I am trying to set the first argument to a method as being optional, followed by any number of args. For example:
def dothis(value=0, *args)
The issue I am running into is that it doesn't seem like this is actually possible? When I call dothis("hey", "how are you", "good") I was hoping it would set value to default to 0, but instead it is just making value="hey". Is there any way to accomplish this behavior?
This is not possible directly in Ruby
There are plenty of options though, depending on what you are doing with your extended params, and what the method is intended to do.
Obvious choices are
1) Take named params using hash syntax
def dothis params
value = params[:value] || 0
list_of_stuff = params[:list] || []
Ruby has nice calling convention around this, you don't need to provide the hash {} brackets
dothis :list => ["hey", "how are you", "good"]
2) Move value to the end, and take an array for the first param
def dothis list_of_stuff, value=0
Called like this:
dothis ["hey", "how are you", "good"], 17
3) Use a code block to provide the list
dothis value = 0
list_of_stuff = yield
Called like this
dothis { ["hey", "how are you", "good"] }
4) Ruby 2.0 introduced named hash parameters, which handle a lot of option 1, above for you:
def dothis value: 0, list: []
# Local variables value and list already defined
# and defaulted if necessary
Called same way as (1):
dothis :list => ["hey", "how are you", "good"]
This post is a little bit old, but I want to contribute if someone is looking for the best solution for that. Since ruby 2.0, you can do that easily with named arguments defined with a hash. The syntax is easy and more readable.
def do_this(value:0, args:[])
puts "The default value is still #{value}"
puts "-----------Other arguments are ---------------------"
for i in args
puts i
end
end
do_this(args:[ "hey", "how are you", "good"])
You can also do the same thing with the greedy keyword **args as a hash, like this:
#**args is a greedy keyword
def do_that(value: 0, **args)
puts "The default value is still #{value}"
puts '-----------Other arguments are ---------------------'
args.each_value do |arg|
puts arg
end
end
do_that(arg1: "hey", arg2: "how are you", arg3: "good")
You will need to use named parameters to accomplish this:
def dothis(args)
args = {:value => 0}.merge args
end
dothis(:value => 1, :name => :foo, :age => 23)
# => {:value=>1, :name=>:foo, :age=>23}
dothis(:name => :foo, :age => 23)
# => {:value=>0, :name=>:foo, :age=>23}
by using value=0 you are actually assigning 0 to value. just to retain the value, you can either use the above mentioned solutions or just simply use value everytime you call this method def dothis(value, digit=[*args]).
The default arguments are used when the arguments are not provided.
I came across the similar issue and I got over it by using:
def check(value=0, digit= [*args])
puts "#{value}" + "#{digit}"
end
and simply call check like this:
dothis(value, [1,2,3,4])
your value would be default and other values belong to other argument.
How can I declare a method with keyword arguments just like rails do. some examples may be
Person.find(:all, :conditions => "...").
How can I use symbols to create methods similar to the above?
I am very new to ruby. Thanks in advance!
Ruby doesn't actually have keyword arguments. Rails is exploiting a feature of Ruby which lets you omit the braces around a hash. For example, with find, what we're really calling is:
Person.find(:all, { :conditions => "...", :offset => 10, :limit => 10 } )
But if the hash is the last argument of the method, you can leave out the braces and it will still be treated as a hash:
Person.find(:all, :conditions => "...", :offset => 10, :limit => 10)
You can use this in your own methods:
def explode(options={})
defaults = { :message => "Kabloooie!", :timer => 10, :count => 1 }
options = defaults.merge(options)
options[:count].times do
sleep options[:timer]
puts options[:message]
end
end
And then call it:
explode :message => "Meh.", :count => 3
Or call it without an argument, resulting in all default values being used:
explode
Since Ruby 2.0, ruby does have keyword arguments.
def my_method(arg1, name: 'defaultName', number: 0)
puts arg1, name, number
end
I agree with accepted answer given by Samir Talwar and christopherwright. The only potential downside is that you get no warnings if you use an incorrect keyword symbol as an argument or when looking up an option, it just ends up ignored. If that's something you're concerned about, the gem hash_keyword_args addresses it. The idiom would be
def explode(opts={})
opts = opts.keyword_args(:message => "Kabloooie!", :timer => 10, :count => 1)
opts.count.times do
sleep opts.timer
puts opts.message
end
end
Notice the use of accessor methods so you'll get a NoMethodError if you mistype a keyword. And the calling behavior is:
explode(:message => "Okay") # works
explode(:msg => "Oops") # raises ArgumentError
The gem also provides a few other features you might or might not care about, such as being able to indicate that a keyword is required. I've been using it happily for a while now.
