Very often in Ruby (and Rails specifically) you have to check if something exists and then perform an action on it, for example:
if #objects.any?
puts "We have these objects:"
#objects.each { |o| puts "hello: #{o}"
end
This is as short as it gets and all is good, but what if you have #objects.some_association.something.hit_database.process instead of #objects? I would have to repeat it second time inside the if expression and what if I don't know the implementation details and the method calls are expensive?
The obvious choice is to create a variable and then test it and then process it, but then you have to come up with a variable name (ugh) and it will also hang around in memory until the end of the scope.
Why not something like this:
#objects.some_association.something.hit_database.process.with :any? do |objects|
puts "We have these objects:"
objects.each { ... }
end
How would you do this?
Note that there's no reason to check that an array has at least one element with any? if you're only going to send each, because sending each to an empty array is a no-op.
To answer your question, perhaps you are looking for https://github.com/raganwald/andand?
Indeed, using a variable pollutes the namespace, but still, I think if (var = value).predicate is is a pretty common idiom and usually is perfectly ok:
if (objects = #objects.some_association.hit_database).present?
puts "We have these objects: #{objects}"
end
Option 2: if you like to create your own abstractions in a declarative fashion, that's also possible using a block:
#objects.some_association.hit_database.as(:if => :present?) do |objects|
puts "We have these objects: #{objects}"
end
Writing Object#as(options = {}) is pretty straigthforward.
What about tap?
#objects.some_association.something.hit_database.process.tap do |objects|
if objects.any?
puts "We have these objects:"
objects.each { ... }
end
end
Edit: If you're using Ruby 1.9, the Object#tap method provides the same functionality as the code listed below.
It sounds like you just want to be able to save a reference to an object without polluting the scope, correct? How about we open up the Object class and add a method do, which will just yield itself to the block:
class Object
def do
yield self if block_given?
return self # allow chaining
end
end
We can then call, for example:
[1,2,3].do { |a| puts a.length if a.any? }
=> 3
[].do { |a| puts a.length if a.any? }
=> nil
Being new to Ruby, I'm having trouble explaining to myself the behavior around method definitions within Ruby.
The example is noted below...
class Foo
def do_something(action)
action.inspect
end
def do_something_else=action
action.inspect
end
end
?> f.do_something("drive")
=> "\"drive\""
?> f.do_something_else=("drive")
=> "drive"
The first example is self explanatory. What Im trying to understand is the behavior of the second example. Other than what looks to be one producing a string literal and the other is not, what is actually happening? Why would I use one over the other?
Generally, do_something is a getter, and do_something= is a setter.
class Foo
attr_accessor :bar
end
is equivalent to
class Foo
def bar
#bar
end
def bar=(value)
#bar = value
end
end
To answer your question about the difference in behavior, methods that end in = always return the right hand side of the expression. In this case returning action, not action.inspect.
class Foo
def do_something=(action)
"stop"
end
end
?> f = Foo.new
?> f.do_something=("drive")
=> "drive"
Both of your methods are actually being defined and called as methods. Quite a lot of things in Ruby can be defined as methods, even the operators such as +, -, * and /. Ruby allows methods to have three special notational suffixes. I made that phrase up all by myself. What I mean by notational suffixes is that the thing on the end of the method will indicate how that method is supposed to work.
Bang!
The first notational suffix is !. This indicates that the method is supposed to be destructive, meaning that it modifies the object that it's called on. Compare the output of these two scripts:
a = [1, 2, 3]
a.map { |x| x * x }
a
And:
a = [1, 2, 3]
a.map! { |x| x * x }
a
There's a one character difference between the two scripts, but they operate differently! The first one will still go through each element in the array and perform the operation inside the block, but the object in a will still be the same [1,2,3] that you started with.
In the second example, however, the a at the end will instead be [1, 4, 9] because map! modified the object in place!
Query
The second notational suffix is ?, and that indicates that a method is used to query an object about something, and means that the method is supposed to return true, false or in some extreme circumstances, nil.
Now, note that the method doesn't have to return true or false... it's just that it'd be very nice if it did that!
Proof:
def a?
true
end
def b?
"moo"
end
Calling a? will return true, and calling b? will return "moo". So there, that's query methods. The methods that should return true or false but sometimes can return other things because some developers don't like other developers.
Setters!
NOW we get to the meat of your (paraphrased) question: what does = mean on the end of a method?
That usually indicates that a method is going to set a particular value, as Erik already outlined before I finished typing this essay of an answer.
However, it may not set one, just like the query methods may not return true or false. It's just convention.
You can call that setter method like this also:
foo.something_else="value"
Or (my favourite):
foo.something_else = "value"
In theory, you can actually ignore the passed in value, just like you can completely ignore any arguments passed into any method:
def foo?(*args)
"moo"
end
>> foo?(:please, :oh, :please, :why, :"won't", :you, :use, :these, :arguments, :i, :got, :just, :for, :you, :question_mark?)
=> "moo"
Ruby supports all three syntaxes for setter methods, although it's very rare to see the one you used!
Well, I hope this answer's been roughly educational and that you understand more things about Ruby now. Enjoy!
You cannot define a return value for assignment methods. The return value is always the same as the value passed in, so that assignment chains (x = y = z = 3) will always work.
Typically, you would omit the brackets when you invoke the method, so that it behaves like a property:
my_value = f.do_something= "drive"
def do_something_else=action
action.inspect
end
This defines a setter method, so do_something_else appears as though we are initializing a attribute. So the value initialized is directly passed,
There has to be a built in way of doing this, right?
class Object
def send_chain(arr)
o=self
arr.each{|a| o=o.send(a) }
return o
end
end
I just ran across this and it really begs for inject:
def send_chain(arr)
arr.inject(self) {|o, a| o.send(a) }
end
Building upon previous answers, in case you need to pass arguments to each method, you can use this:
def send_chain(arr)
Array(arr).inject(self) { |o, a| o.send(*a) }
end
You can then use the method like this:
arr = [:to_i, [:+, 4], :to_s, [:*, 3]]
'1'.send_chain(arr) # => "555"
This method accepts single arguments as well.
No, there isn't a built in way to do this. What you did is simple and concise enough, not to mention dangerous. Be careful when using it.
On another thought, this can be extended to accept arguments as well:
class Object
def send_chain(*args)
o=self
args.each do |arg|
case arg
when Symbol, String
o = o.send arg # send single symbol or string without arguments
when Array
o = o.send *arg # smash the inner array into method name + arguments
else
raise ArgumentError
end
end
return o
end
end
this would let you pass a method name with its arguments in an array, like;
test = MyObject.new
test.send_chain :a_method, [:a_method_with_args, an_argument, another_argument], :another_method
How about this versatile solution without polluting the Object class:
def chain_try(arr)
[arr].flatten.inject(self_or_instance, :try)
end
or
def chain_send(arr)
[arr].flatten.inject(self_or_instance, :send)
end
This way it can take a Symbol, a String or an Array with a mix of both even.🤔
example usage:
chain_send([:method1, 'method2', :method3])
chain_send(:one_method)
chain_send('one_method')
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.