I'm trying to return a list of values based on user defined arguments, from hashes defined in the local environment.
def my_method *args
#initialize accumulator
accumulator = Hash.new(0)
#define hashes in local environment
foo=Hash["key1"=>["var1","var2"],"key2"=>["var3","var4","var5"]]
bar=Hash["key3"=>["var6"],"key4"=>["var7","var8","var9"],"key5"=>["var10","var11","var12"]]
baz=Hash["key6"=>["var13","var14","var15","var16"]]
#iterate over args and build accumulator
args.each do |x|
if foo.has_key?(x)
accumulator=foo.assoc(x)
elsif bar.has_key?(x)
accumulator=bar.assoc(x)
elsif baz.has_key?(x)
accumulator=baz.assoc(x)
else
puts "invalid input"
end
end
#convert accumulator to list, and return value
return accumulator = accumulator.to_a {|k,v| [k].product(v).flatten}
end
The user is to call the method with arguments that are keywords, and the function to return a list of values associated with each keyword received.
For instance
> my_method(key5,key6,key1)
=> ["var10","var11","var12","var13","var14","var15","var16","var1","var2"]
The output can be in any order. I received the following error when I tried to run the code:
undefined method `assoc' for #<Hash:0x10f591518> (NoMethodError)
Please would you point me how to troubleshoot this? In Terminal assoc performs exactly how I expect it to:
> foo.assoc("key1")
=> ["var1","var2"]
I'm guessing you're coming to Ruby from some other language, as there is a lot of unnecessary cruft in this method. Furthermore, it won't return what you expect for a variety of reasons.
`accumulator = Hash.new(0)`
This is unnecessary, as (1), you're expecting an array to be returned, and (2), you don't need to pre-initialize variables in ruby.
The Hash[...] syntax is unconventional in this context, and is typically used to convert some other enumerable (usually an array) into a hash, as in Hash[1,2,3,4] #=> { 1 => 2, 3 => 4}. When you're defining a hash, you can just use the curly brackets { ... }.
For every iteration of args, you're assigning accumulator to the result of the hash lookup instead of accumulating values (which, based on your example output, is what you need to do). Instead, you should be looking at various array concatenation methods like push, +=, <<, etc.
As it looks like you don't need the keys in the result, assoc is probably overkill. You would be better served with fetch or simple bracket lookup (hash[key]).
Finally, while you can call any method in Ruby with a block, as you've done with to_a, unless the method specifically yields a value to the block, Ruby will ignore it, so [k].product(v).flatten isn't actually doing anything.
I don't mean to be too critical - Ruby's syntax is extremely flexible but also relatively compact compared to other languages, which means it's easy to take it too far and end up with hard to understand and hard to maintain methods.
There is another side effect of how your method is constructed wherein the accumulator will only collect the values from the first hash that has a particular key, even if more than one hash has that key. Since I don't know if that's intentional or not, I'll preserve this functionality.
Here is a version of your method that returns what you expect:
def my_method(*args)
foo = { "key1"=>["var1","var2"],"key2"=>["var3","var4","var5"] }
bar = { "key3"=>["var6"],"key4"=>["var7","var8","var9"],"key5"=>["var10","var11","var12"] }
baz = { "key6"=>["var13","var14","var15","var16"] }
merged = [foo, bar, baz].reverse.inject({}, :merge)
args.inject([]) do |array, key|
array += Array(merged[key])
end
end
In general, I wouldn't define a method with built-in data, but I'm going to leave it in to be closer to your original method. Hash#merge combines two hashes and overwrites any duplicate keys in the original hash with those in the argument hash. The Array() call coerces an array even when the key is not present, so you don't need to explicitly handle that error.
I would encourage you to look up the inject method - it's quite versatile and is useful in many situations. inject uses its own accumulator variable (optionally defined as an argument) which is yielded to the block as the first block parameter.
Related
the following code return this error:
block in find_word_lengths': undefined method `[]=' for 3:Integer (NoMethodError)
animals = ['cat', 'horse', 'rabbit', 'deer']
def find_word_lengths(word_list)
word_list.reduce(Hash.new()) do |result, animal|
result[animal] = animal.length
end
end
puts find_word_lengths(animals)
The return value of the block is the accumulator value for the next iteration. That is how a fold works.
