ruby,algorithm,search,recursion , Recursive solution doesn't iterate correctly


Recursive solution doesn't iterate correctly

Question:

Tag: ruby,algorithm,search,recursion

I'm working through a toy problem in Ruby: how to produce all possible 10-digit phone numbers where each successive number is adjacent to the last on the keypad. I've represented the adjacent relationships between numbers, and have a recursive function, but my method isn't iterating through the whole solution space. It's just finding the first solution and returning.

Here's my code:

adjacencies = { 1 => [2, 4],
                2 => [1, 3, 5],
                3 => [2, 6],
                4 => [1, 5, 7],
                5 => [2, 4, 6, 8],
                6 => [3, 5, 9],
                7 => [4, 8],
                8 => [5, 7, 9, 0],
                9 => [6, 8],
                0 => [8]
              }

def append_number(partial_phone_number, to_append, adjacencies)
  phone_length = 10
  partial_phone_number = partial_phone_number + to_append.to_s
  if (partial_phone_number.length == phone_length)
    return partial_phone_number
  else
    adjacencies[to_append].each do |a|
      return append_number(partial_phone_number, a, adjacencies)
    end
  end
end

(0..9).each do |n|
  puts append_number("", n, adjacencies)
end

And here is the output when I run it:

0852121212
1212121212
2121212121
3212121212
4121212121
5212121212
6321212121
7412121212
8521212121
9632121212

Answer:

The first time you enter the adjacencies[to_append].each, you immediately return from the method, therefore the loop will never be executed more than once.

You need to

  1. return a list of phone numbers instead of just a single phone number
  2. build that list somehow in your recursive call

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