Array slicing and ranges in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get the first three elements of the array.

Ruby

arr = [10, 20, 30, 40, 50]
slice = arr[[1]]

Drag options to blanks, or click blank then click option'

A1..3

B0..2

C2..4

D3..5

Attempts:

3 left

💡 Hint

Common Mistakes

Using a range starting at 1 misses the first element.

Using an exclusive range (0...2) only gets two elements.

The range 0..2 selects elements at indexes 0, 1, and 2, which are the first three elements.

2fill in blank

medium

Complete the code to get the last two elements of the array.

Ruby

arr = [5, 10, 15, 20, 25]
slice = arr[[1]]

Drag options to blanks, or click blank then click option'

A-2..-1

B-3..-1

C3..4

D2..3

Attempts:

3 left

💡 Hint

Common Mistakes

Using -3..-1 gets three elements instead of two.

Using positive indexes may be less clear for the last elements.

The range -2..-1 selects the last two elements of the array using negative indexes.

3fill in blank

hard

Fix the error in the code to slice the array from index 1 to 3 inclusive.

Ruby

arr = [2, 4, 6, 8, 10]
slice = arr[[1]]

Drag options to blanks, or click blank then click option'

A1...3

B0..2

C2..4

D1..3

Attempts:

3 left

💡 Hint

Common Mistakes

Using a triple dot range excludes the last element.

Using wrong indexes results in wrong slice.

The range 1..3 includes indexes 1, 2, and 3. The range 1...3 excludes index 3.

4fill in blank

hard

Fill both blanks to create a slice from the second to the fourth element (inclusive).

Ruby

arr = ['a', 'b', 'c', 'd', 'e']
slice = arr[[1]..[2]]

Drag options to blanks, or click blank then click option'

A1

B2

C3

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Using 2 as start misses the second element.

Using 4 as end includes the fifth element.

Indexes 1 to 3 correspond to the second, third, and fourth elements.

5fill in blank

hard

Fill all three blanks to create a slice from the third element to the last element.

Ruby

arr = [100, 200, 300, 400, 500]
slice = arr[[1]..[2]]
puts slice[[3]]

Drag options to blanks, or click blank then click option'

A2

B4

C0

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3 as start misses the third element.

Using 3 as end excludes the last element.

Using 2 as slice index causes wrong output.

The slice from index 2 to 4 includes elements 300, 400, 500. The first element of the slice is at index 0.

Practice

1. What does the expression arr[1..3] do when arr = [10, 20, 30, 40, 50]?

easy

A. Returns a new array with elements at indexes 1, 2, and 3

B. Returns a new array with elements at indexes 1 and 3 only

C. Returns a new array with elements at indexes 1 and 2 only

D. Returns a single element at index 3

Solution

1. Step 1: Understand the range operator '..'

   The range 1..3 includes both 1 and 3, so it covers indexes 1, 2, and 3.

2. Step 2: Apply slicing to the array

   Slicing arr[1..3] returns elements at indexes 1, 2, and 3, which are 20, 30, and 40.

3. Final Answer:

   Returns a new array with elements at indexes 1, 2, and 3 -> Option A

4. Quick Check:

   Range '..' includes end index = elements 20, 30, 40 [OK]

Hint: Remember '..' includes the end index in slicing [OK]

Common Mistakes:

• Thinking '..' excludes the end index
• Confusing indexes with values
• Assuming slicing changes the original array

2. Which of the following is the correct syntax to get elements from index 2 up to but not including index 5 in Ruby array arr?

easy

A. arr[2...5]

B. arr[2:5]

C. arr[2..5]

D. arr[2-5]

Solution

1. Step 1: Identify the range operator for excluding end

   In Ruby, ... excludes the end index, so 2...5 means indexes 2, 3, and 4.

2. Step 2: Check syntax correctness

   Only arr[2...5] uses valid Ruby syntax for slicing excluding the end index.

3. Final Answer:

   arr[2...5] -> Option A

4. Quick Check:

   Use '...' to exclude end index in range [OK]

Hint: Use '...' to exclude the last index in range slicing [OK]

Common Mistakes:

• Using '..' when exclusion is needed
• Using Python-style slicing syntax like ':'
• Using invalid range syntax like '2-5'

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

arr = [5, 10, 15, 20, 25]
slice = arr[1...4]
puts slice.inspect

medium

A. [10, 15, 20, 25]

B. [15, 20, 25]

C. [10, 15, 20]

D. [5, 10, 15]

Solution

1. Step 1: Understand the slicing range

   The range 1...4 includes indexes 1, 2, and 3 but excludes 4.

2. Step 2: Extract elements at these indexes

   Elements at indexes 1, 2, 3 are 10, 15, and 20 respectively.

3. Final Answer:

   [10, 15, 20] -> Option C

4. Quick Check:

   Range '...' excludes end index = [10, 15, 20] [OK]

Hint: Remember '...' excludes the last index in slicing [OK]

Common Mistakes:

• Including element at index 4 mistakenly
• Confusing indexes with values
• Printing original array instead of slice

4. Identify the error in the following Ruby code snippet:

arr = [1, 2, 3, 4, 5]
slice = arr[2..6]
puts slice.inspect

medium

A. Syntax error due to invalid range

B. Range end index 6 is out of bounds but Ruby returns elements up to the end of the array

C. Runtime error because index 6 does not exist

D. slice will be empty array because range is invalid

Solution

1. Step 1: Check the range indexes against array size

   The array has indexes 0 to 4. The range 2..6 goes beyond the last index.

2. Step 2: Understand Ruby slicing behavior for out-of-bounds

   Ruby returns elements from index 2 to the end (indexes 2, 3, 4) and ignores indexes beyond 4 without error.

3. Final Answer:

   Range end index 6 is out of bounds but Ruby returns elements up to the end of the array -> Option B

4. Quick Check:

   Ruby slices safely even if range exceeds array size [OK]

Hint: Ruby slicing ignores out-of-bound indexes without error [OK]

Common Mistakes:

• Expecting runtime or syntax error
• Assuming slice will be empty
• Confusing Ruby with other languages that error here

5. Given arr = [2, 4, 6, 8, 10, 12], which expression returns a new array with every second element starting from index 1 up to index 5?

hard

A. arr[1..5].select.with_index { |_, i| i.even? }

B. arr[1..5].select.with_index { |_, i| i.odd? }

C. arr[1...6].values_at(*arr[1...6].each_index.select(&:even?))

D. arr[1..5].values_at(*arr[1..5].each_index.select(&:odd?))

Solution

1. Step 1: Slice array from index 1 to 5

   arr[1..5] gives [4, 6, 8, 10, 12].

2. Step 2: Select every second element starting from index 1 of the slice

   Indexes inside the slice are 0 to 4. Selecting odd indexes (1, 3) gives elements 6 and 10.

3. Step 3: Check options for correct syntax and output

   arr[1..5].values_at(*arr[1..5].each_index.select(&:odd?)) uses values_at with odd indexes on the slice correctly, returning [6, 10].

4. Final Answer:

   arr[1..5].values_at(*arr[1..5].each_index.select(&:odd?)) -> Option D

5. Quick Check:

   Use values_at with odd indexes on slice for every second element [OK]

Hint: Use values_at with selected indexes for custom slicing [OK]

Common Mistakes:

• Confusing odd/even indexes inside the slice
• Using select without values_at returns elements, not indexes
• Using wrong range operator causing off-by-one errors