Accessing elements (indexing, first, last) in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of accessing the first and last elements?

Consider the Ruby array arr = [10, 20, 30, 40, 50]. What will be printed by the following code?

puts arr.first
puts arr.last

Ruby

arr = [10, 20, 30, 40, 50]
puts arr.first
puts arr.last

A 10\n10

B 50\n10

C 10\n50

D 50\n50

💡 Hint

Remember, first gives the first element and last gives the last element of an array.

The first method returns the first element (10), and last returns the last element (50). So the output is 10 then 50 on separate lines.

❓ Predict Output

intermediate

What is the output of negative indexing in Ruby arrays?

Given the array nums = [5, 10, 15, 20, 25], what will puts nums[-2] print?

Ruby

nums = [5, 10, 15, 20, 25]
puts nums[-2]

A 15

B 20

C 25

D 10

💡 Hint

Negative indices count from the end of the array, starting at -1 for the last element.

Index -1 is the last element (25), so -2 is the second last element (20).

❓ Predict Output

advanced

What happens when accessing an out-of-range index?

What will the following Ruby code print?

arr = [1, 2, 3]
puts arr[5].inspect

Ruby

arr = [1, 2, 3]
puts arr[5].inspect

A nil

B IndexError

C 3

D 0

💡 Hint

Ruby returns nil when you access an index outside the array bounds.

Accessing an index beyond the array length returns nil instead of an error.

❓ Predict Output

advanced

What is the output when using first(n) and last(n) methods?

Given arr = [7, 14, 21, 28, 35], what will the following code print?

p arr.first(3)
p arr.last(2)

Ruby

arr = [7, 14, 21, 28, 35]
p arr.first(3)
p arr.last(2)

A [7, 14]\n[28, 35]

B [7, 14, 21]\n[21, 28]

C [14, 21, 28]\n[28, 35]

D [7, 14, 21]\n[28, 35]

💡 Hint

The first(n) method returns the first n elements, and last(n) returns the last n elements.

The first 3 elements are [7, 14, 21], and the last 2 elements are [28, 35].

🧠 Conceptual

expert

What is the value of result after this code runs?

Consider this Ruby code:

arr = ['a', 'b', 'c', 'd']
result = arr[-1] + arr[0] + arr[-3]

What is the value of result?

Ruby

arr = ['a', 'b', 'c', 'd']
result = arr[-1] + arr[0] + arr[-3]

A "dab"

B "d a b"

C "dcb"

D "dac"

💡 Hint

Remember that negative indices count from the end: -1 is last, -2 is second last, etc.

arr[-1] is 'd', arr[0] is 'a', arr[-3] is 'b'. Concatenating gives 'd' + 'a' + 'b' = "dab".

Practice

1. Which method or syntax in Ruby returns the first element of an array arr?

easy

A. arr.last

B. arr[1]

C. arr[-1]

D. arr.first

Solution

1. Step 1: Understand the purpose of .first

   The .first method returns the first element of an array in Ruby.

2. Step 2: Compare with other options

   arr[1] returns the second element (indexing starts at 0), arr.last and arr[-1] return the last element.

3. Final Answer:

   arr.first -> Option D

4. Quick Check:

   First element = arr.first [OK]

Hint: Use .first to get the first element easily [OK]

Common Mistakes:

• Using arr[1] instead of arr[0] for first element
• Confusing .last with .first
• Using negative index for first element

2. Which of the following is the correct syntax to get the last element of an array arr in Ruby?

easy

A. arr.first

B. arr[0]

C. arr[-1]

D. arr[1]

Solution

1. Step 1: Understand negative indexing in Ruby arrays

   Negative indexes count from the end, so arr[-1] returns the last element.

2. Step 2: Check other options

   arr[0] is the first element, arr.first also returns first, and arr[1] is second element.

3. Final Answer:

   arr[-1] -> Option C

4. Quick Check:

   Last element = arr[-1] [OK]

Hint: Use negative index -1 to get last element [OK]

Common Mistakes:

• Using arr[1] thinking it's last element
• Using arr.first instead of arr.last
• Using arr[-0] which is invalid

3. What is the output of the following Ruby code?

arr = [10, 20, 30, 40, 50]
puts arr[2]
puts arr.first
puts arr[-2]

medium

A. 40
10
30

B. 30
10
40

C. 30
50
40

D. 20
10
50

Solution

1. Step 1: Evaluate arr[2]

   Index 2 is the third element, which is 30.

2. Step 2: Evaluate arr.first and arr[-2]

   arr.first is 10 (first element), arr[-2] is second last element, 40.

3. Final Answer:

   30 10 40 -> Option B

4. Quick Check:

   Index 2 = 30, first = 10, second last = 40 [OK]

Hint: Remember index starts at 0; negative counts from end [OK]

Common Mistakes:

• Confusing index 2 with index 3
• Mixing up first and last elements
• Misunderstanding negative indexes

4. The following Ruby code throws an error. What is the problem?

arr = [5, 10, 15]
puts arr.first()
puts arr[-4]

medium

A. Negative index -4 is out of range and returns nil, no error

B. Missing argument in first() causes error

C. Array is empty, so first() fails

D. Syntax error in calling first()

Solution

1. Step 1: Check arr.first() usage

   first() can be called with or without parentheses; no error here.

2. Step 2: Analyze arr[-4]

   Array has 3 elements, so index -4 is out of range and returns nil, which prints as empty line, no error.

3. Final Answer:

   Negative index -4 is out of range and returns nil, no error -> Option A

4. Quick Check:

   Out of range negative index returns nil [OK]

Hint: Out of range negative index returns nil, no error [OK]

Common Mistakes:

• Assuming out of range index causes error
• Thinking first() needs arguments
• Confusing nil with error

5. Given the array arr = ["a", "b", "c", "d", "e"], which Ruby expression returns a new array with only the first and last elements?

hard

A. [arr.first, arr.last]

B. arr[0, 1] + arr[-1]

C. arr[0] + arr[-1]

D. arr.first + arr.last

Solution

1. Step 1: Understand the goal

   We want a new array containing only the first and last elements.

2. Step 2: Evaluate each option

   [arr.first, arr.last] creates an array with arr.first and arr.last. arr[0, 1] + arr[-1] tries to add an array and a string (wrong). arr[0] + arr[-1] adds two strings, resulting in concatenation, not array. arr.first + arr.last adds two strings, also concatenation.

3. Final Answer:

   [arr.first, arr.last] -> Option A

4. Quick Check:

   Array with first and last elements = [arr.first, arr.last] [OK]

Hint: Use array brackets to combine first and last elements [OK]

Common Mistakes:

• Adding strings instead of creating an array
• Using + with array and string causing error
• Forgetting to wrap elements in array brackets