Assignments in Ruby evaluate to the right-hand side. So, in the first iteration of reduce, the block evaluates to 3 (the length of 'cat'). Which means that in the second iteration of reduce, result is 3, and you are essentially running
3['horse'] = 5
# which is equivalent to
3.[]=('horse', 5)
Which is why you are getting the error message that the Integer 3 does not respond to the message []=.
So, you need to make sure that your block always returns the value that you want to use for the accumulator in the next iteration. Something like this:
word_list.reduce(Hash.new()) do |result, animal|
result.tap {|result| result[animal] = animal.length }
end
This would be the obvious solution, although somewhat cheating.
word_list.reduce(Hash.new()) do |result, animal|
result.merge(animal => animal.length)
end
Would be more idiomatic.
However, when you want to fold into a mutable object, it makes more sense to use Enumerable#each_with_object instead of Enumerable#reduce. each_with_object ignores the result of the block, and simply passes the same object every time. Note that somewhat confusingly, the order of the block parameters is swapped in each_with_object compared to reduce.
word_list.each_with_object(Hash.new()) do |animal, result|
result[animal] = animal.length
end
But I guess the most idiomatic solution would be something like this:
word_list.map {|word| [word, word.length] }.to_h
By the way, in Ruby, it is idiomatic to leave out the parentheses for the argument list if you are not passing any arguments, so Hash.new() should be Hash.new instead. Even more important than being idiomatic is to be consistent – confusingly, you leave out the parentheses for animal.length, but not for Hash.new
Even more idiomatically, you would use the Hash literal notation instead of the Hash::new method, i.e. you should use {} instead of Hash.new.
Consider the following:
(1..10).inject{|memo, n| memo + n}
Question:
How does n know that it is supposed to store all the values from 1..10? I'm confused how Ruby is able to understand that n can automatically be associated with (1..10) right away, and memo is just memo.
I know Ruby code blocks aren't the same as the C or Java code blocks--Ruby code blocks work a bit differently. I'm confused as to how variables that are in between the upright pipes '|' will automatically be assigned to parts of an object. For example:
hash1 = {"a" => 111, "b" => 222}
hash2 = {"b" => 333, "c" => 444}
hash1.merge(hash2) {|key, old, new| old}
How do '|key, old, new|' automatically assign themselves in such a way such that when I type 'old' in the code block, it is automatically aware that 'old' refers to the older hash value? I never assigned 'old' to anything, just declared it. Can someone explain how this works?
The parameters for the block are determined by the method definition. The definition for reduce/inject is overloaded (docs) and defined in C, but if you wanted to define it, you could do it like so (note, this doesn't cover all the overloaded cases for the actual reduce definition):
module Enumerable
def my_reduce(memo=nil, &blk)
# if a starting memo is not given, it defaults to the first element
# in the list and that element is skipped for iteration
elements = memo ? self : self[1..-1]
memo ||= self[0]
elements.each { |element| memo = blk.call(memo, element) }
memo
end
end
This method definition determines what values to use for memo and element and calls the blk variable (a block passed to the method) with them in a specific order.
Note, however, that blocks are not like regular methods, because they don't check the number of arguments. For example: (note, this example shows the usage of yield which is another way to pass a block parameter)
def foo
yield 1
end
# The b and c variables here will be nil
foo { |a, b, c| [a,b,c].compact.sum } # => 1
You can also use deconstruction to define variables at the time you run the block, for example if you wanted to reduce over a hash you could do something like this:
# this just copies the hash
{a: 1}.reduce({}) { |memo, (key, val)| memo[key] = val; memo }
How this works is, calling reduce on a hash implicitly calls to_a, which converts it to a list of tuples (e.g. {a: 1}.to_a = [[:a, 1]]). reduce passes each tuple as the second argument to the block. In the place where the block is called, the tuple is deconstructed into separate key and value variables.
A code block is just a function with no name. Like any other function, it can be called multiple times with different arguments. If you have a method
def add(a, b)
a + b
end
How does add know that sometimes a is 5 and sometimes a is 7?
Enumerable#inject simply calls the function once for each element, passing the element as an argument.
