This probably isn't something you should try at home, but for some reason or another I tried to create an array of methods in Ruby.
I started by defining two methods.
irb(main):001:0> def test1
irb(main):002:1> puts "test!"
irb(main):003:1> end
=> nil
irb(main):004:0> def test2
irb(main):005:1> puts "test2!"
irb(main):006:1> end
=> nil
The weird thing happens when you try to put it into an actual array. It seems to run both methods.
irb(main):007:0> array = [test1, test2]
test!
test2!
=> [nil, nil]
And afterwards, the array is empty.
irb(main):008:0> puts array
=> nil
Can someone explain to me why it runs the methods? Other than that the whole excercise is seriously in need of an exorcist?
What you're storing in your array is the result of calling your methods, not the methods themselves.
def test1
puts "foo!"
end
def test2
puts "bar!"
end
You can store references to the actual methods like this:
> arr = [method(:test1), method(:test2)]
# => [#<Method: Object#test1>, #<Method: Object#test2>]
Later, you can call the referenced methods like this:
> arr.each {|m| m.call }
foo!
bar!
#alestanis explained the reason well. If you were trying to store the methods, then you can do what Lars Haugseth says or you could do the folllowing:
test1 = Proc.new { puts "test!" }
test2 = Proc.new { puts "test2!" }
a = [test1, test2]
This may make your code much more readable.
Here is an irb run.
1.9.3p194 :009 > test1 = Proc.new { puts "test!" }
=> #<Proc:0x00000002798a90#(irb):9>
1.9.3p194 :010 > test2 = Proc.new { puts "test2!" }
=> #<Proc:0x00000002792988#(irb):10>
1.9.3p194 :011 > a = [test1, test2]
=> [#<Proc:0x00000002798a90#(irb):9>, #<Proc:0x00000002792988#(irb):10>]
Your array never contains anything else than two nil values. I tricks you by putting the strings when evaluating. But the return value of each function still is nil.
Your code runs the two methods because you're actually calling the methods when you say "test1" and "test2" - parentheses are optional for ruby method calls.
Since both of your methods just contain a "puts", which returns nil, your resulting array is just an array of two nils.
If you had a square method and wanted to create an array with the square values of 2 and 4, you would write
array = [square(2), square(4)]
Here you are doing exactly the same thing, except that your test methods don't return anything and that's why your final array seems empty (actually, it contains [nil, nil]).
Here's my two-pennies worth. Building on the solutions already posted, this is an example of a working example. What might be handy for some here is that it includes method arguments and the use of self (which refers to the instance of the PromotionalRules class when it is instantiated) and the array of symbols, which is neat - I got that from the Ruby docs on the #send method here. Hope this helps someone!
class PromotionalRules
PROMOTIONS = [:lavender_heart_promotion, :ten_percent_discount]
def apply_promotions total, basket
#total = total
if PROMOTIONS.count > 0
PROMOTIONS.each { |promotion| #total = self.send promotion, #total, basket }
end
#total.round(2)
end
def lavender_heart_promotion total, basket
if two_or_more_lavender_hearts? basket
basket.map { |item| total -= 0.75 if item == 001 }
end
total
end
def two_or_more_lavender_hearts? basket
n = 0
basket.each do |item|
n += 1 if item == 001
end
n >= 2
end
def ten_percent_discount total, *arg
if total > 60.00
total = total - total/10
end
total
end
end
Thanks to everyone for their help. I love the open-source nature of coding - threads just get better and better as people iterate over each other's solutions!
Related
There is a common idiom of using substitutions like:
def with clazz, &block
yield clazz
clazz
end
with Hash.new |hash|
hash.merge!{:a => 1}
end
Is there a way to go further and define #with to have a possibility of doing:
with Hash.new |hash|
merge!{:a => 1}
end
or even:
with Hash.new do
merge!{:a => 1}
end
?
UPDATE
Later accidentally I found exactly what I was looking for (solution similar to the accepted one):
http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/19153
UPDATE 2
It was added to sugar-high/dsl in https://github.com/kristianmandrup/sugar-high
UPDATE 3
docille project on Github exploits this idea very nicely.
