View source for Ruby While and Until Loops

If computers do one thing well it is performing tasks very quickly. One thing computers do even better is performing the same task over and over again until specific criteria are met (or even infinitely if that is what is required). One this that computers are very bad at, however, is doing anything without being told to do it. Given these facts, it should come as no surprise that just about every programming language, Ruby included, provide mechanisms for instructing a computer system to repeat tasks.

In  this chapter of [[Ruby Essentials]] we will look at using ''while'' and ''until'' structures to allow loops to be built into applications. In the next chapter we will look at [[Looping with for and the Ruby Looping Methods|using the for loop and Ruby looping methods]].

== The Ruby While Loop ==

The Ruby ''while'' loop is designed to repeat a task until a particular expression is evaluated to be ''false''. The syntax of a ''while'' loop is as follows:

while ''expression'' do

... ''ruby code here'' ...

end


In the above outline, ''expression'' is a Ruby expression which must evaluate to ''true'' or ''false''. The ''ruby code here'' marker is where the code to executed is placed. This code will be repeatedly executed until the ''expression'' evaluates to ''false''.

For example, we might want to loop until a variable reaches a particular value:

<pre>
i = 0
while i < 5 do
   puts i
   i += 1
end
</pre>

The above code will output the value of i until i is no longer less than 5, resulting in the following output:

<pre>
0
1
2
3
4
</pre>

The ''do'' in this case is actually optional. The following is perfectly valid:

<pre>
i = 0
while i < 5
   puts i
   i += 1
end
</pre>

== Breaking from While Loops ==

It is sometimes necessary to break out of a while loop before the while expression evaluates to true. This can be achieved using the ''break if'' statement:

<pre>
i = 0
while i < 5
   puts i
   i += 1
   break if i == 2
end
</pre>

The above loop when now exit when i equals 2, instead of when i reaches 5.

== unless and until ==

Ruby's ''until'' statement differs from ''while'' in that it loops until a ''true'' condition is met. For example:

<pre>
i = 0
until i == 4
   puts i
   i += 1
end
</pre>

resulting in the following output:

<pre>
0
1
2
3
4
</pre>

The ''until'' keyword can also be used as a statement modifier, as follows:

<pre>
puts i += 1 until i == 5
</pre>

The ''unless'' statement provides an alternative mechanism to the ''if else'' construct. For example we might write an ''if else'' statement as follows:

<pre>
if i < 10
   puts "Student failed"
else
   puts "Student passed"
end
</pre>

The ''unless'' construct inverts this:

<pre>
unless i >= 10
    puts "Student failed"
else
    puts "Student passed"
end
</pre>