Finding gaps in a sorted sequence

ruby-talk
1

Hi Folks,

Given a sorted enumeration I need to find the first gap in a sequence.

e.g.
3 == find_gap [1,2,4,5]
nil == find_gap [1,2,3,4]

Here's the best I can come up with

def first_gap( seq )
  seq.each_cons(2) do |l,r|
    _next = l.next
    return _next if r!= _next
  end
  nil
end

but it seems rather ugly. Anyone have a more elegant implementation?

Cheers,
Pete

2

$ cat bar.rb
def find_gap array
  f = array.first
  l = array.last
  s = l - f + 1

  return nil if s == array.size

  if array.size == 2
    return f + 1
  end

  c = array.size / 2
  if array[c] != f + c
    a = find_gap(array[0..c])
  else
    b = find_gap(array[c..-1])
  end

  a ? a : b
end

p find_gap([1,2,4,5])
p find_gap([1,2,3,4])
p find_gap([1,2,3,4,5,6,7,11,12])
p find_gap([2,3,4,5,6,7,11,12])

$ ruby bar.rb
3
nil
8
8

···

On Thu, Nov 6, 2008 at 12:39 PM, Pete Hodgson <phodgson@lyris.com> wrote:

Given a sorted enumeration I need to find the first gap in a sequence.

e.g.
3 == find_gap [1,2,4,5]
nil == find_gap [1,2,3,4]

3

Pete Hodgson wrote:

Hi Folks,

Given a sorted enumeration I need to find the first gap in a sequence.

e.g.
3 == find_gap [1,2,4,5]
nil == find_gap [1,2,3,4]

Here's the best I can come up with

def first_gap( seq )
  seq.each_cons(2) do |l,r|
    _next = l.next
    return _next if r!= _next
  end
  nil
end

but it seems rather ugly. Anyone have a more elegant implementation?

Cheers,
Pete

The following does not quite meet your specs (it returns an array with
all gaps, empty if there are no gaps). Anyway, here is my try:

def find_gaps( ar )
  (ar.first .. ar.last).to_a - ar
end

p find_gaps( [1,2,4,5] )
p find_gaps( ["a", "b", "d", "e", "h"] )

require 'date'
d1 = Date.new(2008,11,6)
d2 = Date.new(2008,11,7)
d3 = Date.new(2008,11,9)
puts find_gaps( [d1,d2,d3] )

=> [3]
=> ["c", "f", "g"]
=> 2008-11-08

hth,

Siep

···

--
Posted via http://www.ruby-forum.com/\.

4

Pete Hodgson <phodgson@lyris.com> writes:

Hi Folks,

Given a sorted enumeration I need to find the first gap in a sequence.

e.g.
3 == find_gap [1,2,4,5]
nil == find_gap [1,2,3,4]

Here's the best I can come up with

def first_gap( seq )
  seq.each_cons(2) do |l,r|
    _next = l.next
    return _next if r!= _next
  end
  nil
end

but it seems rather ugly. Anyone have a more elegant implementation?

I think your version is pretty readable. However, it appears you do
pay a 60% speed penalty with the Enumerator version. So I guess it
depends on whether conciseness and readability are more important than
efficiency, or not.

--- snip ---
require 'enumerator'
require 'benchmark'
include Benchmark

def first_gap list
  list.each_cons(2) do |l,r|
    _next = l.next
    return _next unless r == _next
  end
  nil
end

def other_gap list
  prev = nil
  list.each do |e|
    if prev && (n = prev.next) != e
      return n
    end
    prev = e
  end
  nil
end

t = (1..100).to_a

[100, 250, 500].each do |n|
  t = (1..n).to_a
  bm(5) do |x|
    x.report('first') { 1_000.times { first_gap(t) } }
    x.report('other') { 1_000.times { other_gap(t) } }
  end
end
--- snip ---

