will print:
3
2
1
Because downto counts down from 3 to 1, printing each number.

beginner

Can upto and downto be used without a block?

No, both methods require a block to tell Ruby what to do with each number in the count. Without a block, they do nothing.

What does 2.upto(4) { |n| puts n } print?

A 2 3 4

B 4 3 2

C 2 4

D Nothing

The upto method counts up from 2 to 4, printing each number.

Which method counts down from a number to a smaller number?

A upto

B times

C downto

D each

downto counts down from a starting number to an ending number.

What happens if you call 5.upto(3) { |n| puts n }?

A Prints 5, 4, 3

B Prints nothing

C Prints 3, 4, 5

D Raises an error

Since 5 is greater than 3, upto does not run the block and prints nothing.

Which of these is true about downto?

A It counts up

B It can be used without a block

C It only works with strings

D It requires a block

downto requires a block to specify what to do for each number.

What will 1.upto(1) { |n| puts n } print?

A 1

B Nothing

C 1 2

D Error

It prints 1 because upto includes the end number.

Explain how the upto method works and give a simple example.

Think about counting forward like climbing stairs.

Describe the difference between upto and downto methods.

One goes up, the other goes down.

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