diff options
Diffstat (limited to 'doc/syntax/control_expressions.rdoc')
| -rw-r--r-- | doc/syntax/control_expressions.rdoc | 60 |
1 files changed, 38 insertions, 22 deletions
diff --git a/doc/syntax/control_expressions.rdoc b/doc/syntax/control_expressions.rdoc index fe1a5fd874..b25f6936eb 100644 --- a/doc/syntax/control_expressions.rdoc +++ b/doc/syntax/control_expressions.rdoc @@ -27,13 +27,13 @@ The +then+ is optional: puts "the test resulted in a true-value" end -This document will include the optional +then+ for all expressions. Many -people omit the +then+ part of the if and other expressions. +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 then + if false puts "the test resulted in a true-value" else puts "the test resulted in a false-value" @@ -46,9 +46,9 @@ You may add an arbitrary number of extra tests to an if expression using a = 1 - if a == 0 then + if a == 0 puts "a is zero" - elsif a == 1 then + elsif a == 1 puts "a is one" else puts "a is some other value" @@ -60,15 +60,17 @@ 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 then + if a == 0 puts "a is zero" - elsif a == 1 then + elsif a == 1 puts "a is one" - elsif a >= 1 then + elsif a >= 1 puts "a is greater than or equal to one" else puts "a is some other value" @@ -77,7 +79,7 @@ In this example only "a is one" is printed: 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 then + if a = object.some_value # do something to a end @@ -89,21 +91,23 @@ expression. The +unless+ expression is the opposite of the +if+ expression. If the value is false the "then" expression is executed: - unless true then + 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 then + 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 then + unless true puts "the value is false" else puts "the value is true" @@ -146,17 +150,18 @@ parse order. Here is an example that shows the difference: This raises the NameError "undefined local variable or method `a'". -When ruby parses this it first encounters +a+ as a method call in the "then" -expression, then later sees +a+ as a local variable in the "test" expression. +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 then 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. +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 as true for +unless+. +The same is true for +unless+. == +case+ Expression @@ -170,7 +175,7 @@ Module#=== and Regexp#=== for examples. Here is an example of using +case+ to compare a String against a pattern: case "12345" - when /^1/ then + when /^1/ puts "the string starts with one" else puts "I don't know what the string starts with" @@ -194,7 +199,7 @@ result as the one above: You may place multiple conditions on the same +when+: case "2" - when /^1/, "2" then + when /^1/, "2" puts "the string starts with one or is '2'" end @@ -202,14 +207,23 @@ 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 then + when a == 1, a == 2 puts "a is one or two" - when a == 3 then + when a == 3 puts "a is three" else puts "I don't know what a is" @@ -282,6 +296,8 @@ The +do+ is optional: 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. |