In this article, it uses the following method.
h = {}
def h.[]=(k, v)
puts "Setting hash key #{k} with #{v.inspect}"
super
end
# 1. The standard ||= approach
h[:x] ||= 10
h[:x] ||= 20
...
I understand that this is a setter like this for =( ) part.
def noise=(noise)
#noise = noise
end
Q1. But I am not sure what .[] part is doing.
Q2. Can you use [] or other non-alphabets in a Ruby method name?
Q1. But I am not sure what .[] part is doing.
Almost everything is an object in Ruby, and we can define a method on anything that's an object. So []= is a method defined on the Hash class (an object) in Ruby, just like + and - are methods on numbers (also objects):
> 4 + 5
# => 9
> 4.+(5)
# => 9
> 10.-(3)
# => 7
Likewise, we can call the .[] or .[]= methods for objects that define it, like an Array:
> arr = ['hi', 'there', 'welcome', 'to', 'StackOverflow']
> arr.[](3)
# => "to"
or like your Hash:
> hash = {:name => 'Bob', :age => 27}
> hash[:name]
# => "Bob"
> hash[:name] = 'Steve'
# => "Steve"
Again, Ruby lets us put methods on anything that's an object, and (almost) everything is an object in Ruby. So we can define that .[] method on any class we like:
> class Foo
> def [](arg)
> arg
> end
> end
> Foo.new[456]
> # => 456
Since instances of objects are also objects, we can define that method to be only on specific instances:
> h = {} # a Hash instance
> def h.[](arg) # we're giving this instance a new method
> "received #{arg}"
> end
> h[123]
# => "received 123"
Other instances of the same class won't get that implementation:
> {:foo => :bar}[123] # other Hash instances don't have this method,
# so they're using the default Hash#[] method
# => nil
.[] is something of a special case in one respect, because we let you skip the parentheses and put the argument right inside the brackets:
> arr[3] == arr.[](3)
# => true
Q2. Can you use [] or other non-alphabets in a Ruby method name?
No, you can't use [] in the name of an arbitrary Ruby method. This is an operator (like + or - in the previous example).
You can only overload specific operators, namely (listed in order of precedence):
!, ~, + (unary)
**
- (unary)
*, /, %
+, - (binary)
<<, >>
&
|, ^
<, <=, =>, >
==, ===, !=, =~, !~, <=>
Otherwise, a Ruby method can contain any mixture of alphanumeric Unicode characters and underscores. (I'm using "alphanumeric" loosely here -- Ruby is very liberal in what it allows, and generally speaking any character that isn't otherwise reserved by the language to tokenize things will work.) It may optionally end with a single ! or ?, and it may not start with a number.
So these are valid method names:
present?
valid_user?
replace!
replace
更换 (but you probably should make your method names using the A-Z Latin alphabet so developers don't hate you)
❨╯°□°❩╯︵┻━┻
┬─┬ノ❨º_ºノ❩
replace_all
REPLACE_1
REPLACE_ALL
Note that while the last two are valid method names, by convention Rubyists usually reserve ALL-CAPS identifiers for constants, not methods.
In Ruby, [] and []= are just operators and they can be overloaded.
Here []= is being defined on the singleton class for h, in other words, it is being defined to work a certain way just for h.
So when you say h[something] = somethingelse the new method gets called.
You are right that this use is quite similar to the general use of setters.
To answer the second question, you can overload the operators shown in this table, but you can't just go ahead and create operators like <-*-> as you can in some languages.
Let's say we have this code:
def something(*someargs)
return *someargs.join(",")
end
Now, I found you can reference *someargs just like any other variable anywhere in the method definition. But I tried this...returning *someargs as a string, separated with a comma. Yet, when I call this method:
a = something(4, 5)
p a.class # => Array
p a #> ["4,5"]
why does something(4,5) still returns an array? If I do something like this:
[4, 5].join(",")
the result will be a string not in an array. So my question would be, how do I make the "something" method return an actual string which contains all the arguments as a string. And it's weird because if I do *someargs.class, the result is "Array", yet it doesn't behave like a typical array...
Try below :
def something(*someargs)
return someargs.join(",")
end
a = something(4, 5)
p a.class # => String
p a # => "4,5"
One example to explain your case:
a = *"12,11"
p a # => ["12,11"]
So when you did return *someargs.join(","), someargs.join(",") created the string as "4,5".But now you are using splat operator(*) again on the evaluated string "4,5" with the assignment operation like a = *"4,5". So finally you are getting ["4,5"].
Read more scenarios with splat operators here - Splat Operator in Ruby
Hope that helps.
An object prepended with a splat *... is not an object. You cannot reference such thing, nor can you pass it as an argument to a method because there is no such thing. However, if you have a method that can take multiple arguments, for example puts, then you can do something like:
puts *["foo", "bar"]
In this case, there is no such thing as *["foo", "bar"]. The splat operator is expanding it into multiple arguments. It is equivalent to:
puts "foo", "bar"
Regarding why someargs remains to be an array after someargs.join(","). That is because join is not a destructive method. It does not do anything to the receiver. Furthermore, an object cannot change its class by a destructive method. The only way to change the reference of someargs from an array to a string is to reassign it.
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')
I'm trying to write a function that would apply the function to just one item if it's not a collection, or else apply that function to each of the collections elements. For example:
replace spaces with underscores
foo 'a bear' => 'a_bear'
foo ['a bear', 'a bee'] => ['a_bear', 'a_bee']
Is this possible?
It depends on how you define "collection". The most natural option would probably be either "any Enumerable" or even "anything with an each method". However this leads to a problem because Strings are Enumerable, too - in ruby 1.8, at least.
If you only need it to work with arrays, it's easy:
def foo(x)
if x.is_a? Array
x.map {|item| foo(item)}
else
# Do something with x
end
end
Personally I would use variable args:
def foo(*args)
args.each { |arg| puts arg }
end
foo("bar") # bar
foo("bar", "foobar") # bar \n foobar
foo(*%w(bar foobar)) # bar \n foobar
a = ["bar", "foobar"]
foo(*a) # bar \n foobar
foo("baz", *a) # baz \n bar \n foobar
a = "bar"
foo(*a) # bar
If you don't know whether or not your argument is a string or an array then just prepend it with a *.
I find this gives the maximum flexibility when dealing with arrays which might instead be a single value as I can enter them as just arguments if I am initializing the array or safely pass in the variable if I know it will either be an array or a single argument. It will choke on hashes though.
You may be interested in the splat operator
def foo(x)
[*x].map {|item| item.gsub(" ", "_")}
end
Unfortunately, this'd return foo("a bear") as ["a_bear"], rather than "a_bear" without the array.
Not sure if I'm misreading the question or not. The below will make it so a function will treat either a single element or an array of elements the same way. Just array-ifies the argument if it's not already an array, and undoes that at the end if necessary.
def foo(x)
x = [x] unless x.is_a? Array
# do array stuff to x
return result.size > 1 ? result : result.first
end
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.