Zip for combining arrays in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the zip method do in Ruby?

The zip method combines elements from two or more arrays into a single array of arrays, pairing elements with the same index together.

intermediate

How does zip handle arrays of different lengths?

If arrays have different lengths, zip fills missing elements with nil for shorter arrays.

beginner

Example: What is the result of [1, 2, 3].zip([4, 5, 6])?

The result is [[1, 4], [2, 5], [3, 6]]. Each element from the first array is paired with the element at the same index from the second array.

intermediate

Can zip combine more than two arrays?

Yes, zip can combine multiple arrays by passing them as arguments. It pairs elements from all arrays by their index.

advanced

How can you use zip with a block?

You can pass a block to zip to process each combined array immediately. The block receives each paired array as an argument.

What does [1, 2].zip([3, 4]) return?

A [[1, 3], [2, 4]]

B [[1, 2], [3, 4]]

C [[1, 3, 4], [2]]

D [[1, 3], [2]]

zip pairs elements by index, so it returns [[1, 3], [2, 4]].

What happens if arrays have different lengths in zip?

A It fills missing elements with nil.

B It raises an error.

C It ignores extra elements.

D It repeats elements from the shorter array.

Ruby fills missing elements with nil when arrays have different lengths.

How do you combine three arrays a, b, and c using zip?

A a.zip(b).map(c)

B a.zip(b).zip(c)

C zip(a, b, c)

D a.zip(b, c)

You pass multiple arrays as arguments: a.zip(b, c).

What does this code output?
[1, 2].zip([3, 4]) { |x| p x }

A Raises an error

B Prints [1, 3] and [2, 4], returns nil

C Returns [[1, 3], [2, 4]] without printing

D Prints [1, 3] and [2, 4], returns [[1, 3], [2, 4]]

The block prints each pair, but zip returns the original array when a block is given.

Which method pairs elements from arrays by their index?

A flatten

B concat

C zip

D map

zip pairs elements from arrays by their index.

Explain how the zip method works to combine arrays in Ruby.

Think about matching elements from each array by their position.

Describe how you can use a block with zip and what effect it has.

Consider what happens inside the block and the method's return value.

Practice

1. What does the Ruby method zip do when used with two arrays?

easy

A. Concatenates the two arrays into one long array.

B. Combines elements from both arrays into pairs based on their positions.

C. Removes duplicate elements from both arrays.

D. Sorts both arrays in ascending order.

Solution

1. Step 1: Understand the purpose of zip

   The zip method pairs elements from arrays by their index positions.

2. Step 2: Compare with other array operations

   Unlike sorting or concatenation, zip creates pairs, not a sorted or merged list.

3. Final Answer:

   Combines elements from both arrays into pairs based on their positions. -> Option B

4. Quick Check:

   Zip pairs elements by position = D [OK]

Hint: Zip pairs elements by index from arrays [OK]

Common Mistakes:

• Thinking zip sorts arrays
• Assuming zip concatenates arrays
• Confusing zip with removing duplicates

2. Which of the following is the correct syntax to zip two arrays a and b in Ruby?

easy

A. a.zip(b)

B. zip(a, b)

C. a.zip = b

D. zip a, b

Solution

1. Step 1: Recall Ruby method call syntax

   In Ruby, methods are called on objects with dot notation, so a.zip(b) is correct.

2. Step 2: Identify incorrect syntax

   The other options do not follow Ruby's method call syntax and will cause errors.

3. Final Answer:

   a.zip(b) -> Option A

4. Quick Check:

   Method call with dot and parentheses = A [OK]

Hint: Use dot notation: array.zip(other_array) [OK]

Common Mistakes:

• Using zip as a standalone function
• Assigning zip like a variable
• Omitting parentheses incorrectly

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

a = [1, 2, 3]
b = ['a', 'b', 'c']
result = a.zip(b)
puts result.inspect

medium

A. [[1, 2, 3], ['a', 'b', 'c']]

B. [1, 'a', 2, 'b', 3, 'c']

C. [[1, 'a'], [2, 'b'], [3, nil]]

D. [[1, 'a'], [2, 'b'], [3, 'c']]

Solution

1. Step 1: Understand zip pairing

   The method pairs elements by index: 1 with 'a', 2 with 'b', 3 with 'c'.

2. Step 2: Check output format

   The result is an array of arrays, each inner array holding one pair.

3. Final Answer:

   [[1, 'a'], [2, 'b'], [3, 'c']] -> Option D

4. Quick Check:

   Zip pairs elements by index = B [OK]

Hint: Zip pairs elements index-wise into nested arrays [OK]

Common Mistakes:

• Expecting flat array instead of nested arrays
• Confusing zip with concatenation
• Misunderstanding nil insertion

4. The following Ruby code is intended to zip two arrays but raises an error. What is the problem?

a = [1, 2]
b = ["x", "y", "z"]
result = a.zip *b
puts result.inspect

medium

A. Using double quotes instead of single quotes causes error.

B. Arrays have different lengths, causing an error.

C. Missing parentheses around argument in zip *b call.

D. The variable result is not defined before use.

Solution

1. Step 1: Check method call syntax

   In Ruby, calling zip without parentheses is allowed only if no ambiguity exists, but here parentheses are recommended.

2. Step 2: Confirm array length and variable usage

   Different lengths do not cause errors; zip fills missing with nil. Variables are defined correctly.

3. Final Answer:

   Missing parentheses around argument in zip *b call. -> Option C

4. Quick Check:

   Use parentheses for method arguments to avoid errors = C [OK]

Hint: Use parentheses with method arguments to avoid syntax errors [OK]

Common Mistakes:

• Assuming different lengths cause errors
• Ignoring parentheses in method calls
• Confusing string quote types with errors

5. Given two arrays names = ["Alice", "Bob", "Carol"] and scores = [85, 92], which Ruby code correctly zips them and replaces missing scores with 0?

hard

A. All of the above

B. names.zip(scores).map { |name, score| [name, score ? score : 0] }

C. names.zip(scores).map { |name, score| [name, score.nil? ? 0 : score] }

D. names.zip(scores).map { |name, score| [name, score || 0] }

Solution

1. Step 1: Understand zip with unequal lengths

   Zipping names and scores pairs elements; missing scores become nil.

2. Step 2: Replace nil with 0 using different Ruby idioms

   All options use valid Ruby ways to replace nil or falsey values with 0.

3. Step 3: Confirm all options work correctly

   Each option correctly maps pairs and replaces missing scores with 0.

4. Final Answer:

   All of the above -> Option A

5. Quick Check:

   All methods replace nil with 0 after zip = A [OK]

Hint: Use map with nil check to replace missing values after zip [OK]

Common Mistakes:

• Ignoring nil values after zip
• Using incorrect nil checks
• Assuming zip fills missing with 0 automatically