= 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.

== Ternary if

You may also write a if-then-else expression using <code>?</code> and
<code>:</code>.  This ternary if:

  input_type = gets =~ /hello/i ? "greeting" : "other"

Is the same as this +if+ expression:

  input_type =
    if gets =~ /hello/i
      "greeting"
    else
      "other"
    end

While the ternary if is much shorter to write than the more verbose form, for
readability it is recommended that the ternary if is only used for simple
conditionals.  Also, avoid using multiple ternary conditions in the same
expression as this can be confusing.

== +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" statement 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" statement, 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 as 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+.

== Modifier Statements

Ruby's grammar differentiates between statements and expressions.  All
expressions are statements (an expression is a type of statement), but
not all statements are expressions.  Some parts of the grammar accept
expressions and not other types of statements, which causes code that
looks similar to be parsed differently.

For example, when not used as a modifier, +if+, +else+, +while+, +until+,
and +begin+ are expressions (and also statements).  However, when
used as a modifier, +if+, +else+, +while+, +until+ and +rescue+
are statements but not expressions.

  if true; 1 end # expression (and therefore statement)
  1 if true      # statement (not expression)

Statements that are not expressions cannot be used in contexts where an
expression is expected, such as method arguments.

  puts( 1 if true )      #=> SyntaxError

You can wrap a statement in parentheses to create an expression.

  puts((1 if true))      #=> 1

If you put a space between the method name and opening parenthesis, you
do not need two sets of parentheses.

  puts (1 if true)       #=> 1, because of optional parentheses for method

This is because this is parsed similar to a method call without
parentheses.  It is equivalent to the following code, without the creation
of a local variable:

  x = (1 if true)
  p x

In a modifier statement, the left-hand side must be a statement and the
right-hand side must be an expression.

So in <code>a if b rescue c</code>, because <code>b rescue c</code> is a
statement that is not an expression, and therefore is not allowed as the
right-hand side of the +if+ modifier statement, the code is necessarily
parsed as <code>(a if b) rescue c</code>.

This interacts with operator precedence in such a way that:

  stmt if v = expr rescue x
  stmt if v = expr unless x

are parsed as:

  stmt if v = (expr rescue x)
  (stmt if v = expr) unless x

This is because modifier +rescue+ has higher precedence than <code>=</code>,
and modifier +if+ has lower precedence than <code>=</code>.

== Flip-Flop

The flip-flop is a 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.
