Array.include? / index / find_index

# Checks whether the array contains the value.
array.include?(value)

# Returns the index of the first element that matches the condition.
array.index(value)
array.index { |element| condition }

# Alias for index.
array.find_index(value)
array.find_index { |element| condition }

# Returns the index of the last element that matches the condition.
array.rindex(value)
array.rindex { |element| condition }

fruits = ["apple", "banana", "orange", "banana", "grape"]

# Use include? to check for existence.
puts fruits.include?("banana")  # true
puts fruits.include?("melon")   # false

# Use index to get the index of the first match.
puts fruits.index("banana")          # 1
puts fruits.index("melon").inspect   # nil (not found)

# Use a block for condition-based searching.
numbers = [10, 25, 3, 47, 8]
puts numbers.index { |n| n > 20 }       # 1 (25 matches first)
puts numbers.find_index { |n| n > 20 }  # 1 (same result)

# Use rindex to get the index of the last match.
puts fruits.rindex("banana")  # 3

# Use the index to retrieve an element.
idx = fruits.index("orange")
if idx
  puts "orange is at index #{idx}."  # orange is at index 2.
end

# Find the position of the first element meeting a condition.
scores = [65, 82, 91, 78, 55]
passing_score = 80
first_pass = scores.index { |score| score >= passing_score }
puts "First passing index: #{first_pass}"  # First passing index: 1

Use include? when you only need to check whether an element exists. When you need the position (index), use index or find_index. Both behave identically, but find_index can make the intent clearer when used with a block.

index and rindex return nil when no match is found. Always check for nil before using the return value in arithmetic or other operations.

To retrieve the matching elements themselves, use select / find. To count matching elements, use count.