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,
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¶ ↑
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”.
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
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 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
1 is not equal to
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.
elsif condition may be followed by a
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
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
:. This ternary if:
input_type = gets =~ /hello/i ? "greeting" : "other"
Is the same as this
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¶ ↑
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
unless just like
Note that the above
unless expression is the same as:
if not true puts "the value is a false-value" end
if expression you may use an
else condition with
unless true puts "the value is false" else puts "the value is true" end
This prints “the value is true” from the
You may not use
elsif with an
The result value of an
unless expression is the last value executed in the expression.
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,
a = 0.zero?.
Since the test is true it executes the “then” expression,
p a. Since the
a in the body was recorded as a method which does not exist the
NameError is raised.
The same is true for
case Expression¶ ↑
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
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
"12345" is compared with
/^1/ by calling
/^1/ === "12345" 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.
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
case "2" when /^1/, "2" puts "the string starts with one or is '2'" end
Ruby will try each condition in turn, so first
/^1/ === "2" returns
"2" === "2" 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
else are optional.
The result value of a
case expression is the last value executed in the expression.
while Loop¶ ↑
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
a < 10 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.
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
break is used to supply a value.
until Loop¶ ↑
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
a > 10 is checked when entering the loop and each time the loop body executes. If the condition is false the loop will continue to execute.
while loop, the
do is optional.
while loop, the result of an
until loop is nil unless
break is used.
for Loop¶ ↑
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.
do is optional.
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.
for loop is rarely used in modern ruby programs.
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
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
ensure, Ruby optimizes away any exception handling overhead.
break Statement¶ ↑
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
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¶ ↑
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¶ ↑
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 for proper usage of
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,
begin are expressions (and also statements). However, when used as a modifier,
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.
a if b rescue c, because
b rescue c 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
(a if b) rescue c.
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
=, and modifier
if has lower precedence than
The flip-flop is a rarely seen conditional expression. It's primary use is for processing text from ruby one-line programs used with
ruby -n or
The form of the flip-flop is an expression that indicates when the flip-flop turns on,
...), then an expression that indicates when the flip-flop will turn off. While the flip-flop is on it will continue to evaluate to
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
n==2. 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
until etc. including the modifier forms.
When you use an inclusive range (
..), 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 
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 (
...), 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.