I will try to explain the problem with a simple example:
def enclose(x)
[x]
end
In my application, enclose does something more complex, but in essence it returns an array, the content of which is solely determined by the value of the parameter x. I could it use it like this:
foo = 'abcd'
....
foo = enclose(foo)
Now to my question: Is it possible to write a method enclose!, which simply replaces the parameter by its enclosed version, so that the example could be written as
foo = 'abcd'
....
enclose!(foo)
Since Ruby passes arguments by reference, I thought hat this could maybe be possible. The naive approach,
def enclose!(x)
x = [x]
end
does not work - I think this is because the assignment creates a new object and leaves the actual parameter untouched.
Is there way, that I can achieve my goal? I think in Smallalk, there would be a method become which would change the object identity, but I didn't find something similar in Ruby.
Since Ruby passes arguments by reference, I thought hat this could maybe be possible.
Ruby is pass-by-value, not pass-by-reference, which you have proven yourself, because otherwise your code would have worked.
I think in Smallalk, there would be a method become which would change the object identity, but I didn't find something similar in Ruby.
There isn't. Ruby has neither pass-by-reference nor become:, what you want simply isn't possible.
There's some other interesting posts about how ruby is pass by value, but the values are references.
What it boils down to is, you can modify the variable an object refers to, but you cannot change it to refer to another object.
> a = [1]
=> [1]
> def add_a(array)
> array << "a"
> end
=> :add_a
> add_a a
=> [1, "a"]
> a
=> [1, "a"]
There is a way to sort of accomplish what you are asking for but it's not quite pretty. Ruby has this concept of a binding (http://ruby-doc.org/core-2.2.0/Binding.html), which is like a CallContext in .NET.
You can do something like this:
def enclose(x)
[x]
end
def enclose!(x, binding)
eval("#{x} = [#{x}]", binding)
end
foo = 'abcd'
enclose!(:foo, binding)
=> ["abcd"]
In the script above, the :foo means you are passing the name of the variable, and the binding (context) where to find its value. Then you're dynamically calling eval to evaluate the assignment operation foo = [foo].
Why does: respond_to? in:
class Wolf
def howl; end
end
Wolf.new.respond_to?(:howl) # => true
not require & while map in:
["1", "2", "3"].map(&:to_i) # => [1, 2, 3]
does? Also, are there any technical names for this?
When you say :method, you're using some nice syntactical sugar in ruby that creates a new Symbol object. When you throw an ampersand before it (&:method), you're using another piece of sugar. This invokes the to_proc method on the symbol.
So, these two things are identical:
method_proc = &:method
sym = :method
method_proc = method.to_proc
What's the difference between that and the other usage? Well, respond_to? has a single argument -- a symbol. So we can pass :method and be all fine and dandy. (Interestingly, objects do respond to the method named method, but that's a far more confusing question).
By comparison, Enumerable's iterators (like map, select, etc) accept a block. When we pass a Proc, it is interpreted properly as that block. So, these two pieces of code are equivalent:
[1,2,3].map { |i| i.even? }
[1,2,3].map(&:even?)
This equivalence is a little confusing, because of course Symbol has no idea that there's an even? method somewhere. To play around with it, I used evenproc = :even?.to_proc to inspect the resulting proc. It's implemented in C (at least in MRI ruby), and isn't willing to give up its source. However, its arity is -1, which means that it accepts one optional arg. My best guess is that it does something like this:
def to_proc
method_name = self.to_s
->(a) { a.send(method_name) }
end
I could dig further, but I think we've already gone way past the question. ;) Good luck!
What are the various meanings of the Ruby sharp/number sign/pound/hash(#) symbol
How many contexts does the symbol # in Ruby have ?
I know that #` represents comment
# a comment
or 'convert to the value':
i = 1
print "#{i}" # simple example
However I also see some Ruby docs describe built-in methods like these:
Array#fill
File::file?
Why do they describe the same thing using 2 different symbols ?
I am new in Ruby. Thanks
This is how instance method described:
Array#fill
So you can:
a = Array.new(2)
=> [nil, nil]
a.fill(42)
=> [42, 42]
This is how class method described:
String::new
s = String.new('abc')
=> "abc"
In Perl, # is used for commenting, and since Perl is an 'ancestor' of Ruby, the role was carried over.
