Array#size

collection.size -> integer

The size method returns the number of elements in a collection. It’s defined on arrays, hashes, strings, and other enumerable objects.

With Arrays

[1, 2, 3].size           # => 3
[].size                 # => 0
Array.new(10).size       # => 10

With Hashes

{a: 1, b: 2}.size       # => 2
{}.size                 # => 0

With Strings

"hello".size            # => 5 (characters)
"hello".length          # => 5
"".size                 # => 0

Practical Examples

Check if Empty

items = []
if items.size == 0
  puts "No items"
end

# Or use empty?
if items.empty?
  puts "No items"
end

Loop Bounds

data = [1, 2, 3, 4, 5]
data.size.times do |i|
  puts "Index: #{i}"
end

Conditional Processing

results = fetch_results
if results.size > 10
  puts "Warning: #{results.size} results found"
end

Chunk Processing

data = (1..100).to_a
chunk_size = data.size / 3

Comparison with length

# size and length are aliases for most collections
"hello".size     # => 5
"hello".length   # => 5
[1,2,3].size     # => 3
[1,2,3].length   # => 3

Performance

# For arrays and strings, size/length are O(1)
# Count iterates, so avoid for size checks
arr = [1,2,3]
arr.size    # O(1), fast
arr.count   # O(n), iterates

Custom Objects

class MyCollection
  def initialize
    @items = []
  end
  
  def <<(item)
    @items << item
  end
  
  def size
    @items.size
  end
end

With Enumerators

# Enumerator with size (when known)
enum = (1..100).each
enum.size  # => 100 (Ruby 2.5+)

See Also

• Array#append
• Array#prepend
• Array#pop