ruby - how to generate the possible sequential combination of letters from an array of strings?

Question

I have an array of strings:

["ABC", "GHI"]

ad I want all the 'letter' combinations, as reads from left to right, i.e.

["AG", "AH", "AI", "BG", "BH", "BI", "CG", "CH", "CI"]

but not

"GA", "GB", "HA", etc.

Similarly,

["ABC", "DEF", "GHI"]

should generate

["ADG", "ADH", "ADI", "AEG", "AEH", "AEI", "AFG", "AFH", "AFI", "BDG", "BDH", 
"BDI", "BEG", "BEH", "BEI", "BFG", "BFH", "BFI", "CDG", "CDH", "CDI", "CEG", 
"CEH", "CEI", "CFG", "CFH" "CFI"]

but not

"DAG", "GAD" or "GFA"

Answer 1

This is what you need:

a = ["ABC","DEF", "GHI"]
a.map(&:chars).reduce(&:product).map(&:join)

By the way, you made some errors in your example output, there should never be strings starting with an E or an F according to your specification. So I suppose instead of "ECH", "ECI", "FCG", "FCH", "FCI" you meant "CEH", "CEI", "CFG", "CFH", "CFI".

Edit:

chars returns an Enumerator, not an array, and in Ruby versions before 2.0 those don't have a product method. So in those versions, just use a to_a like this:

a.map(&:chars).map(&:to_a).reduce(&:product).map(&:join)

Answer 2

a = ["ABC","DEF", "GHI"]
first, *rest = a.map{|s| s.each_char.to_a}
first.product(*rest).map(&:join)
# =>
# ["ADG", "ADH", "ADI", "AEG", "AEH", "AEI", "AFG", "AFH", "AFI", "BDG", "BDH", "BDI",
# "BEG", "BEH", "BEI", "BFG", "BFH", "BFI", "CDG", "CDH", "CDI", "CEG", "CEH", "CEI",
# "CFG", "CFH", "CFI"]

Benchmarks comparing with @ggPeti’s solution:

t = Time.now
50000.times do
  a.map(&:chars).reduce(&:product).map(&:join)
end
puts Time.now - t
# => 2.037303374

t = Time.now
50000.times do
  first, *rest = a.map{|s| s.each_char.to_a}
  first.product(*rest).map(&:join)
end
puts Time.now - t
# => 1.670047516

Answer 3

A recursive solution:

def doit(b)
  recurse(b.map(&:chars))
end

def recurse(a)
  (a.size==2 ? a.first.product(a.last) :a.shift.product(recurse a)).map(&:join)
end

doit(["ABC", "DEF", "GHI"])  
  #=> ["ADG", "ADH", "ADI", "AEG", "AEH", "AEI", "AFG", "AFH", "AFI",
  #    "BDG", "BDH", "BDI", "BEG", "BEH", "BEI", "BFG", "BFH", "BFI",
  #    "CDG", "CDH", "CDI", "CEG", "CEH", "CEI", "CFG", "CFH", "CFI"] 

doit(["ABC", "DE", "FGHI"])
  #=> ["ADF", "ADG", "ADH", "ADI", "AEF", "AEG", "AEH", "AEI",
  #    "BDF", "BDG", "BDH", "BDI", "BEF", "BEG", "BEH", "BEI",
  #    "CDF", "CDG", "CDH", "CDI", "CEF", "CEG", "CEH", "CEI"]