I've just seen this expression in a Ruby/Rails app:
def method(a, b = nil, &c)
c ||= ->(v) { v }
I understand the first part, but not the ->() { ... } syntax. What does it mean?
The variable names have been changed for briefness. I tried searching, but the non-alphanumeric characters are obviously a nightmare for SEO.
It is a lambda literal. Put the block variables inside () and the body inside {}.
->(x, y){x + y}
In the example, ->(v){v} takes a single argument v and returns it, in other words, it is an identity function. If a block is passed to method, then that is assigned to c. If not, the identity function is assigned to c as default.
That is a lambda literal, introduced in Ruby 1.9:
irb> l = ->(v) { v }
# => #<Proc:0x007f4acea30410#(irb):1 (lambda)>
irb> l.call(1)
# => 1
It is equivalent to write:
irb> l = lambda { |v| v }
# => #<Proc:0x00000001daf538#(irb):1 (lambda)>
In the example you posted it is used to provide a default block to the method when none is specified, consider this:
def method(a, &c)
c ||= ->(v) { v }
c.call(a)
end
method(1)
# => 1
method(1) { |v| v * 2 }
# => 2
Related
I want to convert all the values in a nested hash to a utf8 compatible string. I initially thought this would be easy and something like deep_apply should be available for me to use, but I am unable to find anything this simple on a quick google and SO search.
I do not want to write (maintain) a method similar to the lines of Change values in a nested hash . Is there a native API implementation or a shorthand available for this or do I have to write my own method?
I ended up implementing my own approach, that is in no way perfect but works well for my use case and should be easy to maintain. Posting it here for reference to anyone who wants to try it out
def deep_apply object, klasses, &blk
if object.is_a? Array
object.map { |obj_ele| deep_apply(obj_ele, klasses, &blk) }
elsif object.is_a? Hash
object.update(object) {|_, value| deep_apply(value, klasses, &blk) }
elsif klasses.any? { |klass| object.is_a? klass }
blk.call(object)
else
object
end
end
usage:
=> pry(main)> deep_apply({a: [1, 2, "sadsad"]}, [String, Integer]) { |v| v.to_s + "asd" }
=> {:a=>["1asd", "2asd", "sadsadasd"]}
Interesting to learn of the deep_merge approach taken in the answer by "The F". Here is another approach which requires adding a few helper methods.
First, the helper methods:
From the top answer here (converting-a-nested-hash-into-a-flat-hash):
def flat_hash(h,f=[],g={})
return g.update({ f=>h }) unless h.is_a? Hash
h.each { |k,r| flat_hash(r,f+[k],g) }
g
end
From a Github repo called ruby-bury (this functionality was proposed to Ruby core, but rejected)
class Hash
def bury *args
if args.count < 2
raise ArgumentError.new("2 or more arguments required")
elsif args.count == 2
self[args[0]] = args[1]
else
arg = args.shift
self[arg] = {} unless self[arg]
self[arg].bury(*args) unless args.empty?
end
self
end
end
And then a method tying it together:
def change_all_values(hash, &blk)
# the next line makes the method "pure functional"
# but can be removed otherwise.
hash = Marshal.load(Marshal.dump(hash))
flat_hash(hash).each { |k,v| hash.bury(*(k + [blk.call(v)])) }
hash
end
A usage example:
irb(main):063:0> a = {a: 1, b: { c: 1 } }
=> {:a=>1, :b=>{:c=>1}}
irb(main):064:0> b = change_all_values(a) { |val| val + 1 }
=> {:a=>2, :b=>{:c=>2}}
irb(main):066:0> a
=> {:a=>1, :b=>{:c=>1}}
There is deep_merge
yourhash.deep_merge(yourhash) {|_,_,v| v.to_s}
Merge the hash with itself, inspect the value and call to_s on it.
This method requires require 'active_support/core_ext/hash' at the top of file if you are not using ruby on rails.
Obviously, you may handle the conversion of v inside the deep_merge as you like to meet your requirements.
