= Control Expressions

Ruby has a variety of ways to control execution.  All the expressions described
here return a value.

For the tests in these control expressions, +nil+ and +false+ are false-values
and +true+ and any other object are true-values.  In this document "true" will
mean "true-value" and "false" will mean "false-value".

== +if+ Expression

The simplest +if+ expression has two parts, a "test" expression and a "then"
expression.  If the "test" expression evaluates to a true then the "then"
expression is evaluated.

Here is a simple if statement:

  if true then
    puts "the test resulted in a true-value"
  end

This will print "the test resulted in a true-value".

The +then+ is optional:

  if true
    puts "the test resulted in a true-value"
  end

This document will omit the optional +then+ for all expressions as that is the
most common usage of +if+.

You may also add an +else+ expression.  If the test does not evaluate to true
the +else+ expression will be executed:

  if false
    puts "the test resulted in a true-value"
  else
    puts "the test resulted in a false-value"
  end

This will print "the test resulted in a false-value".

You may add an arbitrary number of extra tests to an if expression using
+elsif+.  An +elsif+ executes when all tests above the +elsif+ are false.

  a = 1

  if a == 0
    puts "a is zero"
  elsif a == 1
    puts "a is one"
  else
    puts "a is some other value"
  end

This will print "a is one" as <code>1</code> is not equal to <code>0</code>.
Since +else+ is only executed when there are no matching conditions.

Once a condition matches, either the +if+ condition or any +elsif+ condition,
the +if+ expression is complete and no further tests will be performed.

Like an +if+, an +elsif+ condition may be followed by a +then+.

In this example only "a is one" is printed:

  a = 1

  if a == 0
    puts "a is zero"
  elsif a == 1
    puts "a is one"
  elsif a >= 1
    puts "a is greater than or equal to one"
  else
    puts "a is some other value"
  end

The tests for +if+ and +elsif+ may have side-effects.  The most common use of
side-effect is to cache a value into a local variable:

  if a = object.some_value
    # do something to a
  end

The result value of an +if+ expression is the last value executed in the
expression.

== +unless+ Expression

The +unless+ expression is the opposite of the +if+ expression.  If the value
is false the "then" expression is executed:

  unless true
    puts "the value is a false-value"
  end

This prints nothing as true is not a false-value.

You may use an optional +then+ with +unless+ just like +if+.

Note that the above +unless+ expression is the same as:

  if not true
    puts "the value is a false-value"
  end

Like an +if+ expression you may use an +else+ condition with +unless+:

  unless true
    puts "the value is false"
  else
    puts "the value is true"
  end

This prints "the value is true" from the +else+ condition.

You may not use +elsif+ with an +unless+ expression.

The result value of an +unless+ expression is the last value executed in the
expression.

== Modifier +if+ and +unless+

+if+ and +unless+ can also be used to modify an expression.  When used as a
modifier the left-hand side is the "then" expression and the right-hand side
is the "test" expression:

  a = 0

  a += 1 if a.zero?

  p a

This will print 1.

  a = 0

  a += 1 unless a.zero?

  p a

This will print 0.

While the modifier and standard versions have both a "test" expression and a
"then" expression, they are not exact transformations of each other due to
parse order.  Here is an example that shows the difference:

  p a if a = 0.zero?

This raises the NameError "undefined local variable or method `a'".

When ruby parses this expression it first encounters +a+ as a method call in
the "then" expression, then later it sees the assignment to +a+ in the "test"
expression and marks +a+ as a local variable.

When running this line it first executes the "test" expression, <code>a =
0.zero?</code>.

Since the test is true it executes the "then" expression, <code>p a</code>.
Since the +a+ in the body was recorded as a method which does not exist the
NameError is raised.

The same is true for +unless+.

== +case+ Expression

The +case+ expression can be used in two ways.

The most common way is to compare an object against multiple patterns.  The
patterns are matched using the +===+ method which is aliased to +==+ on
Object.  Other classes must override it to give meaningful behavior.  See
Module#=== and Regexp#=== for examples.

Here is an example of using +case+ to compare a String against a pattern:

  case "12345"
  when /^1/
    puts "the string starts with one"
  else
    puts "I don't know what the string starts with"
  end

Here the string <code>"12345"</code> is compared with <code>/^1/</code> by
calling <code>/^1/ === "12345"</code> which returns +true+.  Like the +if+
expression the first +when+ that matches is executed and all other matches are
ignored.

If no matches are found the +else+ is executed.

The +else+ and +then+ are optional, this +case+ expression gives the same
result as the one above:

  case "12345"
  when /^1/
    puts "the string starts with one"
  end

You may place multiple conditions on the same +when+:

  case "2"
  when /^1/, "2"
    puts "the string starts with one or is '2'"
  end

Ruby will try each condition in turn, so first <code>/^1/ === "2"</code>
returns +false+, then <code>"2" === "2"</code> returns +true+, so "the string
starts with one or is '2'" is printed.

You may use +then+ after the +when+ condition.  This is most frequently used
to place the body of the +when+ on a single line.

  case a
  when 1, 2 then puts "a is one or two
  when 3    then puts "a is three"
  else           puts "I don't know what a is"
  end

The other way to use a +case+ expression is like an if-elsif expression:

  a = 2

  case
  when a == 1, a == 2
    puts "a is one or two"
  when a == 3
    puts "a is three"
  else
    puts "I don't know what a is"
  end

Again, the +then+ and +else+ are optional.