It looks a bit like this:
module Enumerable
def inject(memo)
each do |el|
memo = yield memo, el
end
memo
end
end
And memo is just memo
what do you mean, "just memo"? memo and n take whatever values inject passes. And it is implemented to pass accumulator/memo as first argument and current collection element as second argument.
How do '|key, old, new|' automatically assign themselves
They don't "assign themselves". merge assigns them. Or rather, passes those values (key, old value, new value) in that order as block parameters.
If you instead write
hash1.merge(hash2) {|foo, bar, baz| bar}
It'll still work exactly as before. Parameter names mean nothing [here]. It's actual values that matter.
Just to simplify some of the other good answers here:
If you are struggling understanding blocks, an easy way to think of them is as a primitive and temporary method that you are creating and executing in place, and the values between the pipe characters |memo| is simply the argument signature.
There is no special special concept behind the arguments, they are simply there for the method you are invoking to pass a variable to, like calling any other method with an argument. Similar to a method, the arguments are "local" variables within the scope of the block (there are some nuances to this depending on the syntax you use to call the block, but I digress, that is another matter).
The method you pass the block to simply invokes this "temporary method" and passes the arguments to it that it is designed to do. Just like calling a method normally, with some slight differences, such as there are no "required" arguments. If you do not define any arguments to receive, it will happily just not pass them instead of raising an ArgumentError. Likewise, if you define too many arguments for the block to receive, they will simply be nil within the block, no errors for not being defined.
I have the following code spread out across a bunch of methods:
json_element is passed as an argument to the method.
issues:
The values in the hash change, meaning one could have key but the next time could have search
sometimes the value is nil so it blows up.
The gem I used which creates it has those ['$'] for elements if there's a value, but it errors out if you do json_element['COLLECTION']['$].nil?
json = json_element['JSON']['$'] unless json_element['JSON'].nil?
predicate = json_element['PREDICATE']['$'] unless json_element['PREDICATE'].nil?
key = json_element['KEY']['$'] unless json_element['KEY'].nil?
options = json_element['OPTIONS']['$'] unless json_element['OPTIONS'].nil?
cache_key = json_element['CACHE-KEY']['$'] unless json_element['CACHE-KEY'].nil?
question: how can I extract this whole bit as a method which allows for flexible keys and doesn't error out when a value is nil
I'm not sure I understand this right. If the values in the hash are nil, it shouldn't error out. The variables would just be assigned nil. #nil? also shouldn't error out.
Just refactoring your code into a method:
def process(json_element)
return if json_element.nil?
hash = {} # store the variables in a hash
%w{json predicate key options cache-key}.each do |i|
hash[i.upcase] = json_element[i.upcase]['$'] unless json_element[i.upcase].nil?
end
end
I have a method that accepts either a single object or a collection of objects. What is the proper way of detecting if what's passed in is Enumerable? I'm currently doing the following (which works but I'm not sure it's the correct way):
def foo(bar)
if bar.respond_to? :map
# loop over it
else
# single object
end
end
I would use is_a?.
bar.is_a? Enumerable
But there’s a better way to take a single object or a collection, assuming that the caller knows which one they’re passing in. Use a splat:
def foo(*args)
args.each do |arg|
…
end
end
Then you can call it as foo(single_arg), foo(arg1, arg2), and foo(*argv).
I depends on your exact needs, but it's usually not a great idea to differentiate between a single object and an Enumerable. In particular, a Hash is an Enumerable, but in most cases it should be considered as a single object.
It's usually better to distinguish between a single object and an array-like argument. That's what Ruby often does. The best way to do this is to check if arg.respond_to? :to_ary. If it does, then all methods of Array should be available to you, if not treat it as a single object.
If you really want to check for Enumerable, you could test arg.is_a? Enumerable but consider that a Hash is an Enumerable and so are Lazy enumerators (and calling map on them won't even give you an array!)
If your purpose is to loop over it, then the standard way is to ensure it is an array. You can do it like this without condition.
def foo(bar)
[*bar] # Loop over it. It is ensured to be an array.
end
What about handling single items or a collection in one shot?
[*bar].each { |item| puts item }
This will work whether bar is a single item or an array or hash or whatever. This probably isn't the best for working with hashes, but with arrays it works pretty well.
