Is it possible in Ruby to instantiate another object of a different class using a class method?
I have spent a lot of time to no avail researching Google, the Ruby docs, and Stack Overflow, for an answer to my query, so I am posting this question as a last resort.
Two classes, User, and Blog. In User below I am trying to create a new instance of an object for Blog, and carry over some attributes.
In class User there are 2 methods;
class User
attr_accessor :blogs, :username
def initialize(username)
self.username = username
self.blogs = []
end
def add_blog(date, text)
self.blogs << [Date.parse(date),text]
new_blog = [Date.parse(date),text]
Blog.new(#username,date,text)
end
end
Using the above add_blog I would like to initialize & send a new object to the Blog class.
class Blog
attr_accessor :text, :date, :user
def initialize(user,date,text)
user = user
date = date
text = text
end
end
Yes, it is possible to instantiate another object of a different class using a class method. We ruby programmers are doing it all the time.
Do you want to have the blogs attribute of the user to have an array of blogs? Because your code just puts a date-text tupel into the array.
I think you wanted to do something like this in your user class:
def add_blog(date, text)
parsed_date = Date.parse(date)
new_blog = Blog.new(#username, parsed_date, text)
self.blogs << new_blog
new_blog
end
I have showed it step after step, but you can combine several lines. The last line returns the new blog, you may not need it if you only want the blog be part of the blogs array.
Looks like your code has some flaws. Here is the corrected code:
You were not using # to assign values to instance variables, you were putting a date in #blogs, instead of Blog object.
If you want to pass instance of User to Blog from add_blog, you can use self which represents current instance of User class. If you want to carry just some attributes, then, you can do so by referring to attributes using #attribute_name or self.attribute_name syntax
require "date"
class User
attr_accessor :blogs, :username
def initialize(username)
#username = username
#blogs = []
end
def add_blog(date, text)
#blogs << Blog.new(#username, Date.parse(date),text)
self
end
def to_s
"#{#username} - #{#blogs.collect { |b| b.to_s }}"
end
end
class Blog
attr_accessor :text, :date, :user
def initialize(user,date,text)
#user = user
#date = date
#text = text
end
def to_s
"Blog of #{user}: #{#date} - #{#text}"
end
end
puts User.new("Wand Maker").add_blog("2015-08-15", "Hello")
# => Wand Maker - ["Blog of Wand Maker: 2015-08-15 - Hello"]
Related
So, I'm trying to initialize multiple objects and add them to a list. What I'm wanting to happen is by running Market.new, I want every item from the api added as an object. Below is the code I thought might work. But, it's adding the same object to the list 100x. Is there a way to accomplish this?
def initialize
data = JSON.parse(open(BASE_URL + "markets?vs_currency=usd").read)
i = 0
# looping until we hit the end of the list. adding them all as objects.
while i < data.length
#id = data[i]["id"].to_s
#name = data[i]["name"].to_s
#symbol = data[i]["symbol"].to_s
#price = data[i]["current_price"].to_s
#price_movement_24h = data[i]["price_change_percentage_24h"].to_s
#market_cap = data[i]["market_cap"].to_s
##market << self
i += 1
end
end
This gives me the same object added to the ##market list 100x.
=> [#<Market:0x0000561b3f853f30
#id="iostoken",
#market_cap="43969067",
#name="IOST",
#price="0.0036523",
#price_movement_24h="-0.76702",
#symbol="iost">,
#<Market:0x0000561b3f853f30
#id="iostoken",
#market_cap="43969067",
#name="IOST",
#price="0.0036523",
#price_movement_24h="-0.76702",
#symbol="iost">,
I want to start by saying this is very strange Ruby code and not something you'd typically do. That's not meant as an insult, just to say that Ruby devs tend to follow the same or similar guidelines on structuring objects and this chunk of code feels like it's ported from another language.
The issue you're seeing is due to the fact that within the initialize method you're not creating any new objects but instead updating the instance variables and pushing self into a class variable. self is referencing this instance directly which means the class variable array is filling up with references to the same object. If you're adamant on keeping the code the same then you should instead push a duplicate of your object after you've updated the instance variables.
##market << self.dup
This creates a duplicate object that has a different memory address and reference.
