Ruby syntax confusion [closed] - ruby

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I came across this method definition:
def stock_folder_map
res = {}
folders.each { |ff|
ff.stocks.each { |s|
res[s["id"]] = ff["name"]
}
}
end
Can anybody tell what res = {} and res[s["id"]] = ff["name"] mean?

Answer 1

You are building up a new hash named res. You are going through every stock in every folder, and creating a new element in res where the key is the id of the stock, and the value is the name of the folder.

Answer 2

res = {} is assigning to the variable res a new, empty Hash object.
res[s["id"]] = ff["name"] is assigning to whatever key s["id"] evaluates to, a value, retrieved from another hash, ff.
So if for example [s["id"]] evaluates to :foo and ff["name"] evaluates to "bar", the above is equal to doing:
res[:foo] = "bar"
Now those [] are literals for accessing the hash by providing a key. In this case the keys are:
s["id"] (which is another hash) for the res hash and
"name" for the ff hash.
s is most propably another hash.

Answer 3

res = {} declares a local variable res and assigns an empty hash to it. Hashes are indexed using [] which, in Ruby terminology, means that hash instances respond to the [] message and yield some value corresponding to the key value passed to [].
Meanwhile folders.each yields each element of the folders collection in turn to the loop body, assigning each element in turn to the loop variable ff. Given its usage, ff must respond both to stocks and []. One possible way for ff to do this would be for it be an instance of a class such as ClassOfFF outlined below:
class ClassOfFF
def stocks
# Yield a collection (implementing Enumerable, for example)
end
def [](key)
# Yield the element at "key"
end
end
Each object from the collection returned by stocks must itself respond to []. These objects might also be hashes or instances of another class that explicitly responds to [] like ClassOfFF, for example.
The statement res[s["id"]] = ff["name"] invokes [] on both ff and ss and assigns the value of ff["name"] to the element in res with key s["id"].

Answer 4

res = {} is an Hash object.res[s["id"]] is an Hash of Hash.ff["name"] is another Hash.res[s["id"]] = ff["name"] means you are putting the value of the Hash ff at key "name",to the value (which is a key of res) of the Hash s at key "id".

Answer 5

Some pseudocode that might help.
def stock_folder_map
res = {} # creates a new hash map named 'res'
folders.each { |ff| # for each folder in folders (ff = folder)
ff.stocks.each { |s| # for each stock in ff.stocks (s = stock)
# we see that s is a hash map
res[s["id"]] = ff["name"] # let id = s["id"] i.e. s.get("id") in java
# let name = ff["name"] i.e. ff.get("name") in java
# assign value in res i.e. res.put(id, name) in java
}
}
end
The documentation for Hashes might be useful.