(Disclaimer: I'm the author of the gem.)
You just need to define a method where one of the parameters is a hash. It's actually pretty simple.
def method(arg1, params)
name = params[:name]
number = params[:number]
And then call it like:
method(arg1, :name => 'Eric', :number => 2)
Two notes:
In Ruby, you don't need to surround the parameters hash in {} when you call the method in most cases, unless you have something complicated going on like passing multiple hashes. In that case, make sure you surround those parameters with {}
Ruby is dynamically typed, so you don't need to say that params is a hash when you define the method.
Ruby 2.0 introduced real keyword arguments, and Ruby 2.1 added required keyword arguments.
There's a nice article up at https://chriszetter.com/blog/2012/11/02/keyword-arguments-in-ruby-2-dot-0/ on this, I've borrowed the examples from there:
Ruby 2.0+:
def exclaim(text, exclamation: '!', number: 7)
text + exclamation * number
end
exclaim('hello', number: 4) #=> 'hello!!!!'
# equivalent:
exclaim('hello', {:number => 4}) #=> 'hello!!!!'
Ruby 2.1+:
def exclaim(text, exclamation: '!', number:)
text + exclamation * number
end
exclaim('Yo', number: 5) # => 'Yo!!!!!'
exclaim('Yo') # raises: ArgumentError: missing keyword: number
Since Ruby is typed dynamically, just do :
def my_method(arg1, arg2)
#things
end
example:
my_method(:test, {:somehash => "yay"})
or
my_method :test, :somehash => "yay"
or
my_method(:test, :somehash => "yay")
I have a a hash
foo = {'bar'=>'baz'}
I would like to call foo.bar #=> 'baz'
My motivation is rewriting an activerecord query into a raw sql query (using Model#find_by_sql). This returns a hash with the SELECT clause values as keys. However, my existing code relies on object.method dot notation. I'd like to do minimal code rewrite. Thanks.
Edit: it appears Lua has this feature:
point = { x = 10, y = 20 } -- Create new table
print(point["x"]) -- Prints 10
print(point.x) -- Has exactly the same meaning as line above
>> require 'ostruct'
=> []
>> foo = {'bar'=>'baz'}
=> {"bar"=>"baz"}
>> foo_obj = OpenStruct.new foo
=> #<OpenStruct bar="baz">
>> foo_obj.bar
=> "baz"
>>
What you're looking for is called OpenStruct. It's part of the standard library.
A good solution:
class Hash
def method_missing(method, *opts)
m = method.to_s
if self.has_key?(m)
return self[m]
elsif self.has_key?(m.to_sym)
return self[m.to_sym]
end
super
end
end
Note: this implementation has only one known bug:
x = { 'test' => 'aValue', :test => 'bar'}
x.test # => 'aValue'
If you prefer symbol lookups rather than string lookups, then swap the two 'if' condition
Rather than copy all the stuff out of the hash, you can just add some behaviour to Hash to do lookups.
If you add this defintion, you extend Hash to handle all unknown methods as hash lookups:
class Hash
def method_missing(n)
self[n.to_s]
end
end
Bear in mind that this means that you won't ever see errors if you call the wrong method on hash - you'll just get whatever the corresponding hash lookup would return.
You can vastly reduce the debugging problems this can cause by only putting the method onto a specific hash - or as many hashes as you need:
a={'foo'=>5, 'goo'=>6}
def a.method_missing(n)
self[n.to_s]
end
The other observation is that when method_missing gets called by the system, it gives you a Symbol argument. My code converted it into a String. If your hash keys aren't strings this code will never return those values - if you key by symbols instead of strings, simply substitute n for n.to_s above.
There are a few gems for this. There's my recent gem, hash_dot, and a few other gems with similar names I discovered as I released mine on RubyGems, including dot_hash.
HashDot allows dot notation syntax, while still addressing concerns about NoMethodErrors addressed by #avdi. It is faster, and more traversable than an object created with OpenStruct.
require 'hash_dot'
a = {b: {c: {d: 1}}}.to_dot
a.b.c.d => 1
require 'open_struct'
os = OpenStruct.new(a)
os.b => {c: {d: 1}}
os.b.c.d => NoMethodError
It also maintains expected behavior when non-methods are called.
a.non_method => NoMethodError
Please feel free to submit improvements or bugs to HashDot.
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.