I'm so confused about that..
like this
class Box
attr_accessor :item ,:item2
def initialize(item2)
#item = []
#item2 = item2
end
def add(product)
item << product
end
def empty?
item.empty?
end
def increment(n=1)
item2 +=1
end
end
cart =Box.new(123)
cart.add(1)
puts cart.empty? #false
puts cart.item #1
in the 'add' and 'empty?' methods
I use local variable 'item' right?
why I can get the value from #items ??
and I try this
cart.item2 = 345
puts cart.item2 #345
puts cart.increment #'increment': undefined method `+' for nil:NilClass (NoMethodError)
now I can't get the value?
please fix my brain thx
First, read this answer, which is the most-upvoted Ruby post in StackOverflow history. It will help you understand attr_accessor and its cousins attr_reader and attr_writer.
Besides that, your code has many problems.
First, you should not name an Array with a singular variable name like item. Use a plural items to make its purpose clear.
Second, the name item2 is not good. For your attribute, use something descriptive like counter, and for the variable passed as an argument to initialize it, let's use something descriptive like initial_count.
Third, your increment method takes an optional argument but then ignores it. Wouldn't it be surprising if someone called box.increment(2) and the attribute was incremented by only 1? The intent of this method is to use counter += n instead of counter += 1.
Fourth, to set counter from within the class, we need to use self. So instead of counter += n, we have to do self.counter += n.
Finally, consider whether you want the attributes to be readable and writable from an outside source, or whether you want to reserve write privileges to the object itself. Because you have methods to add things to items and to increment counter, you probably want to conceal write privileges. I would use attr_reader publicly and attr_writer privately.
Incorporating these suggestions, here's the resulting code:
class Box
attr_reader :counter, :items
def initialize(initial_count)
#counter = initial_count
#items = []
end
def add(product)
items << product
end
def empty?
items.empty?
end
def increment(n = 1)
self.counter += n
end
private
attr_writer :counter, :items
end
Now you can do this, all of which makes sense, more or less:
>> cart = Box.new(123)
>> cart.increment(2)
>> cart.counter
#> 125
>> cart.add('A product')
>> cart.add('Another product')
>> cart.items
#> ["A product", "Another product"]
But if you try to set counter or items directly, you'll get an error:
>> cart.counter = 1
#> NoMethodError: private method `counter=' called for #<Box:0x007fc13e17dc50>
This method does not have a description on the APIdock. I know instance_exec in Ruby is similar to the this binding mechanism in JavaScript.
def interpolate(sql, record = nil)
if sql.respond_to?(:to_proc)
owner.instance_exec(record, &sql)
else
sql
end
end
Could someone briefly describe it?
First of all, the check for respond_to?(:to_proc) is necessary to make sure sql might be converted to lambda (by ampersand & to be passed to instance_exec. To simplify things, one might treat sql here as being a lambda already:
def interpolate(sql, record = nil) # assume sql is lambda
owner.instance_exec(record, &sql)
end
As by documentation on instance_exec:
Executes the given block within the context of the receiver...
That said, lambda will be executed as it was the ordinal code, placed somewhere inside instance method of the receiver.
class Owner
def initialize
#records = [:zero, :one, :two]
end
end
record_by_index = ->(idx) { #records[idx] }
Owner.new.instance_exec 1, &record_by_index #⇒ :one
The code above is [more or less] an equivalent to:
class Owner
def initialize
#records = [:zero, :one, :two]
end
def record_by_index idx
#records[idx]
end
end
Owner.new.record_by_index(1) #⇒ :one
The actual parameters of call to instance_exec will be passed to the codeblock. In the context of Owner’s instance we have an access to instance variables, private methods, etc. Hope it helps.
