Get ruby Hash#each to return a built hash

Question

I'm golfing some code while waiting for a coworker, and I shortened a function to this:

def get_data(dir)
 newest = get_most_recent_file_numbers(dir)
 data = {}
 $file_prefixes.each do |type, file_prefix|
 file = File.open("#{dir}/#{file_prefix}_#{newest[type]}.csv")
 file.readline
 data[type] = file.readlines
 end
 data
end

I feel like there must be some method to do away with that last line of just data. How can I get $file_prefixes.each to return data?

If it matters, $file_prefixes looks like this:

$file_prefixes = {
 :fixed_deal => "PC2MMS_CMSS_DEALS_",
 :fixed_offer => "PC2MMS_CMSS_OFFERS_",
 :mobile_deal => "PC2MMS_NGBSS_DEALS_",
 :mobile_offer => "PC2MMS_NGBSS_OFFERS_"
}

and newest is pretty much identical.

My target code might look something like this:

def get_data(dir)
 newest = get_most_recent_file_numbers(dir)
 data = {} # might even be able to get rid of this with clever use of Hash.new
 data = $file_prefixes.each do |type, file_prefix|
 file = File.open("#{dir}/#{file_prefix}_#{newest[type]}.csv")
 file.readline
 ???
 end
end

Disclaimer: I'm aware I could golf more by using single letter names, but I'm just trying to get it succinct and still readable

Answer 1

Some notes:

• Avoid the pattern init + each + update. Use map or other functional methods.
• In Ruby>= 2.1.0 you have Array#to_h. Use Hash[...] in older Rubies.
• Use File.join

I'd write:

def get_data(dir)
 newest = get_most_recent_file_numbers(dir)
 $file_prefixes.map do |type, file_prefix|
 path = File.join(dir, file_prefix + "_" + newest[type] + ".csv")
 [type, File.readlines(path)]
 end.to_h
end

• Devon Parsons: Hmmm... I need to discard the first line of the file, and I don't see a way to sneak that single readline in here without disrupting your logic. Perhaps I can just set the start for readlines at the first line someway?
• Devon Parsons: Aha! I can just change that last line to [type, File.readlines(path)[1,-1]]. Perfect!

Answer 2

tokland's answer is fine, but I thought I'd chime in and suggest using Enumerable#each_with_object instead of converting your data into an array and back.

def get_data(dir)
 newest = get_most_recent_file_numbers(dir)
 $file_prefixes.each_with_object({}) do |mapping, hash|
 file = File.read("#{dir}/#{mapping.first}_#{newest[mapping.last]}
 end
end

In this example, I'm just using each_with_object to construct a new hash and populate it with values. You can also get rid of the need to call file = File.open ... file.readlines by just using File.read to get the same output.