Creating array with two unique entries for every entry from another array in Ruby

Question

I'm trying to take an array like

[ "First.Package", "Second.Package" ]

and turn it into

[ "First.Package.ads", "First.Package.adb", "Second.Package.ads", "Second.Package.adb" ]

I've tried doing the following:

packages = [ "First.Package", "Second.Package" ]
files = []
packages.each do |package|
    files << (package << ".ads")
    files << (package << ".adb")
end
return files

Which does not do what I think it should do. I get:

First.Package.ads.adb
First.Package.ads.adb
Second.Package.ads.adb
Second.Package.ads.adb

I want:

First.Package.ads
First.Package.adb
Second.Package.ads
Second.Package.adb

A different approach arr.flat_map { |str| [str+".ads",str+".adb"] } — Sagar Pandya
– Sagar Pandya, Commented May 31, 2017 at 20:34

Cary Swoveland · Accepted Answer · 2017-05-31 20:49:46Z

5

packages = ["First.Package", "Second.Package"]
suffixes = [".ads", ".adb"]

packages.product(suffixes).map(&:join)
  #=> ["First.Package.ads", "First.Package.adb",
  #    "Second.Package.ads", "Second.Package.adb"]

Note that

packages.product(suffixes)
  #=> [["First.Package", ".ads"], ["First.Package", ".adb"],
  #    ["Second.Package", ".ads"], ["Second.Package", ".adb"]]

See Array#product.

edited May 31, 2017 at 20:49

answered May 31, 2017 at 20:44

Cary Swoveland

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2 Comments

Mark Thomas Over a year ago

This is cool and you get my upvote, but using cartesian product on filenames may just be a little too clever for some teams...

the Tin Man Over a year ago

I'd go with this. It will work if packages or suffixes get new elements without having to write more code.

Bartosz Bonisławski · Accepted Answer · 2017-05-31 20:28:14Z

4

package << ".ads mutates state of package. Use + for string concatenation in places like that, because it doesn't mutate state. It just returns new string(which is what you really want).

packages.each do |package|
    files << (package + ".ads")
    files << (package + ".adb")
end

this should work for your case

answered May 31, 2017 at 20:28

Bartosz Bonisławski

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3 Comments

Cary Swoveland Over a year ago

...and to eliminate files = [] and return files, write packages.each_with_object(files) do |package, files| followed by what you have.

Sagar Pandya Over a year ago

You can also use push here which takes multiple arguments, like files.push(package + ".ads", package + ".adb"). Whether it's better to do so I'm not sure.

the Tin Man Over a year ago

The problem is that additional code has to be written if an additional extension is added. Instead, the code should not care.

hoffm · Accepted Answer · 2017-05-31 20:48:19Z

3

Given

packages = %w(First.Package Second.Package)
extensions = %w(.ads .adb)

You could do it like this:

files = packages.map do |package|
  extensions.map do |extension|
    package + extension
  end
end.flatten

answered May 31, 2017 at 20:48

hoffm

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1 Comment

Sagar Pandya Over a year ago

Consider using flat_map instead of the outer map. This will save you having to flatten at the end

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