Is there a functional programming concept equivalent to the flip-flop operator in Perl or Ruby?

Question

Ruby (and Perl) has a concept of the flip flop:

file = File.open("ordinal")
while file.gets
  print if ($_ =~ /third/) .. ($_ =~ /fifth/)
end

which given a list of ordinals, such as

first
second
third
fourth
fifth
sixth

would start printing when it reached "third" and stop when it reached "fifth":

third
fourth
fifth

Is there a functional programming concept analogous to this, or would this normally be described in terms of takewhiles? I'm not asking about a particular language, just what term you'd use to describe it.

Which languages are you considering for FP? There are many different approaches, and some may meet your needs. Also, are you, in this example, just typing the third - fifth lines? You may want to explain in more detail what you are hoping happens. — James Black, Jun 24 '11 at 00:25

score 8 · Accepted Answer · answered Jun 24 '11 at 04:46

In a functional language such as haskell, you would pass in the flip and flop conditions as predicates, and filter an input list based on it. For example, the following is a definition of flipflop in haskell (don't worry about the implementation if you don't know haskell - the key part is how it is used):

flipflop flip flop = 
  uncurry (++) . second (take 1) . break flop . dropWhile (not . flip)

This is how it can be used:

> flipflop (== 3) (== 5) [1..10]
[3,4,5]

It is an example of making an effectively new language construct just by using higher ordered function.

I don't know if there is a special name for that construct in functional languages.

Victor Moroz · Answer 2 · 2011-06-24T13:36:18.367

Depends on functional language. How about this?

ff_gen =
  lambda{ |first, *conditions| 
    flipflop = false
    condition = first
    lambda{ |v| 
      if condition && condition[v]
        condition = conditions.shift
        flipflop = !flipflop
        true
      else
        flipflop
      end
    }
  }

ff = ff_gen[lambda{|v| v == 3}, lambda{|v| v == 5}, lambda{|v| v == 7}, lambda{|v| v == 11}]

puts (0..20).select{ |i| ff[i] }.inspect # => [3, 4, 5, 7, 8, 9, 10, 11]

Added: Of course, Ruby is not a pure functional language, so I decided to rewrite it in Erlang:

#!/usr/bin/env escript

flipflop(E, {[H|T] = Conditions, FlipFlop}) ->
  case H(E) of
    true ->
      {true, {T, not FlipFlop}};
    false ->
      {FlipFlop, {Conditions, FlipFlop}}
  end;

flipflop(_, {[], FlipFlop}) ->
  {FlipFlop, {[], FlipFlop}}.

flipflop_init(Conditions) ->
  {[], {Conditions, false}}.

main([]) ->
  {L, _} = 
    lists:foldl(
      fun(E, {L2, FFState}) -> 
        case flipflop(E, FFState) of
          {true, FFState2} ->
            {[E|L2], FFState2};
          {false, FFState2} ->
            {L2, FFState2}
        end
      end,
      flipflop_init([
        fun(E) -> E == 3 end, 
        fun(E) -> E == 5 end, 
        fun(E) -> E == 7 end, 
        fun(E) -> E == 11 end
      ]), 
      lists:seq(0,20)
    ),
  io:format("~p~n", [lists:reverse(L)]),
  ok.

Note: In fact, classic flip-flop should work like dropwhile(!first) -> takewhile(!second), so Ruby's flip-flop is ad hoc one (compare with flip-flop in electronics).

Do you know if there's a term describing the concept? – Andrew Grimm Jun 24 '11 at 03:22 — Andrew Grimm, Jun 24 '11 at 03:22

score 0 · Answer 3 · answered Jun 24 '11 at 07:04

Same as @nanothief's solution, but in Scala:

def flipFlop[A](flip: A => Boolean, flop: A => Boolean, seq: Seq[A]): Seq[A] = {
  val (p, q) = seq.dropWhile(!flip(_)).span(!flop(_))
  p ++ q.take(1)
}

Sample runs:

> flipFlop[Int](_ == 3, _ == 5, Nil)
List()

> flipFlop[Int](_ == 3, _ == 5, 1 to 19)
Vector(3, 4, 5)

Is there a functional programming concept equivalent to the flip-flop operator in Perl or Ruby?

3 Answers3