If you are referring to the way in which Rails does routing then I think you need to do something like this
def with(instance, &block)
instance.instance_eval(&block)
instance
end
with(Hash.new) do
merge!({:a => 1})
merge!({:b => 1})
end
This is how I can see it being done in the Rails source anyway start by looking at the draw method in action_pack/lib/action_dispatch/routing/route_set
Isn't your pseudo-Ruby:
with Hash.new do |hash|
merge!{:a => 1}
end
The same thing as using 1.9's tap? For example:
>> x = Hash[:a, :b].tap { |h| h.merge!({:c => :d}) }
=> {:a=>:b, :c=>:d}
You still have to name the block argument of course.
You can use the ruby builtin tap:
Hash.new.tap do |hash|
hash.merge! a: 1
end
This can even be "abused" for multiple objects:
[one_long_name, another_long_name].tap do |(a,b)|
a.prop = b.prop
end
Of course both don't give you exactly what with would do according to your example: The block won't be evaluated within the instance of the object. But I prefer a lot to use tap with multiple objects, plus tap return self, so it can be chained:
[one_long_name, another_long_name].tap {|(a,b)| a.prop = b.prop }.inspect
Excuse I am newbie in ruby.
My problem is about argument passes by value and reference.
I am coding this method
def show_as_tree(parents)
array = []
iterate_categories(parents, array)
end
def iterate_categories(parents, array)
parents.each do |p|
#return p.description or "-#{p.description} if the node is root or not
p.description = category_name(p)
#add to array
array << p
#call iterate categories with children of parent node and same array
iterate_categories(p.children, array)
end
end
however the array content is only the parent nodes.
I need understand the ruby mechanism for references and how could fix my problem?
I'm pretty sure Ruby just creates a copy of your array. Therefore what you should be doing is having
array << iterate_categories(p.children, new_array)
And return your array at the end of the function.
Just did a quick example: (Updating code based on Wayne Conrad's answer… it is correct)
class Person
attr_accessor :name, :children
end
class Test
def iterate_categories(parents,array)
parents.each do |p|
array << p.name
if !p.children.nil?
iterate_categories(p.children,array)
end
end
end
def iterate_categories_test
p1 = Person.new
p1.name = "Bob"
p2 = Person.new
p2.name = "Joe"
p3 = Person.new
p3.name = "Ann"
p4 = Person.new
p4.name = "John"
p1.children = [p2,p3]
p3.children = [p4]
array = []
iterate_categories([p1],array)
puts array
end
end
Then:
>> a = Test.new
>> a.iterate_categories_test
Bob
Joe
Ann
John
=> nil
Hopefully that helps.
Your show_as_tree method should return the array.
def show_as_tree(parents)
array = []
iterate_categories(parents, array)
array
end
Without array as the last line, the return value of show_as_tree is the return value of iterate_categories, which happens to be parents. That's why it looks like only parents is getting added to array. That's an illusion: It was parents being returned, not array.
Ruby does not make copies of its arguments. It passes references by value. That means it is the same array being acted upon throughout your functions.
I don't quite get the code, the formatting seems all wrong but I'll try to help.
When you pass an argument to ruby, it seems to give the method a copy, not a reference. Fortunately, you don't need to worry about that, as ruby lets you return multiple things in one call.
For example, in irb I ran something like this:
ruby-1.9.2-p180 :005 > def stuff(a, b)
ruby-1.9.2-p180 :006?> c = a + b
ruby-1.9.2-p180 :007?> [c, a]
ruby-1.9.2-p180 :008?> end
=> nil
ruby-1.9.2-p180 :009 > a, b = stuff(1, 2)
=> [3, 1]
ruby-1.9.2-p180 :010 > a
=> 3
ruby-1.9.2-p180 :011 > b
=> 1
That way you can easily return multiple values without problem
I hope I answered your question.
This question already has answers here:
Ruby Style: How to check whether a nested hash element exists
(16 answers)
How to avoid NoMethodError for nil elements when accessing nested hashes? [duplicate]
(4 answers)
Closed 7 years ago.
I'm working a little utility written in ruby that makes extensive use of nested hashes. Currently, I'm checking access to nested hash elements as follows:
structure = { :a => { :b => 'foo' }}
# I want structure[:a][:b]
value = nil
if structure.has_key?(:a) && structure[:a].has_key?(:b) then
value = structure[:a][:b]
end
Is there a better way to do this? I'd like to be able to say:
value = structure[:a][:b]
And get nil if :a is not a key in structure, etc.