The "#{}" syntax is called 'interpolation' and the pound was picked most likely because interpolation is similar in a sense to commenting, because you are changing the context of your code (in this case to another context for execution)
The # following a Class name is just meant to indicate the following identifier is a method of that Class, and is just a convention. Read more about it here: Why are methods in Ruby documentation preceded by a hash sign?
The :: is interesting, it acts similarly to the . in that you can call methods via both
Car::is_hybrid?
and
car.is_hybrid?
As you will see in most code though . is preferred for methods.
One case where :: is often preferred is where you have constant in the class and you will see this in system calls such as Math::PI or ones you create, e.g. ThePentagon::NUMBER_OF_BUILDING_SIDES
Just to show you as an example,that Ruby shows instance method preceded with the symbol # and class methods preceded with the symbol ..
class Foo
def self.foo;end
def bar;end
end
p Foo.method(:foo) # => #<Method: Foo.foo>
p Foo.new.method(:bar) # => #<Method: Foo#bar>
I'd like to write the method (define_variables) which can get a block and use the variables defined in it. Is it possible? For example, I'd like to get 5 in output:
module A
def self.define_variables
yield
puts a # not 5 :(
end
end
A::define_variables do
a = 5
end
Maybe there is some tricks with eval, but haven't found anyone yet.
In short, no. After you've called yield those variables defined in the block are gone (sort of, as we shall see), except for what is returned—that's just how scope works. In your example, the 5 is still there in that it is returned by the block, and thus puts yield would print 5. Using this you could return a hash from the block {:a => 5}, and then access multiple "variables" that way. In Ruby 1.8 (in IRb only) you can do:
eval "a = 5"
a # => 5
Though I don't know of anyway to eval the contents of a block. Regardless, in Ruby 1.9 the scope of eval was isolated and this will give you a NameError. You can do an eval within the context of a Binding though:
def foo
b = yield
eval(a, b) + 2
end
foo do
a = 5
binding
end # => 7
It seems to me that what you're trying to do is emulate macros in Ruby, which is just not possible (at least not pure Ruby), and I discourage the use of any of the "workarounds" I've mentioned above.
Agreed that this is a bit backwards, and Andrew's explanation is correct. If your use case is defining variables, however, there are already class_variable_set and instance_variable_set methods that are great for this:
module A
def self.define_variables(vars = {})
vars.each { |n, v| class_variable_set n, v }
puts ##a
end
end
A::define_variables :##a => 5
The above is more of an example of how it would work within the code you've posted rather than a recommendation.
Locked. This question and its answers are locked because the question is off-topic but has historical significance. It is not currently accepting new answers or interactions.
Continuing the "Hidden features of ..." meme, let's share the lesser-known but useful features of Ruby programming language.
Try to limit this discussion with core Ruby, without any Ruby on Rails stuff.
See also:
Hidden features of C#
Hidden features of Java
Hidden features of JavaScript
Hidden features of Ruby on Rails
Hidden features of Python
(Please, just one hidden feature per answer.)
Thank you
From Ruby 1.9 Proc#=== is an alias to Proc#call, which means Proc objects can be used in case statements like so:
def multiple_of(factor)
Proc.new{|product| product.modulo(factor).zero?}
end
case number
when multiple_of(3)
puts "Multiple of 3"
when multiple_of(7)
puts "Multiple of 7"
end
Peter Cooper has a good list of Ruby tricks. Perhaps my favorite of his is allowing both single items and collections to be enumerated. (That is, treat a non-collection object as a collection containing just that object.) It looks like this:
[*items].each do |item|
# ...
end
Don't know how hidden this is, but I've found it useful when needing to make a Hash out of a one-dimensional array:
fruit = ["apple","red","banana","yellow"]
=> ["apple", "red", "banana", "yellow"]
Hash[*fruit]
=> {"apple"=>"red", "banana"=>"yellow"}
One trick I like is to use the splat (*) expander on objects other than Arrays. Here's an example on a regular expression match:
match, text, number = *"Something 981".match(/([A-z]*) ([0-9]*)/)
Other examples include:
a, b, c = *('A'..'Z')
Job = Struct.new(:name, :occupation)
tom = Job.new("Tom", "Developer")
name, occupation = *tom
Wow, no one mentioned the flip flop operator:
1.upto(100) do |i|
puts i if (i == 3)..(i == 15)
end
One of the cool things about ruby is that you can call methods and run code in places other languages would frown upon, such as in method or class definitions.