I am coming from a C# background and trying to learn Ruby and Ruby on Rails. I have the following Car class - note the build_xml method I need in order to build XML in that syntax and then pass to a WebService
class Car
##array = Array.new
#this will allow us to get list of all instances of cars created if needed
def self.all_instances
##array
end
def initialize(id, model_number, engine_size, no_doors)
# Instance variables
#id = id
#model_number = model_number
#engine_size = engine_size
#no_doors = no_doors
##array << self
end
def build_car_xml
car = { 'abc:Id'=> #id, 'abc:ModelNo' => #model_number, 'abc:ES' => #engine_size, 'abc:ND' => #no_doors}
cars = {'abc:Car' => [car] }
end
end
In another class then I was using this as below:
car1 = Car.new('1', 18, 3.0, 4)
request = car1.build_car_xml
This works as expected and the request is formatted how I need and the webservice returns the results. I now want to expand this however so I can pass in an array of cars and produce the request XML - however I am struggling to get this part working.
So far I have been trying the following (for now I am ok with just the Id changing as it is the only parameter required to be unique):
car_array = []
(1..10).each do |i|
car_array << Car.new(i.to_s, 18, 3.0, 4)
end
Am I correct in saying that I would need to define a new build_car_xml method on my Car class that can take an array of cars and then build the xml so my request call would be something like:
request = Car.build_car_xml(car_array)
What i am unsure of is 1) - is this the correct way of doing things in Ruby and 2) how to construct the method so that it is Building the XML in the correct format in the way it was when I call it on the single object - i.e - I need the namespaces added before the actual value.
def build_car_xml(car_array)
#here is where I am unsure how to contruct this method
end
Possible solution ('abc:Car' is a wrong name, should be Cars if you want it to hold an array):
class Car
...
def self.build_cars_xml(cars)
{ 'abc:Car' => cars.map(&:build_car_xml) }
end
def build_car_xml
{ 'abc:Id'=> #id, 'abc:ModelNo' => #model_number, 'abc:ES' => #engine_size, 'abc:ND' => #no_doors }
end
end
cars =
(1..10).map do |i|
Car.new(i.to_s, 18, 3.0, 4)
end
Car.build_cars_xml(cars)
It doesn't meet your requirements as instance build_car_xml doesn't generate Car namespace, but for me it's some inconsistency. Your XML is actually a collection, even if it has just one element, instance method should not be responsible for collection. Car.build_cars_xml([Car.new(...)] looks more logical to me.
my resque worker class is:
require 'resque'
require 'resque/job_with_status'
class PatstatResqueWorker < Resque::JobWithStatus
#queue = :my_worker_q
def self.perform(query, label)
puts "query:"
puts options['query']
puts "label:"
puts options['label']
end
end
and my controller part, where I call this resque is...
class MyController < ApplicationController
def resque
job_id = PatstatResqueWorker.create(:query => #query, :label => "yes")
status = Resque::Plugins::Status::Hash.get(job_id)
end
end
and its not working :(
if i remove the parameter from resque function it says Wrong number of arguments (2 for 0) and if i add the parameter section back it says options not defined :(
Could you help?
The reason you're getting the "options not defined" error is that you haven't defined options in the method that uses it. Your self.perform method expects to receive two distinct arguments, query and label, but the code inside the method expects to have an options hash. You've got to choose one or the other.
Either do this:
def self.perform(query, label)
# use the parameters we've already defined
puts "query:"
puts query
puts "label:"
puts label
end
# call it like this
PatstatResqueWorker.create(#query, "yes")
Or else do this:
# change the method signature to match what you're doing
def self.perform(options)
puts "query:"
puts options['query']
puts "label:"
puts options['label']
end
# call it like this, with string keys
PatstatResqueWorker.create('query' => #query, 'label' => "yes")
Notice that with the hash version, I changed the call to use strings for the hash keys instead of symbols. You can use symbols if you want, but you'd have to change it in the body of the method as well (i.e. options[:query] instead of options['query']). You've just got to be consistent.
New to Ruby and ROR and loving it each day, so here is my question since I have not idea how to google it (and I have tried :) )
we have method
def foo(first_name, last_name, age, sex, is_plumber)
# some code
# error happens here
logger.error "Method has failed, here are all method arguments #{SOMETHING}"
end
So what I am looking for way to get all arguments passed to method, without listing each one. Since this is Ruby I assume there is a way :) if it was java I would just list them :)
Output would be:
Method has failed, here are all method arguments {"Mario", "Super", 40, true, true}
In Ruby 1.9.2 and later you can use the parameters method on a method to get the list of parameters for that method. This will return a list of pairs indicating the name of the parameter and whether it is required.
e.g.