In rails console:
2.3.0 :001 > h1 = { a: true, b: { c: [1, 2, 3] } }
=> {:a=>true, :b=>{:c=>[1, 2, 3]}}
2.3.0 :002 > h1.deep_merge(h1) { |_,_,v| v.to_s}
=> {:a=>"true", :b=>{:c=>"[1, 2, 3]"}}
Well, it's quite simple to write it - so why don't write your own and be absolutely sure how does it behave in all situations ;)
def to_utf8(h)
if h.is_a? String
return h.force_encoding('utf-8')
elsif h.is_a? Symbol
return h.to_s.force_encoding('utf-8').to_sym
elsif h.is_a? Numeric
return h
elsif h.is_a? Array
return h.map { |e| to_utf8(e) }.to_s
else
return h.to_s.force_encoding('utf-8')
end
return hash.to_a.map { |e| result.push(to_utf8(e[0], e[1])) }.to_h
end
You may want to check if all behavior and conversions are correct - and change it if necessary.
hash = {
"d" => {
"o" => {
"g" => {
"s" => {}
},
"l" => {
"l" => {}
},
"o" => {
"m" => {}
}
}
},
"b" => {
"o"=>{
"o"=>{
"m"=>{}
}
}
}
}
trie.print(hash)
Within the Trie class there is method called print to print trie:
def print(trie)
trie.each do |k,v|
#res.concat(k)
print(trie[k]) if trie[k].length > 0
unless trie[k].length > 0
#result << #res unless trie[k].length > 0
#res = ""
p #result
end
end
end
The above method prints:
["dogs", "ll", "om", "boom"]
But I want to print:
["dogs" , "doll", "doom" , "boom"]
I think we don't have to pass the prefix.
def compose(trie)
trie.flat_map do |k, v|
v.empty? ? k : compose(v).map{|sv| "#{k}#{sv}"}
end
end
I've renamed the function to compose to avoid clashing with Kernel#print. The reason for that is that I'm calling this function from the inside, where it should be callable without pointing to an object explicitly. The approach you're using doesn't "reuse" traversed prefixes. The most common way to do this is to use recursion and build up that prefix in the arguments.
I've got this recursive function. Recursion is a common approach to processing trees. It accepts a "subtrie" and a prefix it's placed below (defaults to empty string, if none given). Recursion base: if we got an empty subtrie, we return a single-element array of a built up prefix at this point. Higher levels will return arrays of prefixes built from a given "subtrie".
def compose(trie, prefix="")
trie.flat_map do |k, v|
new_prefix = prefix + k
results = compose(v, new_prefix)
results.empty? ? new_prefix : results
end
end
Note flat_map, otherwise (with map) it will output a deeply nested array structured as your trie with leaves replaced with built up prefixes.
UPD: the new version returns an empty array for empty subtrie.
If I run, per the docs
a = [:code]
a.collect { |x| x.to_s } # => ["code"]
However if I run
a = [:code]
a.collect({ |x| x.to_s }) # => SyntaxError
As far as I know, ruby has optional parens. How is my syntax getting screwed up here? This is an even bigger problem for me because I want to chain another function after this one, so I require the parens.
You aren't passing the block as a parameter to the parenthesis.
a.collect { |x| x.to_s }
is the same as
a.collect() {|x| x.to_s }
is the same as
a.collect() do |x|
x.to_s
end
And all of that is fairly close to this as well:
block = -> (x) {x.to_s} # Shortcut 'stabby' syntax for lambda{|x| x.to_s}
a.collect(&block) # Or a.collect &block
I am using Ruby on Rails 3.2.2 and Ruby 1.9.3. I would like to compare two Hashes (A and B) so to return true if a Hash (A) include all keys/values of the other Hash (B).