For instance, to create a class that has an unknown superclass until run time, i.e. is random, you could do the following:
class RandomSubclass < [Array, Hash, String, Fixnum, Float, TrueClass].sample
end
RandomSubclass.superclass # could output one of 6 different classes.
This uses the 1.9 Array#sample method (in 1.8.7-only, see Array#choice), and the example is pretty contrived but you can see the power here.
Another cool example is the ability to put default parameter values that are non fixed (like other languages often demand):
def do_something_at(something, at = Time.now)
# ...
end
Of course the problem with the first example is that it is evaluated at definition time, not call time. So, once a superclass has been chosen, it stays that superclass for the remainder of the program.
However, in the second example, each time you call do_something_at, the at variable will be the time that the method was called (well, very very close to it)
Another tiny feature - convert a Fixnum into any base up to 36:
>> 1234567890.to_s(2)
=> "1001001100101100000001011010010"
>> 1234567890.to_s(8)
=> "11145401322"
>> 1234567890.to_s(16)
=> "499602d2"
>> 1234567890.to_s(24)
=> "6b1230i"
>> 1234567890.to_s(36)
=> "kf12oi"
And as Huw Walters has commented, converting the other way is just as simple:
>> "kf12oi".to_i(36)
=> 1234567890
Hashes with default values! An array in this case.
parties = Hash.new {|hash, key| hash[key] = [] }
parties["Summer party"]
# => []
parties["Summer party"] << "Joe"
parties["Other party"] << "Jane"
Very useful in metaprogramming.
Another fun addition in 1.9 Proc functionality is Proc#curry which allows you to turn a Proc accepting n arguments into one accepting n-1. Here it is combined with the Proc#=== tip I mentioned above:
it_is_day_of_week = lambda{ |day_of_week, date| date.wday == day_of_week }
it_is_saturday = it_is_day_of_week.curry[6]
it_is_sunday = it_is_day_of_week.curry[0]
case Time.now
when it_is_saturday
puts "Saturday!"
when it_is_sunday
puts "Sunday!"
else
puts "Not the weekend"
end
Download Ruby 1.9 source, and issue make golf, then you can do things like this:
make golf
./goruby -e 'h'
# => Hello, world!
./goruby -e 'p St'
# => StandardError
./goruby -e 'p 1.tf'
# => 1.0
./goruby19 -e 'p Fil.exp(".")'
"/home/manveru/pkgbuilds/ruby-svn/src/trunk"
Read the golf_prelude.c for more neat things hiding away.
Boolean operators on non boolean values.
&& and ||
Both return the value of the last expression evaluated.
Which is why the ||= will update the variable with the value returned expression on the right side if the variable is undefined. This is not explicitly documented, but common knowledge.
However the &&= isn't quite so widely known about.
string &&= string + "suffix"
is equivalent to
if string
string = string + "suffix"
end
It's very handy for destructive operations that should not proceed if the variable is undefined.
The Symbol#to_proc function that Rails provides is really cool.
Instead of
Employee.collect { |emp| emp.name }
You can write:
Employee.collect(&:name)
One final one - in ruby you can use any character you want to delimit strings. Take the following code:
message = "My message"
contrived_example = "<div id=\"contrived\">#{message}</div>"
If you don't want to escape the double-quotes within the string, you can simply use a different delimiter:
contrived_example = %{<div id="contrived-example">#{message}</div>}
contrived_example = %[<div id="contrived-example">#{message}</div>]
As well as avoiding having to escape delimiters, you can use these delimiters for nicer multiline strings:
sql = %{
SELECT strings
FROM complicated_table
WHERE complicated_condition = '1'
}
Use a Range object as an infinite lazy list:
Inf = 1.0 / 0
(1..Inf).take(5) #=> [1, 2, 3, 4, 5]
More info here: http://banisterfiend.wordpress.com/2009/10/02/wtf-infinite-ranges-in-ruby/
I find using the define_method command to dynamically generate methods to be quite interesting and not as well known. For example:
((0..9).each do |n|
define_method "press_#{n}" do
#number = #number.to_i * 10 + n
end
end
The above code uses the 'define_method' command to dynamically create the methods "press1" through "press9." Rather then typing all 10 methods which essentailly contain the same code, the define method command is used to generate these methods on the fly as needed.
module_function
Module methods that are declared as module_function will create copies of themselves as private instance methods in the class that includes the Module:
module M
def not!