If you're looking to write more idiomatic code you'd want to use multiple objects and not rely on class variables at all. If you're not interpolating a variable in a string use single quotes instead of double quotes. Keep object methods simple and focused on specific tasks. These are just a few things Ruby developers consider when writing code, but find what works best for you.
Take something like this for instance:
class Market
attr_accessor :id, :name, :symbol, :price, :price_movement_24h, :market_cap
def initialize(data = {})
#id = data['id'].to_s
#name = data['name'].to_s
#symbol = data['symbol'].to_s
#price = data['current_price'].to_s
#price_movement_24h = data['price_change_percentage_24h'].to_s
#market_cap = data['market_cap'].to_s
end
end
class ImportService
def self.from_api(url)
response = JSON.parse(open(url).read) || []
response.map { |data| Market.new(data) }
end
end
You could then call this as such:
#market_data = ImportService.from_api(BASE_URL + 'markets?vs_currency=usd')
When adding self to the ##market, you should change the code to this
##market << self.dup
However, I don't think it's a good practice to use a class variable here and add self to init an array of object. Instead, you should create a new class (for example MarketImporter)
class Market
attr_accessor :id, :name, :symbol, :price, :price_movement_24h, :market_cap
def initialize(data = {})
#id = data["id"].to_s
#name = data["name"].to_s
#symbol = data["symbol"].to_s
#price = data["current_price"].to_s
#price_movement_24h = data["price_change_percentage_24h"].to_s
#market_cap = data["market_cap"].to_s
end
end
class MarketImporter
attr_accessor :markets
def initialize
data = JSON.parse(open(BASE_URL + "markets?vs_currency=usd").read)
#markets = data.collect { |item| Market.new(item) }
end
end
Then you can init the collection by
MarketImporter.new
I have a class Book with an instance variable title and a method title
class Book
attr_accessor :title
def title(title)
#title = title.capitalize
end
end
I create a book object
b = Book.new
And then I set the title
b.title("inferno")
Now when I try to print the title field variable value
p b.title
Ruby thinks I'm trying to call the method
"title": wrong number of arguments
Any idea how to print the field variable value?
Thanks
All attr_accessor :title does is create wrapper methods, identical to def title() and def title=(value). It's just a syntactic sugar method generation. You can then set the value with:
b.title = "Whatever"
and access with:
b.title
If you just wanted a reader or a writer, you could use attr_reader :title or attr_writer :title separately.
If you want to make your own reader/writer methods manually, all the above do is create:
def title
#title
end
def title=(value)
#title = value
end
You can't have two methods with the same name and different arity in Ruby, so when you define your own method (as in your question), you're overwriting the reader method with your writer. That leaves you with two ways to set and no way to read.
You could use attr_reader and a custom writer:
def title=(value)
#title = value.capitalize
end
You can always name the method anything you want, like def capitalize_and_set_title(value). It might be more clear than the magic of attr_accessor and operator overloading.
I think that what you want in order to set the title is:
def title=(str)
super str.try(:capitalize)
end
If you want to be able to set the value with title(some_value) and read it with title (without arguments), you could do e.g.:
class Book
def title(title = :no_value_given)
if title == :no_value_given
#title
else
#title = title.capitalize
end
end
end
This relies on a default argument value for when you call it without arguments.
The reason I did not use nil as a default argument value is so that you can set the value to nil.
I have a module FDParser that reads a csv file and returns a nice array of hashes that each look like this:
{
:name_of_investment => "Zenith Birla",
:type => "half-yearly interest",
:folio_no => "52357",
:principal_amount => "150000",
:date_of_commencement => "14/05/2010",
:period => "3 years",
:rate_of_interest => "11.25"
}
Now I have an Investment class that accepts the above hash as input and transforms each attribute according to what I need.
class Investment
attr_reader :name_of_investment, :type, :folio_no,
:principal_amount, :date_of_commencement,
:period, :rate_of_interest
def initialize(hash_data)
#name = hash_data[:name_of_investment]
#type = hash_data[:type]
#folio_no = hash_data[:folio_no]
#initial_deposit = hash_data[:principal_amount]
#started_on =hash_data[:date_of_commencement]
#term = hash_data[:period]
#rate_of_interest = hash_data[:rate_of_interest]
end
def type
#-- custom transformation here
end
end
I also have a Porfolio class with which I wish to manage a collection of investment objects. Here is what the Portfolio class looks like:
class Portfolio
include Enumerable
attr_reader :investments
def initialize(investments)
#investments = investments
end
def each &block
#investments.each do |investment|
if block_given?
block.call investment
else
yield investment
end
end
end
end
Now what I want is to loop over the investment_data yielded by the module and dynamically create instances of the investment class and then send those instances as input to the Portfolio class.
