Find/detect for first match in Ruby - Mini Project: Build & Apply

Find the First Even Number in a List

📖 Scenario: Imagine you have a list of numbers from a game score sheet. You want to find the first even number to see when the player first scored an even point.

🎯 Goal: You will write a Ruby program that finds the first even number in a list using the find method.

📋 What You'll Learn

Create an array called scores with the exact numbers: 3, 7, 8, 10, 15

Create a variable called is_even that holds a lambda to check if a number is even

Use the find method with is_even to get the first even number from scores

Print the found even number using puts

💡 Why This Matters

🌍 Real World

Finding the first item that meets a condition is common in data filtering, like finding the first available appointment or the first product in stock.

💼 Career

Many programming jobs require filtering lists or arrays to find specific data quickly, making this skill very useful.

1

Create the list of scores

Create an array called scores with these exact numbers: 3, 7, 8, 10, 15

Ruby

# Create the array scores with the numbers 3, 7, 8, 10, 15
# Your code here

Hint

Use square brackets [] to create an array and separate numbers with commas.

2

Create a lambda to check even numbers

Create a variable called is_even that holds a lambda which returns true if a number is even

Ruby

scores = [3, 7, 8, 10, 15]
# Create a lambda called is_even to check if a number is even
# Your code here

Hint

Use ->(num) { num.even? } to create a lambda that checks if num is even.

3

Find the first even number

Use the find method on scores with the is_even lambda to get the first even number and store it in first_even

Ruby

scores = [3, 7, 8, 10, 15]
is_even = ->(num) { num.even? }
# Use find with is_even to get the first even number and store in first_even
# Your code here

Hint

Use scores.find(&is_even) to find the first number where is_even returns true.

4

Print the first even number

Print the variable first_even using puts to show the first even number found

Ruby

scores = [3, 7, 8, 10, 15]
is_even = ->(num) { num.even? }
first_even = scores.find(&is_even)
# Print the first_even variable
# Your code here

Hint

Use puts first_even to print the value.

Practice

1.

What does the Ruby method find (also known as detect) do when called on an array?

easy

A. It removes the first element that matches the condition in the block.

B. It returns all elements that match the condition in the block.

C. It returns the first element that matches the condition in the block.

D. It sorts the array based on the block's condition.

Solution

1. Step 1: Understand the purpose of find

   The find method searches through the array elements one by one.

2. Step 2: Identify what find returns

   It returns the first element for which the block condition is true, then stops searching.

3. Final Answer:

   It returns the first element that matches the condition in the block. -> Option C

4. Quick Check:

   find = first matching element [OK]

Hint: Remember: find returns only the first match, not all matches [OK]

Common Mistakes:

• Thinking find returns all matches
• Confusing find with methods that modify the array
• Assuming find sorts the array

2.

Which of the following is the correct syntax to find the first even number in an array nums using find?

nums = [1, 3, 4, 6]

easy

A. nums.find { |n| n.even? }

B. nums.find(n.even?)

C. nums.find => n.even?

D. nums.find |n| n.even?

Solution

1. Step 1: Recall the block syntax for find

   The block must be passed with curly braces or do-end and use a block variable with pipes.

2. Step 2: Check each option's syntax

   nums.find { |n| n.even? } uses correct block syntax: { |n| n.even? }. Others have syntax errors.

3. Final Answer:

   nums.find { |n| n.even? } -> Option A

4. Quick Check:

   Correct block syntax = nums.find { |n| n.even? } [OK]

Hint: Use curly braces and pipes for blocks with find [OK]

Common Mistakes:

• Missing pipes around block variable
• Passing condition without a block
• Using wrong arrow or syntax for block

3.

What is the output of the following Ruby code?

words = ["apple", "banana", "cherry", "date"]
result = words.find { |w| w.length > 5 }
puts result

medium

A. "cherry"

B. nil

C. "apple"

D. "banana"

Solution

1. Step 1: Check each word's length

   "apple" (5), "banana" (6), "cherry" (6), "date" (4).

2. Step 2: Find first word with length > 5

   "banana" is the first word longer than 5 characters.

3. Final Answer:

   "banana" -> Option D

4. Quick Check:

   First word with length > 5 = "banana" [OK]

Hint: Check elements in order; find stops at first match [OK]

Common Mistakes:

• Choosing the last matching word instead of first
• Confusing length condition
• Assuming find returns all matches

4.

Identify the error in this Ruby code that tries to find the first negative number:

numbers = [3, 5, -2, 7]
result = numbers.find do |n|
  n < 0
end
puts result

medium

A. There is no error; the code works correctly.

B. The variable n is not defined in the block.

C. The block syntax is incorrect; should use curly braces.

D. The method find does not accept a block.

Solution

1. Step 1: Check block syntax

   The do...end block with variable n is valid syntax for find.

2. Step 2: Confirm method usage

   find accepts a block and returns the first element matching n < 0.

3. Final Answer:

   There is no error; the code works correctly. -> Option A

4. Quick Check:

   Valid block syntax and method usage = There is no error; the code works correctly. [OK]

Hint: Remember: do-end blocks are valid for find [OK]

Common Mistakes:

• Thinking only curly braces work for blocks
• Assuming variable must be declared outside block
• Believing find can't take blocks

5.

You have an array of hashes representing people:

people = [
  {name: "Alice", age: 18},
  {name: "Bob", age: 17},
  {name: "Carol", age: 22}
]

Which code finds the first person who is an adult (age 18 or older) and returns their name?

hard

A. people.detect { |p| p[:age] > 18 }[:name]

B. people.find { |p| p[:age] >= 18 }[:name]

C. people.find { |p| p[:age] > 18 }[:name]

D. people.detect { |p| p[:age] >= 18 }[:age]

Solution

1. Step 1: Understand the condition for adult

   Adult means age 18 or older, so condition is p[:age] >= 18.

2. Step 2: Find first matching person and get name

   Use find or detect with condition, then access [:name].

3. Step 3: Check options carefully

   people.find { |p| p[:age] >= 18 }[:name] uses correct condition and returns name. people.detect { |p| p[:age] >= 18 }[:age] returns age, not name. Options A and C use > 18, excluding age 18.

4. Final Answer:

   people.find { |p| p[:age] >= 18 }[:name] -> Option B

5. Quick Check:

   Correct condition and return name = people.find { |p| p[:age] >= 18 }[:name] [OK]

Hint: Use >= 18 to include age 18, then get [:name] [OK]

Common Mistakes:

• Using > 18 excludes age 18
• Returning age instead of name
• Confusing find and detect (both work but check return)