Ruby for Keyword

last modified October 18, 2026

This tutorial explains how to use Ruby's for keyword for iteration. The for loop provides a clean syntax for traversing collections.

The for keyword creates a loop that iterates over a collection. It automatically assigns each element to a loop variable. The loop runs until all elements are processed.

While Rubyists often prefer each, for remains familiar to developers coming from other languages. Unlike a block passed to each, for does not create a new scope: the loop variable is an ordinary local variable in the surrounding scope and remains available after the loop.

Basic for Loop with Range

This simple example demonstrates iterating over a numeric range. The loop variable takes each value in sequence.

basic_for_range.rb

for i in 1..5
  puts "Current number: #{i}"
end

puts "Final value: #{i}"

The loop runs from 1 to 5 inclusive. Because for does not introduce a new scope, i is still accessible after the loop; block-scoped iterators such as each behave differently.

Iterating Over an Array

Arrays work naturally with for loops. Each element becomes available through the loop variable in turn.

for_array.rb

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

for fruit in fruits
  puts "Fruit: #{fruit.capitalize}"
end

puts "Last fruit: #{fruit}"

The loop processes each array element sequentially. The variable fruit holds each value. Again, it persists after the loop completes.

Nested for Loops

for loops can be nested to handle multi-dimensional data. Each level gets its own loop variable.

nested_for.rb

matrix = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9]
]

for row in matrix
  for cell in row
    print "#{cell} "
  end
  puts
end

The outer loop handles each row, while the inner loop processes individual cells. This pattern works for any nested enumerable structure.

for with Hash

A hash yields each key-value pair as a two-element array. With two loop variables, Ruby destructures the pair automatically.

for_hash.rb

person = {name: "John", age: 30, city: "New York"}

for key, value in person
  puts "#{key}: #{value}"
end

The loop unpacks each key-value pair into key and value. With a single loop variable, each item would be a two-element array such as [:name, "John"].

Modifying Elements in a for Loop

A for loop yields each element rather than a copy of it, but assigning to the loop variable only rebinds that variable. To change the collection, mutate the element or assign through its index.

modifying_for.rb

numbers = [1, 2, 3, 4, 5]

for num in numbers
  num *= 2
end

puts numbers.inspect

num *= 2 creates a new integer and assigns it to num; it does not change the array. Since integers are immutable, there is no element to mutate in place. Use map! to replace the elements, or iterate over indices and assign to numbers[i].

Break and Next in for Loops

Control flow keywords work similarly in for as in other loops. break exits entirely, while next skips ahead.

control_flow.rb

for i in 1..10
  next if i % 2 == 0
  break if i > 7
  puts "Odd number: #{i}"
end

next skips every even number. break ends the loop at the first odd number greater than 7, which is 9, so the last value printed is 7.

for vs each

This example contrasts for with Ruby's preferred each iterator. The main differences involve variable scope and return values.

for_vs_each.rb

# for loop version
for x in [1, 2, 3]
  puts x
end
puts "x after for: #{x}"

# each version
[1, 2, 3].each do |y|
  puts y
end
puts "y after each: #{y}" rescue puts "y undefined"

The for variable remains in the surrounding scope. The block variable y is local to the block, so referencing it afterward raises a NameError, which the rescue modifier catches. A for expression also returns the collection it iterated over.

Source

Ruby Keywords Documentation

This tutorial covered Ruby's for keyword with examples showing range iteration, array processing, nested loops, and control flow.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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