Ruby has a few other loop constructs too. This is how to do a while loop:
while tired sleep end
Or, here’s another way to put it:
sleep while tired
Even though the syntax of these two examples is different, they perform the same function. In the first example, the code between while and end (here a call to a method named sleep) executes just as long as the Boolean condition (which, in this case, is the value returned by a method called tired) evaluates to true. As in for loops, the keyword do may optionally be placed between the test condition and the code to be executed when these appear on separate lines; the do keyword is obligatory when the test condition and the code to be executed appear on the same line.
In the second version of the loop (sleep while tired), the code to be executed (sleep) precedes the test condition (while tired). This syntax is called a while modifier. When you want to execute several expressions using this syntax, you can put them between the begin and end keywords:
begin sleep snore end while tired
Here is an example showing the various alternative syntaxes:
$hours_asleep = 0
def tired
if $hours_asleep >= 8 then
$hours_asleep = 0
return false
else
$hours_asleep += 1
return true
end
end
def snore
puts('snore....')
end
def sleep
puts("z" * $hours_asleep )
end
while tired do sleep end # a single-line while loop
while tired # a multiline while loop
sleep
end
sleep while tired # single-line while modifier
begin # multiline while modifier
sleep
snore
end while tiredThe last example in the previous code (the multiline while modifier) needs close consideration because it introduces some important new behavior. When a block of code delimited by begin and end precedes the while test, that code always executes at least once. In the other types of while loop, the code may never execute at all if the Boolean condition initially evaluates to false.
Usually a while loops executes zero or more times since the Boolean test is evaluated before the loop executes; if the test returns false at the outset, the code inside the loop never runs. However, when the while test follows a block of code enclosed between begin and end, the loop executes one or more times as the Boolean expression is evaluated after the code inside the loop executes.
These examples should help clarify:
2loops.rb
x = 100
# The code in this loop never runs
while (x < 100) do puts('x < 100') end
# The code in this loop never runs
puts('x < 100') while (x < 100)
# But the code in loop runs once
begin puts('x < 100') end while (x < 100)