'not!'
end
module_function :not!
end
class C
include M
def fun
not!
end
end
M.not! # => 'not!
C.new.fun # => 'not!'
C.new.not! # => NoMethodError: private method `not!' called for #<C:0x1261a00>
If you use module_function without any arguments, then any module methods that comes after the module_function statement will automatically become module_functions themselves.
module M
module_function
def not!
'not!'
end
def yea!
'yea!'
end
end
class C
include M
def fun
not! + ' ' + yea!
end
end
M.not! # => 'not!'
M.yea! # => 'yea!'
C.new.fun # => 'not! yea!'
Short inject, like such:
Sum of range:
(1..10).inject(:+)
=> 55
Warning: this item was voted #1 Most Horrendous Hack of 2008, so use with care. Actually, avoid it like the plague, but it is most certainly Hidden Ruby.
Superators Add New Operators to Ruby
Ever want a super-secret handshake operator for some unique operation in your code? Like playing code golf? Try operators like
-~+~-
or
<---
That last one is used in the examples for reversing the order of an item.
I have nothing to do with the Superators Project beyond admiring it.
I'm late to the party, but:
You can easily take two equal-length arrays and turn them into a hash with one array supplying the keys and the other the values:
a = [:x, :y, :z]
b = [123, 456, 789]
Hash[a.zip(b)]
# => { :x => 123, :y => 456, :z => 789 }
(This works because Array#zip "zips" up the values from the two arrays:
a.zip(b) # => [[:x, 123], [:y, 456], [:z, 789]]
And Hash[] can take just such an array. I've seen people do this as well:
Hash[*a.zip(b).flatten] # unnecessary!
Which yields the same result, but the splat and flatten are wholly unnecessary--perhaps they weren't in the past?)
Auto-vivifying hashes in Ruby
def cnh # silly name "create nested hash"
Hash.new {|h,k| h[k] = Hash.new(&h.default_proc)}
end
my_hash = cnh
my_hash[1][2][3] = 4
my_hash # => { 1 => { 2 => { 3 =>4 } } }
This can just be damn handy.
Destructuring an Array
(a, b), c, d = [ [:a, :b ], :c, [:d1, :d2] ]
Where:
a #=> :a
b #=> :b
c #=> :c
d #=> [:d1, :d2]
Using this technique we can use simple assignment to get the exact values we want out of nested array of any depth.
Class.new()
Create a new class at run time. The argument can be a class to derive from, and the block is the class body. You might also want to look at const_set/const_get/const_defined? to get your new class properly registered, so that inspect prints out a name instead of a number.
Not something you need every day, but quite handy when you do.
create an array of consecutive numbers:
x = [*0..5]
sets x to [0, 1, 2, 3, 4, 5]
A lot of the magic you see in Rubyland has to do with metaprogramming, which is simply writing code that writes code for you. Ruby's attr_accessor, attr_reader, and attr_writer are all simple metaprogramming, in that they create two methods in one line, following a standard pattern. Rails does a whole lot of metaprogramming with their relationship-management methods like has_one and belongs_to.
But it's pretty simple to create your own metaprogramming tricks using class_eval to execute dynamically-written code.
The following example allows a wrapper object to forwards certain methods along to an internal object:
class Wrapper
attr_accessor :internal
def self.forwards(*methods)
methods.each do |method|
define_method method do |*arguments, &block|
internal.send method, *arguments, &block
end
end
end
forwards :to_i, :length, :split
end
w = Wrapper.new
w.internal = "12 13 14"
w.to_i # => 12
w.length # => 8
w.split('1') # => ["", "2 ", "3 ", "4"]
The method Wrapper.forwards takes symbols for the names of methods and stores them in the methods array. Then, for each of those given, we use define_method to create a new method whose job it is to send the message along, including all arguments and blocks.