~/temp$ ruby first_gap.rb
           user system total real
first 0.700000 0.250000 0.950000 ( 0.957255)
other 0.430000 0.170000 0.600000 ( 0.604145)
           user system total real
first 1.720000 0.630000 2.350000 ( 2.354915)
other 1.070000 0.420000 1.490000 ( 1.490321)
           user system total real
first 3.470000 1.250000 4.720000 ( 4.838116)
other 2.120000 0.840000 2.960000 ( 3.018840)

···

--
Brian Adkins
http://www.lojic.com/

5

irb(main):001:0> g = [1,2,3,5,6,8,9,10]
=> [1, 2, 3, 5, 6, 8, 9, 10]

irb(main):002:0> gap = g.inject {|a, e| e == a.next ? e : (break a.next)}
=> 4

martin

···

On Thu, Nov 6, 2008 at 9:39 AM, Pete Hodgson <phodgson@lyris.com> wrote:

Hi Folks,

Given a sorted enumeration I need to find the first gap in a sequence.

e.g.
3 == find_gap [1,2,4,5]
nil == find_gap [1,2,3,4]

6

unsubscribe

7

PH = Pete Hodgson
SK = Siep Korteling

Given a sorted enumeration I need to find the first gap in a

sequence.

e.g.
3 == find_gap [1,2,4,5]
nil == find_gap [1,2,3,4]

The following does not quite meet your specs (it returns an array

with

all gaps, empty if there are no gaps). Anyway, here is my try:

def find_gaps( ar )
(ar.first .. ar.last).to_a - ar
end

Given the above definition, it's easy to get Pete's behavior exactly:

def find_gap( ar )
    find_gaps(ar).first
end

And then you have a choice of methods to use, depending on whether you
need the full list or just the first gap.

8

Martin DeMello <martindemello@gmail.com> writes:

Hi Folks,

Given a sorted enumeration I need to find the first gap in a sequence.

e.g.
3 == find_gap [1,2,4,5]
nil == find_gap [1,2,3,4]

irb(main):001:0> g = [1,2,3,5,6,8,9,10]
=> [1, 2, 3, 5, 6, 8, 9, 10]

irb(main):002:0> gap = g.inject {|a, e| e == a.next ? e : (break a.next)}
=> 4

Beautiful. Excellent use of inject :slight_smile: I would've thought this would be
better performing also, but it lags the 'each' version by quite a bit:

           user system total real
OP 3.490000 1.240000 4.730000 ( 4.755358)
Brian 2.170000 0.830000 3.000000 ( 3.000508)
Martin 3.320000 1.240000 4.560000 ( 4.582286)

I still think I like it best though because it seems to be the most
natural Ruby solution to the original problem.

···

On Thu, Nov 6, 2008 at 9:39 AM, Pete Hodgson <phodgson@lyris.com> wrote:

--
Brian Adkins
http://www.lojic.com/

9

Martin DeMello <martindemello@gmail.com> writes:

<snip>

I still think I like it best though because it seems to be the most
natural Ruby solution to the original problem.

Well not quite, it does not return nil if there is no gap, one has to
check if the return value is the last element of the array
and replace it with nil in that case.
One should know that inject is quite slow, but no reason not to use it
unless performance is a problem.

Cheers
Robert

···

On Fri, Nov 7, 2008 at 9:28 AM, Brian Adkins <lojicdotcom@gmail.com> wrote:
--
C'est véritablement utile puisque c'est joli.

Antoine de Saint Exupéry

10

Robert Dober <robert.dober@gmail.com> writes:

Martin DeMello <martindemello@gmail.com> writes:

<snip>

I still think I like it best though because it seems to be the most
natural Ruby solution to the original problem.

Well not quite, it does not return nil if there is no gap,

D'oh! I missed that. Ok, I guess I like my version best again then :slight_smile:

···

On Fri, Nov 7, 2008 at 9:28 AM, Brian Adkins <lojicdotcom@gmail.com> wrote:

one has to
check if the return value is the last element of the array
and replace it with nil in that case.
One should know that inject is quite slow, but no reason not to use it
unless performance is a problem.

Cheers
Robert
--
C'est véritablement utile puisque c'est joli.

Antoine de Saint Exupéry

--
Brian Adkins
http://www.lojic.com/