Traditionally, you really had to do something like this:
structure[:a] && structure[:a][:b]
However, Ruby 2.3 added a method Hash#dig that makes this way more graceful:
structure.dig :a, :b # nil if it misses anywhere along the way
There is a gem called ruby_dig that will back-patch this for you.
Hash and Array have a method called dig.
value = structure.dig(:a, :b)
It returns nil if the key is missing at any level.
If you are using a version of Ruby older than 2.3, you can install a gem such as ruby_dig or hash_dig_and_collect, or implement this functionality yourself:
module RubyDig
def dig(key, *rest)
if value = (self[key] rescue nil)
if rest.empty?
value
elsif value.respond_to?(:dig)
value.dig(*rest)
end
end
end
end
if RUBY_VERSION < '2.3'
Array.send(:include, RubyDig)
Hash.send(:include, RubyDig)
end
The way I usually do this these days is:
h = Hash.new { |h,k| h[k] = {} }
This will give you a hash that creates a new hash as the entry for a missing key, but returns nil for the second level of key:
h['foo'] -> {}
h['foo']['bar'] -> nil
You can nest this to add multiple layers that can be addressed this way:
h = Hash.new { |h, k| h[k] = Hash.new { |hh, kk| hh[kk] = {} } }
h['bar'] -> {}
h['tar']['zar'] -> {}
h['scar']['far']['mar'] -> nil
You can also chain indefinitely by using the default_proc method:
h = Hash.new { |h, k| h[k] = Hash.new(&h.default_proc) }
h['bar'] -> {}
h['tar']['star']['par'] -> {}
The above code creates a hash whose default proc creates a new Hash with the same default proc. So, a hash created as a default value when a lookup for an unseen key occurs will have the same default behavior.
EDIT: More details
Ruby hashes allow you to control how default values are created when a lookup occurs for a new key. When specified, this behavior is encapsulated as a Proc object and is reachable via the default_proc and default_proc= methods. The default proc can also be specified by passing a block to Hash.new.
Let's break this code down a little. This is not idiomatic ruby, but it's easier to break it out into multiple lines:
1. recursive_hash = Hash.new do |h, k|
2. h[k] = Hash.new(&h.default_proc)
3. end
Line 1 declares a variable recursive_hash to be a new Hash and begins a block to be recursive_hash's default_proc. The block is passed two objects: h, which is the Hash instance the key lookup is being performed on, and k, the key being looked up.
Line 2 sets the default value in the hash to a new Hash instance. The default behavior for this hash is supplied by passing a Proc created from the default_proc of the hash the lookup is occurring in; ie, the default proc the block itself is defining.
Here's an example from an IRB session:
irb(main):011:0> recursive_hash = Hash.new do |h,k|
irb(main):012:1* h[k] = Hash.new(&h.default_proc)
irb(main):013:1> end
=> {}
irb(main):014:0> recursive_hash[:foo]
=> {}
irb(main):015:0> recursive_hash
=> {:foo=>{}}
When the hash at recursive_hash[:foo] was created, its default_proc was supplied by recursive_hash's default_proc. This has two effects:
The default behavior for recursive_hash[:foo] is the same as recursive_hash.
The default behavior for hashes created by recursive_hash[:foo]'s default_proc will be the same as recursive_hash.
So, continuing in IRB, we get the following:
irb(main):016:0> recursive_hash[:foo][:bar]
=> {}
irb(main):017:0> recursive_hash
=> {:foo=>{:bar=>{}}}
irb(main):018:0> recursive_hash[:foo][:bar][:zap]
=> {}
irb(main):019:0> recursive_hash
=> {:foo=>{:bar=>{:zap=>{}}}}
I made rubygem for this. Try vine.
Install:
gem install vine
Usage:
hash.access("a.b.c")
I think one of the most readable solutions is using Hashie:
require 'hashie'
myhash = Hashie::Mash.new({foo: {bar: "blah" }})
myhash.foo.bar
=> "blah"
myhash.foo?
=> true
# use "underscore dot" for multi-level testing
myhash.foo_.bar?
=> true
myhash.foo_.huh_.what?
