Ruby defining operator procedure - ruby

how can a write a class in ruby that has a procedures that i can call like this:
a = MyObj.new()
b = MyObj.new()
c = a * b
d = a / b
e = a - b
this is nicer than:
c = a.multiply(b)
...
thanks

class Foo
attr_accessor :value
def initialize( v )
self.value = v
end
def *(other)
self.class.new(value*other.value)
end
end
a = Foo.new(6)
#=> #<Foo:0x29c9920 #value=6>
b = Foo.new(7)
#=> #<Foo:0x29c9900 #value=7>
c = a*b
#=> #<Foo:0x29c98e0 #value=42>
You can find the list of operators that may be defined as methods here:
http://phrogz.net/ProgrammingRuby/language.html#operatorexpressions

You already got an answer on how to define the binary operators, so just as little addendum here's how you can define the unary - (like for negative numbers).
> class String
.. def -#
.. self.swapcase
.. end
.. end #=> nil
>> -"foo" #=> "FOO"
>> -"FOO" #=> "foo"

Just create methods whose name is the operator you want to overload, for example:
class MyObj
def / rhs
# do something and return the result
end
def * rhs
# do something and return the result
end
end

In Ruby, the * operator (and other such operators) are really just calling a method with the same name as the operator. So to override *, you could do something like this:
class MyObj
def *(obj)
# Do some multiplication stuff
true # Return whatever you want
end
end
You can use a similar technique for other operators, like / or +. (Note that you can't create your own operators in Ruby, though.)

Related

How Ruby Method Modify in Place

How does one write Ruby methods for modification in place?
I want to accomplish the following:
def fulljoin(ruby_array)
r = ''
ruby_array.each {|item| r += "'#{ item }', "}
r.chop!.chop!
end
a = ['Alex', 'Bert', 'Charlie']
a = fulljoin(a) # => 'Alex', 'Bert', 'Charlie'
But I want to modify the array a in place:
a.fulljoin!
What is the syntax to accomplish this?
Initially a is an Array. If you could write method a.fulljoin! with desirable result, a would become a String, but it's not possible in Ruby.
But a.fulljoin! can convert a to Array with single member a[0] - a
String you need. And it will be as close to your goal as possible:
class Array
def fulljoin!
r = "'#{self.join("', '")}'"
self.clear
self[0] = r
end
end
a = ["Alex", "Bert", "Charlie"]
a.fulljoin!
p a
=> ["'Alex', 'Bert', 'Charlie'"]
P.S.: As suggested by #engineersmnky, method fulljoin! can be simplified to:
class Array
def fulljoin!
self.replace(["'#{self.join("', '")}'"])
end
end

Ruby: How to chain methods specified in an array (or split string) of methods?

How is it possible to chain methods in Ruby when the method calls are specified as an array?
Example:
class String
def bipp(); self.to_s + "-bippity"; end
def bopp(); self.to_s + "-boppity"; end
def drop(); self.to_s + "-dropity"; end
end
## this produces the desired output
##
puts 'hello'.bipp.bopp.drop #=> hello-bippity-boppity-dropity
## how do we produce the same desired output here?
##
methods = "bipp|bopp|drop".split("|")
puts 'world'.send( __what_goes_here??__ ) #=> world-bippity-boppity-droppity
[Note to Ruby purists: stylistic liberties were taken with this example. For notes on preferred usage regarding semicolons, parenthesis, comments and symbols, please feel free to consult Ruby style guides (e.g., https://github.com/styleguide/ruby)]
Try this:
methods = "bipp|bopp|drop".split("|")
result = 'world'
methods.each {|meth| result = result.send(meth) }
puts result
or, using inject:
methods = "bipp|bopp|drop".split("|")
result = methods.inject('world') do |result, method|
result.send method
end
or, more briefly:
methods = "bipp|bopp|drop".split("|")
result = methods.inject('world', &:send)
By the way - Ruby doesn't need semicolons ; at the end of each line!
methods = "bipp|bopp|drop".split("|")
result = 'world'
methods.each {|meth| result = result.method(meth).call }
puts result #=> world-bippity-boppity-dropity
or
methods = "bipp|bopp|drop".split("|")
methods.each_with_object('world') {|meth,result| result.replace(result.method(meth).call)} #=> world-bippity-boppity-dropity

