[Ruby] Nested loops _ String

# Nested Loops 

arr = ["a", "b", "c", "d"]

# below we iterate through all possible w/duplicates

arr.each do |ele1|
  arr.each do |ele2|
    puts ele1 + ele2
  end
end
-----------------------------
#=>
aa
ab
ac
ad
ba
bb
bc
bd
ca
cb
cc
cd
da
db
dc
dd

# below we iterate through only unique pairs

arr.each_with_index do |ele1, idx1|
  arr.each_with_index do |ele2, idx2|
    if idx2 > idx1
      puts ele1 + ele2
      puts idx1.to_s + "  " + idx2.to_s
      puts "------"
    end
  end
end

# if idx2 is greater than idx1, then idx2 is never referring to what idx1 referred to previously.
# Right, basically idx2 is always going to a new element and never trying to touch what idx1 already touched previously.
-----------------------------------------
#=>
ab
0  1
------
ac
0  2
------
ad
0  3
------
bc
1  2
------
bd
1  3
------
cd
2  3
------