=> false
value = structure[:a][:b] rescue nil
Solution 1
I suggested this in my question before:
class NilClass; def to_hash; {} end end
Hash#to_hash is already defined, and returns self. Then you can do:
value = structure[:a].to_hash[:b]
The to_hash ensures that you get an empty hash when the previous key search fails.
Solution2
This solution is similar in spirit to mu is too short's answer in that it uses a subclass, but still somewhat different. In case there is no value for a certain key, it does not use a default value, but rather creates a value of empty hash, so that it does not have the problem of confusion in assigment that DigitalRoss's answer has, as was pointed out by mu is too short.
class NilFreeHash < Hash
def [] key; key?(key) ? super(key) : self[key] = NilFreeHash.new end
end
structure = NilFreeHash.new
structure[:a][:b] = 3
p strucrture[:a][:b] # => 3
It departs from the specification given in the question, though. When an undefined key is given, it will return an empty hash instread of nil.
p structure[:c] # => {}
If you build an instance of this NilFreeHash from the beginning and assign the key-values, it will work, but if you want to convert a hash into an instance of this class, that may be a problem.
You could just build a Hash subclass with an extra variadic method for digging all the way down with appropriate checks along the way. Something like this (with a better name of course):
class Thing < Hash
def find(*path)
path.inject(self) { |h, x| return nil if(!h.is_a?(Thing) || h[x].nil?); h[x] }
end
end
Then just use Things instead of hashes:
>> x = Thing.new
=> {}
>> x[:a] = Thing.new
=> {}
>> x[:a][:b] = 'k'
=> "k"
>> x.find(:a)
=> {:b=>"k"}
>> x.find(:a, :b)
=> "k"
>> x.find(:a, :b, :c)
=> nil
>> x.find(:a, :c, :d)
=> nil
This monkey patch function for Hash should be easiest (at least for me). It also doesn't alter structure i.e. changing nil's to {}. It would still also apply even if you're reading a tree from a raw source e.g. JSON. It also doesn't need to produce empty hash objects as it goes or parse a string. rescue nil was actually a good easy solution for me as I'm brave enough for such a low risk but I find it to essentially have a drawback with performance.
class ::Hash
def recurse(*keys)
v = self[keys.shift]
while keys.length > 0
return nil if not v.is_a? Hash
v = v[keys.shift]
end
v
end
end
Example:
> structure = { :a => { :b => 'foo' }}
=> {:a=>{:b=>"foo"}}
> structure.recurse(:a, :b)
=> "foo"
> structure.recurse(:a, :x)
=> nil
What's also good is that you can play around saved arrays with it:
> keys = [:a, :b]
=> [:a, :b]
> structure.recurse(*keys)
=> "foo"
> structure.recurse(*keys, :x1, :x2)
=> nil
The XKeys gem will read and auto-vivify-on-write nested hashes (::Hash) or hashes and arrays (::Auto, based on the key/index type) with a simple, clear, readable, and compact syntax by enhancing #[] and #[]=. The sentinel symbol :[] will push onto the end of an array.
require 'xkeys'
structure = {}.extend XKeys::Hash
structure[:a, :b] # nil
structure[:a, :b, :else => 0] # 0 (contextual default)
structure[:a] # nil, even after above
structure[:a, :b] = 'foo'
structure[:a, :b] # foo
You can use the andand gem, but I'm becoming more and more wary of it:
>> structure = { :a => { :b => 'foo' }} #=> {:a=>{:b=>"foo"}}
>> require 'andand' #=> true
>> structure[:a].andand[:b] #=> "foo"
>> structure[:c].andand[:b] #=> nil
There is the cute but wrong way to do this. Which is to monkey-patch NilClass to add a [] method that returns nil. I say it is the wrong approach because you have no idea what other software may have made a different version, or what behavior change in a future version of Ruby can be broken by this.
A better approach is to create a new object that works a lot like nil but supports this behavior. Make this new object the default return of your hashes. And then it will just work.
Alternately you can create a simple "nested lookup" function that you pass the hash and the keys to, which traverses the hashes in order, breaking out when it can.
I would personally prefer one of the latter two approaches. Though I think it would be cute if the first was integrated into the Ruby language. (But monkey-patching is a bad idea. Don't do that. Particularly not to demonstrate what a cool hacker you are.)