Another way to ensure that something is an Array is with the Array "function (technically still a method):
def foo(bar)
Array(bar).map { |o| … }
end
Array will leave an array an array, and convert single elements to an array:
Array(["foo"]) # => ["foo"]
Array("foo") # => ["foo"]
Array(nil) # => []
This is for an already existing public API that I cannot break, but I do wish to extend.
Currently the method takes a string or a symbol or anything else that makes sense when passed as the first parameter to send
I'd like to add the ability to send a list of strings, symbols, et cetera. I could just use is_a? Array, but there are other ways of sending lists, and that's not very ruby-ish.
I'll be calling map on the list, so the first inclination is to use respond_to? :map. But a string also responds to :map, so that won't work.
How about treating them all as Arrays? The behavior you want for Strings is the same as for an Array containing only that String:
def foo(obj, arg)
[*arg].each { |method| obj.send(method) }
end
The [*arg] trick works because the splat operator (*) turns a single element into itself or an Array into an inline list of its elements.
Later
This is basically just a syntactically sweetened version or Arnaud's answer, though there are subtle differences if you pass an Array containing other Arrays.
Later still
There's an additional difference having to do with foo's return value. If you call foo(bar, :baz), you might be surprised to get [baz] back. To solve this, you can add a Kestrel:
def foo(obj, arg)
returning(arg) do |args|
[*args].each { |method| obj.send(method) }
end
end
which will always return arg as passed. Or you could do returning(obj) so you could chain calls to foo. It's up to you what sort of return-value behavior you want.
A critical detail that was overlooked in all of the answers: strings do not respond to :map, so the simplest answer is in the original question: just use respond_to? :map.
Since Array and String are both Enumerables, there's not an elegant way to say "a thing that's an Enumberable, but not a String," at least not in the way being discussed.
What I would do is duck-type for Enumerable (responds_to? :[]) and then use a case statement, like so:
def foo(obj, arg)
if arg.respond_to?(:[])
case arg
when String then obj.send(arg)
else arg.each { |method_name| obj.send(method_name) }
end
end
end
or even cleaner:
def foo(obj, arg)
case arg
when String then obj.send(arg)
when Enumerable then arg.each { |method| obj.send(method) }
else nil
end
end
Perhaps the question wasn't clear enough, but a night's sleep showed me two clean ways to answer this question.
1: to_sym is available on String and Symbol and should be available on anything that quacks like a string.
if arg.respond_to? :to_sym
obj.send(arg, ...)
else
# do array stuff
end
2: send throws TypeError when passed an array.
begin
obj.send(arg, ...)
rescue TypeError
# do array stuff
end
I particularly like #2. I severely doubt any of the users of the old API are expecting TypeError to be raised by this method...
Let's say your function is named func
I would make an array from the parameters with
def func(param)
a = Array.new
a << param
a.flatten!
func_array(a)
end
You end up with implementing your function func_array for arrays only
with func("hello world") you'll get a.flatten! => [ "hello world" ]
with func(["hello", "world"] ) you'll get a.flatten! => [ "hello", "world" ]
Can you just switch behavior based on parameter.class.name? It's ugly, but if I understand correctly, you have a single method that you'll be passing multiple types to - you'll have to differentiate somehow.
Alternatively, just add a method that handles an array type parameter. It's slightly different behavior so an extra method might make sense.
Use Marshal to serialize your objects before sending these.
If you don't want to monkeypatch, just massage the list to an appropriate string before the send. If you don't mind monkeypatching or inheriting, but want to keep the same method signature:
class ToBePatched
alias_method :__old_takes_a_string, :takes_a_string
#since the old method wanted only a string, check for a string and call the old method
# otherwise do your business with the map on things that respond to a map.
def takes_a_string( string_or_mappable )
return __old_takes_a_string( string_or_mappable ) if String === string_or_mappable
raise ArgumentError unless string_or_mappable.responds_to?( :map )
# do whatever you wish to do
end
end
Between those 3 types I'd do this
is_array = var.respond_to?(:to_h)
is_string = var.respond_to?(:each_char)
is_symbol = var.respond_to?(:to_proc)
Should give a unique answer for [], :sym, 'str'