A great resource for metaprogramming issues is Why the Lucky Stiff's "Seeing Metaprogramming Clearly".
use anything that responds to ===(obj) for case comparisons:
case foo
when /baz/
do_something_with_the_string_matching_baz
when 12..15
do_something_with_the_integer_between_12_and_15
when lambda { |x| x % 5 == 0 }
# only works in Ruby 1.9 or if you alias Proc#call as Proc#===
do_something_with_the_integer_that_is_a_multiple_of_5
when Bar
do_something_with_the_instance_of_Bar
when some_object
do_something_with_the_thing_that_matches_some_object
end
Module (and thus Class), Regexp, Date, and many other classes define an instance method :===(other), and can all be used.
Thanks to Farrel for the reminder of Proc#call being aliased as Proc#=== in Ruby 1.9.
The "ruby" binary (at least MRI's) supports a lot of the switches that made perl one-liners quite popular.
Significant ones:
-n Sets up an outer loop with just "gets" - which magically works with given filename or STDIN, setting each read line in $_
-p Similar to -n but with an automatic puts at the end of each loop iteration
-a Automatic call to .split on each input line, stored in $F
-i In-place edit input files
-l Automatic call to .chomp on input
-e Execute a piece of code
-c Check source code
-w With warnings
Some examples:
# Print each line with its number:
ruby -ne 'print($., ": ", $_)' < /etc/irbrc
# Print each line reversed:
ruby -lne 'puts $_.reverse' < /etc/irbrc
# Print the second column from an input CSV (dumb - no balanced quote support etc):
ruby -F, -ane 'puts $F[1]' < /etc/irbrc
# Print lines that contain "eat"
ruby -ne 'puts $_ if /eat/i' < /etc/irbrc
# Same as above:
ruby -pe 'next unless /eat/i' < /etc/irbrc
# Pass-through (like cat, but with possible line-end munging):
ruby -p -e '' < /etc/irbrc
# Uppercase all input:
ruby -p -e '$_.upcase!' < /etc/irbrc
# Same as above, but actually write to the input file, and make a backup first with extension .bak - Notice that inplace edit REQUIRES input files, not an input STDIN:
ruby -i.bak -p -e '$_.upcase!' /etc/irbrc
Feel free to google "ruby one-liners" and "perl one-liners" for tons more usable and practical examples. It essentially allows you to use ruby as a fairly powerful replacement to awk and sed.
The send() method is a general-purpose method that can be used on any Class or Object in Ruby. If not overridden, send() accepts a string and calls the name of the method whose string it is passed. For example, if the user clicks the “Clr” button, the ‘press_clear’ string will be sent to the send() method and the ‘press_clear’ method will be called. The send() method allows for a fun and dynamic way to call functions in Ruby.
%w(7 8 9 / 4 5 6 * 1 2 3 - 0 Clr = +).each do |btn|
button btn, :width => 46, :height => 46 do
method = case btn
when /[0-9]/: 'press_'+btn
when 'Clr': 'press_clear'
when '=': 'press_equals'
when '+': 'press_add'
when '-': 'press_sub'
when '*': 'press_times'
when '/': 'press_div'
end
number.send(method)
number_field.replace strong(number)
end
end
I talk more about this feature in Blogging Shoes: The Simple-Calc Application
Fool some class or module telling it has required something that it really hasn't required:
$" << "something"
This is useful for example when requiring A that in turns requires B but we don't need B in our code (and A won't use it either through our code):
For example, Backgroundrb's bdrb_test_helper requires 'test/spec', but you don't use it at all, so in your code:
$" << "test/spec"
require File.join(File.dirname(__FILE__) + "/../bdrb_test_helper")
Defining a method that accepts any number of parameters and just discards them all
def hello(*)
super
puts "hello!"
end
The above hello method only needs to puts "hello" on the screen and call super - but since the superclass hello defines parameters it has to as well - however since it doesn't actually need to use the parameters itself - it doesn't have to give them a name.
private unless Rails.env == 'test'
# e.g. a bundle of methods you want to test directly
Looks like a cool and (in some cases) nice/useful hack/feature of Ruby.