So far I tried:
FDParser.investment_data.each_with_index do |data, index|
"inv#{index+1}" = Investment.new(data)
end
But obviously this doesn't work because I get a string instead of an object instance. What is the right way to send a collection of instances to a enumerable collection class that can then manage them?
I'm not sure what "send as input to the Portfolio class" means; classes themselves don't accept "input". But if you're just trying to add Investment objects to the #investments instance variable inside an instance of Portfolio, try this:
portfolio = Portfolio.new([])
FDParser.investment_data.each do |data|
portfolio.investments << Investment.new(data)
end
Note that the array literal [] and the return value of portfolio.investments point to the self-same Array object here. This means you could equivalently do this, which arguably is a little clearer:
investments = []
FDParser.investment_data.each do |data|
investments << Investment.new(data)
end
Portfolio.new(investments)
And if you want to play a little code golf, it shrinks further if you use map.
investments = FDParser.investment_data.map {|data| Investment.new(data) }
Portfolio.new(investments)
I think this is a little harder to read than the previous option, though.
Having some trouble when it comes to initializing variables within a class (instance variables etc.) and I was wondering if anybody could clarify the proper syntax for me.
Sample code:
Class Pets
attr_accessor :name
def initialize(name)
#name=name
end
def name=(name)
#name = name
#I believe this is where I change #name instance variable
end
#in this space I could create more <methods> for Class.new.<method>
end
My question is do I need to have attr_accessor as well as def initialize and def name=?
In addition, if I have multiple attr_accessors do I need to add them as arguments to def initialize, e.g.:
Class Pets
attr_accessor :name :age :color
def initialize(name, age, color)
#name = name
#age = age
#color = color
#and if this is the case do I need methods for each (name= age= color= etc.)
end
One last thing:
If someone could confirm or deny my thought process on the name= age= and color= type of methods within the classes. Am I correct in thinking method= is necessary to change the instance variable? I am a bit unsure about what the method= is for and why I cannot change the instance variable within initialize.
attr_accessor :symbol do the same as attr_writer :symbol and attr_reader :symbol, i.e. it creates both reader (def symbol; #symbol; end) and writer (def symbol=(value); #symbol = value; end).
Initialize is a method called every time new instance of the class is being created. It is not the same as new method as some classes may have its own custom factory methods. You don't need to define your initialize method, only problem is that then symbol reader would return nil, as the local variable would not been set.
In ruby everything is a method. In case of objects, object.attr = value is just a short for object.attr=(value) where attr= is just another method. (Similarly << operator is defined as a method on Array class, attr_accessor is a method defined on class "Class").
To piggy back on what what said earlier, recall that if you want your attributes to be accessible outside your class (you want to write over the attribute value or you want to read it) you will need to use the attr_accessor (or attr_writer or attr_reader).
If I had a class like ...
class Calendar
attr_reader :event_name, :all_events
def initialize
#event_name = event_name
#all_events = []
end
def create_event(event_name)
puts "#{event_name} has been added to your calendar."
#all_events << event_name
p #all_events
end
def see_all_events
puts "Here are your events --"
#all_events.each {|event| puts "- #{event}"}
end
end
my_calendar=Calendar.new
my_calendar.create_event("interview")
my_calendar.see_all_events
my_calendar.all_events
I can read all my events either with the method see_all_events or by calling all_events on my class Calendar object. If for some reason I did not want a see_all_events method but instead only wanted it to be seen by calling all_events on my object I can only do this because of attr_reader.
Basically the point here is to remember exactly how you want your users to interact with your object attributes. If it needs to be private and only accessed via methods then you should be weary of using attr_accessor or attr_writer or attr_reader (depending on the situation).
