From: Colin Bartlett <colinb2r@...>
Date: 2009-10-09T00:21:30+09:00
Subject: Re: On Ranges as conditions

On Wed, Oct 7, 2009 at 1:08 PM, Ken Bloom <kbloom@gmail.com> wrote:
> Pretty cool! Thanks for pointing this syntax out! I don't recall seeing
> it in Pickaxe.
It's definitely in the first (2001) edition as "Ranges as conditions":
_A range used in a boolean expression acts as a flip-flop.
_ It has two states, set and unset, and is initially unset.
_ On each call, the range cycles through the state machine ...
_ The range returns true if it is in the set state at the end of the call,
_ and false otherwise.
_The two-dot form of a range behaves slightly differently than the
three-dot form.
_ When the two-dot form first makes the transition from unset to set,
_it immediately evaluates the end condition and makes the transition
accordingly.
_This means that if expr1 and expr2 both evaluate to true on the same call,
_the two-dot form will finish the call in the unset state.
_However, it still returns true for this call.
.
*But* - as Rick DeNatale and Markus Roberts posted - in these expressions
the ".." and "..." don't create ranges but create "flip-flop" expressions,
so I think it's misleading to call these "ranges",
and I think the explanations in "The Ruby Programming Language" are better
because they make that clear.
>
> One question:
>
> irb(main):013:0> a=%w{foo start bar end baz start foo bar end baz}
> => ["foo", "start", "bar", "end", "baz", "start", "foo", "bar", "end",
> "baz"]
> irb(main):014:0> a.each {|i| puts i + (if i =~ /start/ .. i =~ /end/ then
> " triggered" else "" end) }
> foo
> start triggered
> bar triggered
> end triggered
> baz
> start triggered
> foo triggered
> bar triggered
> end triggered
> baz
> => ["foo", "start", "bar", "end", "baz", "start", "foo", "bar", "end",
> "baz"]
> irb(main):015:0> a.each {|i| puts i + (if i =~ /start/ ... i =~ /end/
> then " triggered" else "" end) }
> foo
> start triggered
> bar triggered
> end triggered
> baz
> start triggered
> foo triggered
> bar triggered
> end triggered
> baz
>
> Shouldn't the last version (using ... instead of ..) logically omit the
> end line from the triggered section?
>
If I understand your question correctly, according to "Programming Ruby"
and "The Ruby Programming Language" the ".." and "..." refer to
*when* the flip-flop expressions are or are not evaluated,
as the quote above from Pickaxe ("Programming Ruby") says.
Does the following (irb) code explain this?

An = [1, 2, 3, 5, 2.0, 3.0, 5.0] #=> [1, 2, 3, 5, 2.0, 3.0, 5.0]
def ffn(b, e, dots = "..")
  aa = []
  ffop = "x==#{b} #{dots} x>=#{e}"
  expr = "An.each { |x| aa << x if #{ffop} }"
  puts "expr #=> " + expr
  eval expr
  aa
end
#=> nil
ffn 2, 3, ".."
# expr #=> An.each { |x| aa << x if x==2 .. x>=3 }
#=> [2, 3, 2.0, 3.0]
ffn 2, 3, "..."
# expr #=> An.each { |x| aa << x if x==2 ... x>=3 }
#=> [2, 3, 2.0, 3.0]
fn 2, 2, ".."
# expr #=> An.each { |x| aa << x if x==2 .. x>=2 }
#=> [2, 2.0]
ffn 2, 2, "..."
# expr #=> An.each { |x| aa << x if x==2 ... x>=2 }
#=> [2, 3, 2.0, 3.0]