String Interpolation in Ruby

Gluing strings and variables together with + works in Ruby, but it's rarely how idiomatic Ruby code does it. Interpolation is cleaner, faster, and handles type conversion for you.

The basics

name = "Ada"
age = 36

# Avoid: concatenation
puts "Hello, " + name + "! You are " + age.to_s + " years old."

# Prefer: interpolation
puts "Hello, #{name}! You are #{age} years old."

Anything inside #{} is evaluated as a real Ruby expression, and the result is converted to a string automatically — no explicit .to_s needed, and no risk of the classic "can't concatenate a String with an Integer" error that concatenation causes if you forget one.

Interpolation works with any expression

items = ["apple", "banana", "cherry"]

puts "You have #{items.length} items: #{items.join(", ")}"
puts "Total after tax: #{(19.99 * 1.08).round(2)}"
puts "Status: #{age >= 18 ? "adult" : "minor"}"

Method calls, arithmetic, even a ternary — all of it can go inside #{}. Keep it readable, though: if the expression gets long or nested, pull it out into a named variable first and interpolate the variable instead.

# Harder to read
puts "Discount: #{items.select { |i| i.start_with?("a") }.length}"

# Clearer
a_items = items.select { |i| i.start_with?("a") }
puts "Discount: #{a_items.length}"

Single quotes don't interpolate

name = "Ada"
puts 'Hello, #{name}'   # literally prints: Hello, #{name}
puts "Hello, #{name}"   # prints: Hello, Ada

This is the single biggest reason to default to double-quoted strings in Ruby: forgetting that single quotes are literal is an easy mistake, and the bug (a raw #{...} showing up in your output) is confusing the first time you see it.

Multi-line strings and heredocs

Interpolation works across multiple lines too, and for longer blocks of text Ruby has heredoc syntax:

name = "Ada"
message = <<~TEXT
  Hello, #{name}.

  Thanks for signing up. Your account is ready.
TEXT

puts message

The <<~ (squiggly heredoc) strips leading indentation based on the least-indented line, which is what lets you indent the heredoc body naturally inside your code without that indentation leaking into the actual string.

String formatting as an alternative

For more controlled formatting — padding, fixed decimal places — format (or its alias sprintf) is worth knowing, though interpolation covers most everyday cases:

puts format("Total: $%.2f", 19.9)   # Total: $19.90

Between #{} interpolation for everyday cases and format for precise layout, you rarely need + concatenation in real Ruby code.