Initialize value of self in the standartd class accessor

How I can do something like this:
class Some < String
def m1(a, b)
self = a + b
end
end
s = Some.new("hello")
s.m1("one ", "two")
p s # => "one two"
That depends on how exactly you define "something like".
If you want to make it so that all variables that point to the given Some object, now instead point to the string that is the result of a + b, that's not possible.
If you want to change the string contents of the given Some object, you can use the replace method, i.e. replace(a+b).
To illustrate the difference between using replace and reassignment:
class Some < String
def m1(a, b)
replace( a + b )
end
end
s1 = Some.new("hello")
p s1.object_id # some number
s1.m1("one ", "two")
p s1 # "one two"
p s1.object_id # the same number as above
p s1.class # Some
s2 = Some.new("hello")
p s2.object_id # some number
s2 = "one " + "two"
p s2 # "one two"
p s2.object_id # a different number
p s2.class # String
The latter behavior is not achievable using a method.
Ruby have delegate in standart library for this situations. You can
safely override standart classes. It is recommended using ! in
destructive method names.
require 'delegate'
class MyStr < DelegateClass(String)
def initialize dnm=""
#str = dnm
super(#str)
end
def m1!(a,b)
#str.replace(a + b)
end
end
s = MyStr.new("deneme")
s.m1!("de", "ne")
Something like this?:
class Some < String
def m1(a, b)
self.clear << a << b
end
end
some = Some.new("bye")
some.m1("hello ","world")
p some #=>hello world

How does Ruby's Array.| compare elements for equality?

Here's some example code:
class Obj
attr :c, true
def == that
p '=='
that.c == self.c
end
def <=> that
p '<=>'
that.c <=> self.c
end
def equal? that
p 'equal?'
that.c.equal? self.c
end
def eql? that
p 'eql?'
that.c.eql? self.c
end
end
a = Obj.new
b = Obj.new
a.c = 1
b.c = 1
p [a] | [b]
It prints 2 objects but it should print 1 object. None of the comparison methods get called. How is Array.| comparing for equality?
Array#| is implemented using hashs. So in order for your type to work well with it (as well as with hashmaps and hashsets), you'll have to implement eql? (which you did) and hash (which you did not). The most straight forward way to define hash meaningfully would be to just return c.hash.
Ruby's Array class is implemented in C, and from what I can tell, uses a custom hash table to check for equality when comparing objects in |. If you wanted to modify this behavior, you'd have to write your own version that uses an equality check of your choice.
To see the full implementation of Ruby's Array#|: click here and search for "rb_ary_or(VALUE ary1, VALUE ary2)"
Ruby is calling the hash functions and they are returning different values, because they are still just returning the default object_id. You will need to def hash and return something reflecting your idea of what makes an Obj significant.
>> class Obj2 < Obj
>> def hash; t = super; p ['hash: ', t]; t; end
>> end
=> nil
>> x, y, x.c, y.c = Obj2.new, Obj2.new, 1, 1
=> [#<Obj2:0x100302568 #c=1>, #<Obj2:0x100302540 #c=1>, 1, 1]
>> p [x] | [y]
["hash: ", 2149061300]
["hash: ", 2149061280]
["hash: ", 2149061300]
["hash: ", 2149061280]
[#<Obj2:0x100302568 #c=1>, #<Obj2:0x100302540 #c=1>]

