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Diffstat (limited to 'doc/syntax/assignment.rdoc')
| -rw-r--r-- | doc/syntax/assignment.rdoc | 62 |
1 files changed, 46 insertions, 16 deletions
diff --git a/doc/syntax/assignment.rdoc b/doc/syntax/assignment.rdoc index 7424d4885f..3988f82e5f 100644 --- a/doc/syntax/assignment.rdoc +++ b/doc/syntax/assignment.rdoc @@ -1,6 +1,6 @@ = Assignment -In Ruby assignment uses the <code>=</code> (equals sign) character. This +In Ruby, assignment uses the <code>=</code> (equals sign) character. This example assigns the number five to the local variable +v+: v = 5 @@ -8,6 +8,9 @@ example assigns the number five to the local variable +v+: Assignment creates a local variable if the variable was not previously referenced. +An assignment expression result is always the assigned value, including +{assignment methods}[rdoc-ref:@Assignment+Methods]. + == Local Variable Names A local variable name must start with a lowercase US-ASCII letter or a @@ -92,8 +95,9 @@ Now any reference to +big_calculation+ is considered a local variable and will be cached. To call the method, use <code>self.big_calculation</code>. You can force a method call by using empty argument parentheses as shown above -or by using an explicit receiver like <code>self.</code>. Using an explicit -receiver may raise a NameError if the method's visibility is not public. +or by using an explicit receiver like <code>self</code>. Using an explicit +receiver may raise a NameError if the method's visibility is not public or the +receiver is the literal <code>self</code>. Another commonly confusing case is when using a modifier +if+: @@ -103,12 +107,34 @@ Rather than printing "true" you receive a NameError, "undefined local variable or method `a'". Since ruby parses the bare +a+ left of the +if+ first and has not yet seen an assignment to +a+ it assumes you wish to call a method. Ruby then sees the assignment to +a+ and will assume you are referencing a local -method. +variable. The confusion comes from the out-of-order execution of the expression. First the local variable is assigned-to then you attempt to call a nonexistent method. +== Local Variables and eval + +Using +eval+ to evaluate Ruby code will allow access to local variables defined +in the same scope, even if the local variables are not defined until after the +call to +eval+. However, local variables defined inside the call to +eval+ +will not be reflected in the surrounding scope. Inside the call to +eval+, +local variables defined in the surrounding scope and local variables defined +inside the call to +eval+ will be accessible. However, you will not be able +to access local variables defined in previous or subsequent calls to +eval+ in +the same scope. Consider each +eval+ call a separate nested scope. Example: + + def m + eval "bar = 1" + lvs = eval "baz = 2; ary = [local_variables, foo, baz]; x = 2; ary" + eval "quux = 3" + foo = 1 + lvs << local_variables + end + + m + # => [[:baz, :ary, :x, :lvs, :foo], nil, 2, [:lvs, :foo]] + == Instance Variables Instance variables are shared across all methods for the same object. @@ -136,9 +162,7 @@ Here is an example of instance variable usage: p object1.value # prints "some value" p object2.value # prints "other value" -An uninitialized instance variable has a value of +nil+. If you run Ruby with -warnings enabled you will get a warning when accessing an uninitialized -instance variable. +An uninitialized instance variable has a value of +nil+. The +value+ method has access to the value set by the +initialize+ method, but only for the same object. @@ -255,7 +279,7 @@ An uninitialized global variable has a value of +nil+. Ruby has some special globals that behave differently depending on context such as the regular expression match variables or that have a side-effect when -assigned to. See the {global variables documentation}[rdoc-ref:globals.rdoc] +assigned to. See the {global variables documentation}[rdoc-ref:language/globals.md] for details. == Assignment Methods @@ -279,7 +303,7 @@ to an instance variable most people use Module#attr_accessor: end When using method assignment you must always have a receiver. If you do not -have a receiver Ruby assumes you are assigning to a local variable: +have a receiver, Ruby assumes you are assigning to a local variable: class C attr_accessor :value @@ -319,6 +343,9 @@ This prints: local_variables: @value: 42 +Note that the value returned by an assignment method is ignored whatever, +since an assignment expression result is always the assignment value. + == Abbreviated Assignment You can mix several of the operators and assignment. To add 1 to an object @@ -354,7 +381,7 @@ Here is an example: p a # prints 1 -Note that these two operators behave more like <code>a || a = 0<code> than +Note that these two operators behave more like <code>a || a = 0</code> than <code>a = a || 0</code>. == Implicit Array Assignment @@ -374,7 +401,11 @@ assigning. This is similar to multiple assignment: p a # prints [1, 2, 3] -You can splat anywhere in the left-hand side of the assignment: + b = *1 + + p b # prints [1] + +You can splat anywhere in the right-hand side of the assignment: a = 1, *[2, 3] @@ -382,7 +413,7 @@ You can splat anywhere in the left-hand side of the assignment: == Multiple Assignment -You can assign multiple values on the left-hand side to multiple variables: +You can assign multiple values on the right-hand side to multiple variables: a, b = 1, 2 @@ -408,8 +439,8 @@ You can use multiple assignment to swap two values in-place: p new_value: new_value, old_value: old_value # prints {:new_value=>1, :old_value=>2} -If you have more values on the left hand side of the assignment than variables -on the right hand side the extra values are ignored: +If you have more values on the right hand side of the assignment than variables +on the left hand side, the extra values are ignored: a, b = 1, 2, 3 @@ -422,7 +453,7 @@ the assignment. p a: a, b: b # prints {:a=>1, :b=>[2, 3]} -The <code>*</code> can appear anywhere on the right-hand side: +The <code>*</code> can appear anywhere on the left-hand side: *a, b = 1, 2, 3 @@ -452,4 +483,3 @@ Since each decomposition is considered its own multiple assignment you can use p a: a, b: b, c: c, d: d # prints {:a=>1, :b=>2, :c=>[3, 4], :d=>[5, 6]} - |