If you do
def foo(x, y)
end
then
method(:foo).parameters # => [[:req, :x], [:req, :y]]
You can use the special variable __method__ to get the name of the current method. So within a method the names of its parameters can be obtained via
args = method(__method__).parameters.map { |arg| arg[1].to_s }
You could then display the name and value of each parameter with
logger.error "Method failed with " + args.map { |arg| "#{arg} = #{eval arg}" }.join(', ')
Note: since this answer was originally written, in current versions of Ruby eval can no longer be called with a symbol. To address this, an explicit to_s has been added when building the list of parameter names i.e. parameters.map { |arg| arg[1].to_s }
Since Ruby 2.1 you can use binding.local_variable_get to read value of any local variable, including method parameters (arguments). Thanks to that you can improve the accepted answer to avoid evil eval.
def foo(x, y)
method(__method__).parameters.map do |_, name|
binding.local_variable_get(name)
end
end
foo(1, 2) # => 1, 2
One way to handle this is:
def foo(*args)
first_name, last_name, age, sex, is_plumber = *args
# some code
# error happens here
logger.error "Method has failed, here are all method arguments #{args.inspect}"
end
This is an interesting question. Maybe using local_variables? But there must be a way other than using eval. I'm looking in Kernel doc
class Test
def method(first, last)
local_variables.each do |var|
puts eval var.to_s
end
end
end
Test.new().method("aaa", 1) # outputs "aaa", 1
If you need arguments as a Hash, and you don't want to pollute method's body with tricky extraction of parameters, use this:
def mymethod(firstarg, kw_arg1:, kw_arg2: :default)
args = MethodArguments.(binding) # All arguments are in `args` hash now
...
end
Just add this class to your project:
class MethodArguments
def self.call(ext_binding)
raise ArgumentError, "Binding expected, #{ext_binding.class.name} given" unless ext_binding.is_a?(Binding)
method_name = ext_binding.eval("__method__")
ext_binding.receiver.method(method_name).parameters.map do |_, name|
[name, ext_binding.local_variable_get(name)]
end.to_h
end
end
This may be helpful...
def foo(x, y)
args(binding)
end
def args(callers_binding)
callers_name = caller[0][/`.*'/][1..-2]
parameters = method(callers_name).parameters
parameters.map { |_, arg_name|
callers_binding.local_variable_get(arg_name)
}
end
You can define a constant such as:
ARGS_TO_HASH = "method(__method__).parameters.map { |arg| arg[1].to_s }.map { |arg| { arg.to_sym => eval(arg) } }.reduce Hash.new, :merge"
And use it in your code like:
args = eval(ARGS_TO_HASH)
another_method_that_takes_the_same_arguments(**args)
If the function is inside some class then you can do something like this:
class Car
def drive(speed)
end
end
car = Car.new
method = car.method(:drive)
p method.parameters #=> [[:req, :speed]]
If you would change the method signature, you can do something like this:
def foo(*args)
# some code
# error happens here
logger.error "Method has failed, here are all method arguments #{args}"
end
Or:
def foo(opts={})
# some code
# error happens here
logger.error "Method has failed, here are all method arguments #{opts.values}"
end
In this case, interpolated args or opts.values will be an array, but you can join if on comma. Cheers
It seems like what this question is trying to accomplish could be done with a gem I just released, https://github.com/ericbeland/exception_details. It will list local variables and vlaues (and instance variables) from rescued exceptions. Might be worth a look...
Before I go further, you're passing too many arguments into foo. It looks like all of those arguments are attributes on a Model, correct? You should really be passing the object itself. End of speech.
You could use a "splat" argument. It shoves everything into an array. It would look like:
def foo(*bar)
...
log.error "Error with arguments #{bar.joins(', ')}"
end