Not that I would do it, but you can Monkeypatch in NilClass#[]:
> structure = { :a => { :b => 'foo' }}
#=> {:a=>{:b=>"foo"}}
> structure[:x][:y]
NoMethodError: undefined method `[]' for nil:NilClass
from (irb):2
from C:/Ruby/bin/irb:12:in `<main>'
> class NilClass; def [](*a); end; end
#=> nil
> structure[:x][:y]
#=> nil
> structure[:a][:y]
#=> nil
> structure[:a][:b]
#=> "foo"
Go with #DigitalRoss's answer. Yes, it's more typing, but that's because it's safer.
In my case, I needed a two-dimensional matrix where each cell is a list of items.
I found this technique which seems to work. It might work for the OP:
$all = Hash.new()
def $all.[](k)
v = fetch(k, nil)
return v if v
h = Hash.new()
def h.[](k2)
v = fetch(k2, nil)
return v if v
list = Array.new()
store(k2, list)
return list
end
store(k, h)
return h
end
$all['g1-a']['g2-a'] << '1'
$all['g1-a']['g2-a'] << '2'
$all['g1-a']['g2-a'] << '3'
$all['g1-a']['g2-b'] << '4'
$all['g1-b']['g2-a'] << '5'
$all['g1-b']['g2-c'] << '6'
$all.keys.each do |group1|
$all[group1].keys.each do |group2|
$all[group1][group2].each do |item|
puts "#{group1} #{group2} #{item}"
end
end
end
The output is:
$ ruby -v && ruby t.rb
ruby 1.9.2p0 (2010-08-18 revision 29036) [x86_64-linux]
g1-a g2-a 1
g1-a g2-a 2
g1-a g2-a 3
g1-a g2-b 4
g1-b g2-a 5
g1-b g2-c 6
I am currently trying out this:
# --------------------------------------------------------------------
# System so that we chain methods together without worrying about nil
# values (a la Objective-c).
# Example:
# params[:foo].try?[:bar]
#
class Object
# Returns self, unless NilClass (see below)
def try?
self
end
end
class NilClass
class MethodMissingSink
include Singleton
def method_missing(meth, *args, &block)
end
end
def try?
MethodMissingSink.instance
end
end
I know the arguments against try, but it is useful when looking into things, like say, params.
I have a few arrays of Ruby objects of class UserInfo:
class UserInfo
attr_accessor :name, :title, :age
end
How can I merge these arrays into one array? A user is identified by its name, so I want no duplicate names. If name, title, age, etc. are equal I'd like to have 1 entry in the new array. If names are the same, but any of the other details differ I probably want those 2 users in a different array to manually fix the errors.
Thanks in advance
Redefine equality comparison on your object, and you can get rid of actual duplicates quickly with Array#uniq
class UserInfo
attr_accessor :name, :title, :age
def == other
name==other.name and title==other.title and age==other.age
end
end
# assuming a and b are arrays of UserInfo objects
c = a | b
# c will only contain one of each UserInfo
Then you can sort by name and look for name-only duplicates
d = c.sort{ |p,q| p.name <=> q.name } #sort by name
name = ""
e = []
d.each do |item|
if item.name == name
e[-1] = [e[-1],item].flatten
else
e << item
end
end
A year ago I monkey patched a kind of cryptic instance_variables_compare on Object. I guess you could use that.
class Object
def instance_variables_compare(o)
Hash[*self.instance_variables.map {|v|
self.instance_variable_get(v)!=o.instance_variable_get(v) ?
[v,o.instance_variable_get(v)] : []}.flatten]
end
end
A cheesy example
require 'Date'
class Cheese
attr_accessor :name, :weight, :expire_date
def initialize(name, weight, expire_date)
#name, #weight, #expire_date = name, weight, expire_date
end
end
stilton=Cheese.new('Stilton', 250, Date.parse("2010-12-02"))
gorgonzola=Cheese.new('Gorgonzola', 250, Date.parse("2010-12-17"))
irb is my weapon of choice
>> stilton.instance_variables_compare(gorgonzola)
=> {"#name"=>"Gorgonzola", "#expire_date"=>#<Date: 4910305/2,0,2299161>}
>> gorgonzola.instance_variables_compare(stilton)
=> {"#name"=>"Stilton", "#expire_date"=>#<Date: 4910275/2,0,2299161>}
>> stilton.expire_date=gorgonzola.expire_date
=> #<Date: 4910305/2,0,2299161>
>> stilton.instance_variables_compare(gorgonzola)
=> {"#name"=>"Gorgonzola"}
>> stilton.instance_variables_compare(stilton)
=> {}
As you can see the instance_variables_compare returns an empty Hash if the two objects has the same content.