conditional chaining in ruby

Is there a good way to chain methods conditionally in Ruby?
What I want to do functionally is
if a && b && c
my_object.some_method_because_of_a.some_method_because_of_b.some_method_because_of_c
elsif a && b && !c
my_object.some_method_because_of_a.some_method_because_of_b
elsif a && !b && c
my_object.some_method_because_of_a.some_method_because_of_c
etc...
So depending on a number of conditions I want to work out what methods to call in the method chain.
So far my best attempt to do this in a "good way" is to conditionally build the string of methods, and use eval, but surely there is a better, more ruby, way?
You could put your methods into an array and then execute everything in this array
l= []
l << :method_a if a
l << :method_b if b
l << :method_c if c
l.inject(object) { |obj, method| obj.send(method) }
Object#send executes the method with the given name. Enumerable#inject iterates over the array, while giving the block the last returned value and the current array item.
If you want your method to take arguments you could also do it this way
l= []
l << [:method_a, arg_a1, arg_a2] if a
l << [:method_b, arg_b1] if b
l << [:method_c, arg_c1, arg_c2, arg_c3] if c
l.inject(object) { |obj, method_and_args| obj.send(*method_and_args) }
You can use tap:
my_object.tap{|o|o.method_a if a}.tap{|o|o.method_b if b}.tap{|o|o.method_c if c}
Sample class to demonstrate chaining methods that return a copied instance without modifying the caller.
This might be a lib required by your app.
class Foo
attr_accessor :field
def initialize
#field=[]
end
def dup
# Note: objects in #field aren't dup'ed!
super.tap{|e| e.field=e.field.dup }
end
def a
dup.tap{|e| e.field << :a }
end
def b
dup.tap{|e| e.field << :b }
end
def c
dup.tap{|e| e.field << :c }
end
end
monkeypatch: this is what you want to add to your app to enable conditional chaining
class Object
# passes self to block and returns result of block.
# More cumbersome to call than #chain_if, but useful if you want to put
# complex conditions in the block, or call a different method when your cond is false.
def chain_block(&block)
yield self
end
# passes self to block
# bool:
# if false, returns caller without executing block.
# if true, return result of block.
# Useful if your condition is simple, and you want to merely pass along the previous caller in the chain if false.
def chain_if(bool, &block)
bool ? yield(self) : self
end
end
Sample usage
# sample usage: chain_block
>> cond_a, cond_b, cond_c = true, false, true
>> f.chain_block{|e| cond_a ? e.a : e }.chain_block{|e| cond_b ? e.b : e }.chain_block{|e| cond_c ? e.c : e }
=> #<Foo:0x007fe71027ab60 #field=[:a, :c]>
# sample usage: chain_if
>> cond_a, cond_b, cond_c = false, true, false
>> f.chain_if(cond_a, &:a).chain_if(cond_b, &:b).chain_if(cond_c, &:c)
=> #<Foo:0x007fe7106a7e90 #field=[:b]>
# The chain_if call can also allow args
>> obj.chain_if(cond) {|e| e.argified_method(args) }
Although the inject method is perfectly valid, that kind of Enumerable use does confuse people and suffers from the limitation of not being able to pass arbitrary parameters.
A pattern like this may be better for this application:
object = my_object
if (a)
object = object.method_a(:arg_a)
end
if (b)
object = object.method_b
end
if (c)
object = object.method_c('arg_c1', 'arg_c2')
end
I've found this to be useful when using named scopes. For instance:
scope = Person
if (params[:filter_by_age])
scope = scope.in_age_group(params[:filter_by_age])
end
if (params[:country])
scope = scope.in_country(params[:country])
end
# Usually a will_paginate-type call is made here, too
#people = scope.all
Use #yield_self or, since Ruby 2.6, #then!
my_object.
then{ |o| a ? o.some_method_because_of_a : o }.
then{ |o| b ? o.some_method_because_of_b : o }.
then{ |o| c ? o.some_method_because_of_c : o }
Here's a more functional programming way.
Use break in order to get tap() to return the result. (tap is in only in rails as is mentioned in the other answer)
'hey'.tap{ |x| x + " what's" if true }
.tap{ |x| x + "noooooo" if false }
.tap{ |x| x + ' up' if true }
# => "hey"
'hey'.tap{ |x| break x + " what's" if true }
.tap{ |x| break x + "noooooo" if false }
.tap{ |x| break x + ' up' if true }
# => "hey what's up"
Maybe your situation is more complicated than this, but why not:
my_object.method_a if a
my_object.method_b if b
my_object.method_c if c
I use this pattern:
class A
def some_method_because_of_a
...
return self
end
def some_method_because_of_b
...
return self
end
end
a = A.new
a.some_method_because_of_a().some_method_because_of_b()
If you're using Rails, you can use #try. Instead of
foo ? (foo.bar ? foo.bar.baz : nil) : nil
write:
foo.try(:bar).try(:baz)
or, with arguments:
foo.try(:bar, arg: 3).try(:baz)
Not defined in vanilla ruby, but it isn't a lot of code.
What I wouldn't give for CoffeeScript's ?. operator.
I ended up writing the following:
class Object
# A naïve Either implementation.
# Allows for chainable conditions.
# (a -> Bool), Symbol, Symbol, ...Any -> Any
def either(pred, left, right, *args)
cond = case pred
when Symbol
self.send(pred)
when Proc
pred.call
else
pred
end
if cond
self.send right, *args
else
self.send left
end
end
# The up-coming identity method...
def itself
self
end
end
a = []
# => []
a.either(:empty?, :itself, :push, 1)
# => [1]
a.either(:empty?, :itself, :push, 1)
# => [1]
a.either(true, :itself, :push, 2)
# => [1, 2]

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