For example, given I have
params.inspect
# => { "action"=>"...", "controller"=>"...", "key_param1"=>"value_param1", , "key_param2"=>"value_param2", "key_param3"=>"value_param3", ... }
my_hash1.inspect
# => { "key_param1"=>"value_param1", "key_param2"=>"value_param2" }
my_hash2.inspect
# => { "key_param4"=>"value_param4", "key_param1"=>"value_param1" }
my_hash3.inspect
# => {}
Then I am looking for a method (or something like that) in order to make
params.has_same_keys_and_values_as?(my_hash1)
# => true
params.has_same_keys_and_values_as?(my_hash2)
# => false
params.has_same_keys_and_values_as?(my_hash3)
# => true
Assuming that Hash#keys and Hash#values return values in the same order:
params.values_at(*my_hash.keys) == my_hash.values
I think you can use:
a.slice(*b.keys) == b
where a and b are your hashes. note that slice is a rails method and not ruby.
in plain ruby you can write:
a.keep_if{|k, v| b[k]} == b
class Hash
def >=(b)
eq = true
b.each { |k, v| eq &= !(self.include? k) ? false : ( ( ((self[k]&&v).is_a? Hash) && !((v||self[k]).empty?) ) ? self[k]>=v : true)}
return eq
end
end
params = { "action"=>"...", "controller"=>"...", "key_param1"=>"value_param1", "key_param2"=>"value_param2", "key_param3"=>"value_param3" }
my_hash1 = { "key_param1"=>"value_param1", "key_param2"=>"value_param2" }
my_hash2 = { "key_param4"=>"value_param4", "key_param1"=>"value_param1" }
my_hash3 = {}
p params >= my_hash1 #true
p params >= my_hash2 #false
p params >= my_hash3 #true
It'll work with "deep" hashes as well:
b = {1 => {2 => {} }, 4 => {} }
a = {1 => {2 => {3 => {} }}, 4 => {}, 5 => "123" }
p a >= b #true
p b >= a #false
P.S.
Whether one hash includes another hash
EDIT: This is assuming that the values/keys are not in the same order in both hashes.
You could iterate over each key in hash1 and use has_key? on hash2. Keep in mind this is naive solution that could be slow for large datasets.
Checkout has_key? and has_value? here: http://www.ruby-doc.org/core-1.9.3/Hash.html#method-i-has_key-3F
You could loop as follows:
hash1.each_key { |key|
if hash2.has_key?(key)
do whatever
endif
}
better way, there's an active support method for this, hash.diff, wrap it with .empty? to check if they are the same
{:one => 1}.diff({:one => 1}).empty?
=> true
{:one => 1}.diff({:one => 2}).empty?
=> false
http://as.rubyonrails.org/classes/ActiveSupport/CoreExtensions/Hash/Diff.html
Please help me understand WHY the following works.
class Dog
def bark
"woof"
end
end
bark_string = Dog.new.bark
puts bark_string # "woof" - a string at this point
ref_to_bark = -> { bark_string } # the string a moment ago is now the method again
ref_to_bark.call # "woof"
Why does wrapping a reference to a method in a proc/lambda return a reference to the original method? It baffles me.
It doesn't. ref_to_bark just returns bark_string, the bark method is not called.
Lambdas (and blocks, and procs) in Ruby are closures; this means that local variables available in the same scope as the lambda is defined are accessible inside the lambda. For example:
foo = 42
l = lambda{ p foo }
l.call()
#=> 42
The above should not be any more surprising than the fact that this code works:
x = 17
[1,2,3].map do |n|
n+x # Whoa, you can use _x_ here?!
end
#=> [18,19,20]
It's slightly more surprising when you do something like this:
def make_adder( x )
->(y){ x+y }
end
add10 = make_adder(10)
z = add10.call(32) #=> 42
Again, the local variable x (the parameter passed to the method) is "closed over" by the lambda, its value preserved for reference whenever the lambda is invoked.
So in your example the lambda is just "capturing" the bark_string variable and returning its value later on. Your method is never invoked a second time.
Note that a closure captures the variable itself, not just the object referred to by the variable:
x = "hello"
y = x # reference the same _object_
l = ->{ x } # close over the _variable_ itself
x = "world" # change the variable to point to a new object
p y, #=> "hello" (same object as the original)
l[] #=> "world" (new object)
A lambda defined using -> is called lambda literal. Sometimes called stabby lambda. The following will also return the same result.
ref_to_bark = lambda { bark_string }
Or
ref_to_bark = lambda { "woof" }