I see code like:
class Person
def initialize(name)
#name = name
end
end
I understand this allows me to do things like person = Person.new and to use #name elsewhere in my class like other methods. Then, I saw code like:
class Person
attr_accessor :name
end
...
person = Person.new
person.name = "David"
I'm just at a loss with these two methods mesh. What are the particular uses of def initialize(name)? I suppose attr_accessor allows me to read and write. That implies they are two separate methods. Yes? Want clarifications on def initialize and attr_accessor and how they mesh.
initialize and attr_accessor have nothing to do with each other. attr_accessor :name creates a couple of methods:
def name
#name
end
def name=(val)
#name = val
end
If you want to set name upon object creation, you can do it in the initializer:
def initialize(name)
#name = name
# or
# self.name = name
end
But you don't have to do that. You can set name later, after creation.
p = Person.new
p.name = "David"
puts p.name # >> "David"
Here is the answer you are looking for Classes and methods. Read it carefully.
Here is a good documentation from the link:
Classes and methods
Now we are ready to create our very own Address class. Let's start simple. Let's start with an address that only contains the "street" field.
This is how you define a class:
class Address
def initialize(street)
#street = street
end
end
Let's go through this:
The class keyword defines a class.
By defining a method inside this class, we are associating it with this class.
The initialize method is what actually constructs the data structure. Every class must contain an initialize method.
#street is an object variable. Similar to the keys of a hash. The # sign distinguishes #street as an object variable. Every time you create an object of the class Address, this object will contain a #street variable.
Let's use this class to create an address object.
address = Addres.new("23 St George St.")
That's it. address is now an object of the class Address
Reading the data in an object
Suppose that we want to read the data in the address object. To do this, we need to write a method that returns this data:
class Address
def initialize(street)
#street = street
end
# Just return #street
def street
#street
end
end
Now the method Address#street lets you read the street of the address. In irb:
>> address.street
=> "23 St George St."
A property of an object, which is visible outside, is called an attribute. In this case, street is is an attribute. In particular, it is a readable attribute. Because this kind of attribute is very common, Ruby offers you a shortcut through the attr_reader keyword:
class Address
attr_reader :street
def initialize(street)
#street = street
end
end
Changing the data in an object
We can also define a method to change the data in an object.
class Address
attr_reader :street
def initialize(street)
#street = street
end
def street=(street)
#street = street
end
end
Ruby is pretty smart in its use of the street= method:
address.street = "45 Main St."
Notice that you can put spaces betten street and =. Now that we can change the address data, we can simplify the initialize method, and have it simply default the street to the empty string "".
class Address
attr_reader :street
def initialize
#street = ""
end
def street=(street)
#street = street
end
end
address = Address.new
address.street = "23 St George St."
This might not seem like much of a simplification, but when we add the city, state and zip fields, and more methods this will make the class definition a bit simpler.
Now, street is also a writable attribute. As before, you can declare it as such with attr_writer:
class Address
attr_reader :street
attr_writer :street
def initialize
#street = ""
end
end
Accessing data
Very often, you have attributes that are both readable and writable attributes. Ruby lets you lump these together with attr_accessor. I guess these would be called "accessible attributes", but I have never seen them be called that.
class Address
attr_accessor :street
def initialize
#street = ""
end
end
With this knowledge, it is now easy to define the entire addressbook structure. As it turns out, attr_accessor and friends all accept multiple arguments.
class Address
attr_accessor :street, :city, :state, :zip
def initialize
#street = #city = #state = #zip = ""
end
end
I think you consider initialize as a constructor. To be precise, it is not. The default constructor is the new method on the class, and initialize is called by that method. If you do not define initialize, you can still create an object with new because initialize is not the constructor itself. In that case, the default initialize does nothing. If you do define initialize, then that is called right after the object creation.
The statement #foo = ... and attr_accessor :foo are different. The former assigns a value to the instance variable #foo, whereas the latter lets you access #foo via methods foo and foo=. Without the latter, you can still access #foo by directly describing so.
Unlike C++,Java instance variables in Ruby are private by default(partially as they can be accessed by using a.instance_variable_get :#x)
eg:
class Dda
def initialize task
#task = task
#done = false
end
end
item = Dda.new "Jogging" # This would call the initializer and task = Jogging would
be set for item
item.task # would give error as their is no function named task to access the instance
variable.
Although we have set the value to item but we won't be able to do anything with it as instace variables are private in ruby.
code for getter:
def task
#task
end
#for getter
def task=task
#task = task
end
Using getter would ensure that item.task returns it's value
And using setter gives us the flexibility to provide values to instance variables at any time.