The Ruby Programming Language

Loops

This section documents Ruby’s simple looping statements: while, until, and for. Ruby also includes the ability to define custom looping constructs known as iterators. Iterators (see Iterators and Enumerable Objects) are probably more commonly used than Ruby’s built-in looping statements; they are documented later in this chapter.

while and until

Ruby’s basic looping statements are while and until. They execute a chunk of code while a certain condition is true, or until the condition becomes true. For example:

x = 10               # Initialize a loop counter variable
while x >= 0 do      # Loop while x is greater than or equal to 0
  puts x             #   Print out the value of x
  x = x - 1          #   Subtract 1 from x
end                  # The loop ends here

# Count back up to 10 using an until loop
x = 0                # Start at 0 (instead of -1)
until x > 10 do      # Loop until x is greater than 10
  puts x
  x = x + 1
end                  # Loop ends here

The loop condition is the Boolean expression that appears between the while or until and do keywords. The loop body is the Ruby code that appears between the do and the end keyword. The while loop evaluates its condition. If the value is anything other than false or nil, it executes its body, and then loops to evaluate its condition again. In this way, the body is executed repeatedly, zero or more times, while the condition remains true (or, more strictly, non-false and non-nil).

The until loop is the reverse. The condition is tested and the body is executed if the condition evaluates to false or nil. This means that the ...