An array of cheese
stilton2=Cheese.new('Stilton', 210, Date.parse("2010-12-02"))
gorgonzola2=Cheese.new('Gorgonzola', 250, Date.parse("2010-12-17"))
arr=[]<<stilton<<stilton2<<gorgonzola<<gorgonzola2
One hash without problems and one with
h={}
problems=Hash.new([])
arr.each {|c|
if h.has_key?(c.name)
if problems.has_key?(c.name)
problems[c.name]=problems[c.name]<<c
elsif h[c.name].instance_variables_compare(c) != {}
problems[c.name]=problems[c.name]<<c<<h[c.name]
h.delete(c.name)
end
else
h[c.name]=c
end
}
Now the Hash h contains the objects without merging problems and the problems hash contains those that has instance variables that differs.
>> h
=> {"Gorgonzola"=>#<Cheese:0xb375e8 #name="Gorgonzola", #weight=250, #expire_date=#<Date: 2010-12-17 (4911095/2,0,2299161)>>}
>> problems
=> {"Stilton"=>[#<Cheese:0xf54c30 #name="Stilton", #weight=210, #expire_date=#<Date: 2010-12-02 (4911065/2,0,2299161)>>, #<Cheese:0xfdeca8 #name="Stilton", #weight=250,#expire_date=#<Date: 2010-12-02 (4911065/2,0,2299161)>>]}
As far as I can see you will not have to modify this code at all to support an array of UserInfo objects.
It would most probably be much faster to compare the properties directly or with a override of ==. This is how you override ==
def ==(other)
return self.weight == other.weight && self.expire_date == other.expire_date
end
and the loop changes into this
arr.each {|c|
if h.has_key?(c.name)
if problems.has_key?(c.name)
problems[c.name]=problems[c.name]<<c
elsif h[c.name] != c
problems[c.name]=problems[c.name]<<c<<h[c.name]
h.delete(c.name)
end
else
h[c.name]=c
end
}
Finally you might want to convert the Hash back to an Array
result = h.values
Here's another potential way. If you have a way of identifying each UserInfo, say a to_str method that prints out the values:
def to_str()
return "#{#name}:#{#title}:#{#age}"
end
You can use inject and a hash
all_users = a + b # collection of users to "merge"
res = all_users.inject({})do |h,v|
h[v.to_str] = v #save the value indexed on the string output
h # return h for the next iteration
end
merged = res.values #the unique users
I have a method that returns a hash, or nil:
def person_of_age(age)
some_hash = #array_of_hashes.select { |h| h.age == age }.last
return some_hash
end
I want to use this hash like so:
my_height = 170
my_age = 30
if my_height < self.person_of_age(my_age)['height']
puts "You are shorter than another person I know of the same age!"
end
Now, if the hash returns nil, ruby doesn't like me using ['height']:
undefined method `[]' for nil:NilClass (NoMethodError)
Fair enough, but how can I use ||= to avoid this problem?
If the method does return nil, let's just say I want the 'height' to be 0.
I have tried things along the lines of, to no avail:
if my_height < self.person_of_age(age)||={ 'height' => 0 }['height']
#...
if my_height < self.person_of_age(age)['height'] ||= 0
Clearly my example is running a little thin, and there are other ways around this problem, but if ||= can be used I'd love to know how.
Thank you!
obvious:
(self.person_of_my_age(age) || {'height' => 0})['height']
but somehow this does not feel so right
You can do it like this:
if my_height < self.person_of_age(age).nil? ? 0 : self.person_of_age(age)['height']
Or you can rewrite person_of_age method:
def person_of_age(age)
some_hash = #array_of_hashes.select { |h| h.age == age }.last || {:height => 0}
return some_hash
end
or simpler
def person_of_age(age)
#array_of_hashes.select { |h| h.age == age }.last || {:height => 0}
end
I'm not sure if this helps in your circumstance, but one capability Hash has is
hash.fetch(key) { block_for_code_evaluated_if_key_absent }