The result value of a +case+ expression is the last value executed in the
expression.

== +while+ Loop

The +while+ loop executes while a condition is true:

  a = 0

  while a < 10 do
    p a
    a += 1
  end

  p a

Prints the numbers 0 through 10.  The condition <code>a < 10</code> is checked
before the loop is entered, then the body executes, then the condition is
checked again.  When the condition results in false the loop is terminated.

The +do+ keyword is optional.  The following loop is equivalent to the loop
above:

  while a < 10
    p a
    a += 1
  end

The result of a +while+ loop is +nil+ unless +break+ is used to supply a
value.

== +until+ Loop

The +until+ loop executes while a condition is false:

  a = 0

  until a > 10 do
    p a
    a += 1
  end

  p a

This prints the numbers 0 through 11.  Like a while loop the condition <code>a
> 10</code> is checked when entering the loop and each time the loop body
executes.  If the condition is false the loop will continue to execute.

Like a +while+ loop the +do+ is optional.

Like a +while+ loop the result of an +until+ loop is nil unless +break+ is
used.

== +for+ Loop

The +for+ loop consists of +for+ followed by a variable to contain the
iteration argument followed by +in+ and the value to iterate over using #each.
The +do+ is optional:

  for value in [1, 2, 3] do
    puts value
  end

Prints 1, 2 and 3.

Like +while+ and +until+, the +do+ is optional.

The +for+ loop is similar to using #each, but does not create a new variable
scope.

The result value of a +for+ loop is the value iterated over unless +break+ is
used.

The +for+ loop is rarely used in modern ruby programs.

== Modifier +while+ and +until+

Like +if+ and +unless+, +while+ and +until+ can be used as modifiers:

  a = 0

  a += 1 while a < 10

  p a # prints 10

+until+ used as a modifier:

  a = 0

  a += 1 until a > 10

  p a # prints 11

You can use +begin+ and +end+ to create a +while+ loop that runs the body once
before the condition:

  a = 0

  begin
    a += 1
  end while a < 10

  p a # prints 10

If you don't use +rescue+ or +ensure+ Ruby optimizes away any exception
handling overhead.

== +break+ Statement

Use +break+ to leave a block early.  This will stop iterating over the items in +values+ if one of them is even:

  values.each do |value|
    break if value.even?

    # ...
  end

You can also terminate from a +while+ loop using +break+:

  a = 0

  while true do
    p a
    a += 1

    break if a < 10
  end

  p a

This prints the numbers 0 and 1.

+break+ accepts a value that supplies the result of the expression it is
"breaking" out of:

  result = [1, 2, 3].each do |value|
    break value * 2 if value.even?
  end

  p result # prints 4

== +next+ Statement

Use +next+ to skip the rest of the current iteration:

  result = [1, 2, 3].map do |value|
    next if value.even?

    value * 2
  end

  p result # prints [2, nil, 6]

+next+ accepts an argument that can be used the result of the current block
iteration:

  result = [1, 2, 3].map do |value|
    next value if value.even?

    value * 2
  end

  p result # prints [2, 2, 6]

== +redo+ Statement

Use +redo+ to redo the current iteration:

  result = []

  while result.length < 10 do
    result << result.length

    redo if result.last.even?

    result << result.length + 1
  end

  p result

This prints [0, 1, 3, 3, 5, 5, 7, 7, 9, 9, 11]

In Ruby 1.8 you could also use +retry+ where you used +redo+.  This is no
longer true, now you will receive a SyntaxError when you use +retry+ outside
of a +rescue+ block.  See {Exceptions}[rdoc-ref:syntax/exceptions.rdoc]
for proper usage of +retry+.

== Flip-Flop

The flip-flop is rarely seen conditional expression.  It's primary use is
for processing text from ruby one-line programs used with <code>ruby -n</code>
or <code>ruby -p</code>.

The form of the flip-flop is an expression that indicates when the
flip-flop turns on, <code>..</code> (or <code>...</code>), then an expression
that indicates when the flip-flop will turn off.  While the flip-flop is on it
will continue to evaluate to +true+, and +false+ when off.

Here is an example:


  selected = []

  0.upto 10 do |value|
    selected << value if value==2..value==8
  end

  p selected # prints [2, 3, 4, 5, 6, 7, 8]

In the above example the on condition is <code>n==2</code>.  The flip-flop
is initially off (false) for 0 and 1, but becomes on (true) for 2 and remains
on through 8.  After 8 it turns off and remains off for 9 and 10.

The flip-flop must be used inside a conditional such as +if+, +while+,
+unless+, +until+ etc. including the modifier forms.

When you use an inclusive range (<code>..</code>) the off condition is
evaluated when the on condition changes:

  selected = []

  0.upto 5 do |value|
    selected << value if value==2..value==2
  end

  p selected # prints [2]

Here both sides of the flip-flop are evaluated so the flip-flop turns on and
off only when +value+ equals 2.  Since the flip-flop turned on in the
iteration it returns true.

When you use an exclusive range (<code>...</code>) the off condition is
evaluated on the following iteration:

  selected = []

  0.upto 5 do |value|
    selected << value if value==2...value==2
  end

  p selected # prints [2, 3, 4, 5]

Here the flip-flop turns on when +value+ equals 2 but doesn't turn off on the
same iteration.  The off condition isn't evaluated until the following
iteration and +value+ will never be two again.