Easy question

ruby-talk
1

I want to maintain a list of two-dimensional points and store, at list level, the min and max values for x and y.

The minx/maxx/miny/maxy should be updated as each point is pushed onto the list.

The following works, but it's probably not leveraging Ruby:

def add_point( p )
   @points.push(p)
    if !@minx || @minx > p.x
        @minx=p.x
    end
    if ....

Thanks in advance.

2

There's not a lot of changes you can make to something as simple as a conditional, actually. Even if you improve the aesthetics, there's little practical impact in re-writing simple conditionals.

That said, you can compress each statement into a single line using the if/unless as a modifier (like Perl). I tend to prefer 'unless' in this case, but that's just me.

@minx = p.x unless @minx and p.x > @minx
@minx = p.x if !@minx or @minx > p.x

matthew smillie.

···

On Jun 8, 2006, at 14:20, Robert Mela wrote:

I want to maintain a list of two-dimensional points and store, at list level, the min and max values for x and y.
The minx/maxx/miny/maxy should be updated as each point is pushed onto the list.

The following works, but it's probably not leveraging Ruby:

def add_point( p )
  @points.push(p)
   if !@minx || @minx > p.x
       @minx=p.x
   end
   if ....

3

Robert Mela wrote:

def add_point( p )
  @points.push(p)
   if !@minx || @minx > p.x
       @minx=p.x
   end
   if ....

@points is an array of arrays, right?

   def add_point(p)
     @points.push(p)
     @max_x = @points.inject{|max, cur| cur[0] > max[0] ? cur : max}[0]
     @min_x = @points.inject{|min, cur| cur[0] < min[0] ? cur : max}[0]
     @max_y = @points.inject{|max, cur| cur[1] > max[1] ? cur : max}[1]
     @min_y = @points.inject{|min, cur| cur[1] < min[1] ? cur : max}[1]
   end

Depending on how often you update the array, and how often you want the min/max values, you ought to consider putting each #inject in its own method instead:

   def max_x
     @points.inject{|max, cur| cur[0] > max[0] ? cur : max}[0]
   end

Cheers,
Daniel

4

You could have functions that query the points in the system instead
of keeping track as you add them.

def min_x
  (@points.min{|a,b| a.x <=> b.x}).x
end

def min_y
  (@points.min{|a,b| a.y <=> b.y}).y
end

def max_x
  (@points.max{|a,b| a.x <=> b.x}.x
end

def max_y
  (@points.min{|a,b| a.y <=> b.y}).y
end

5

1)

@points is an array of point objects

class Point
  :attr_reader :x, :y
   def initialize( x, y )
      @x, @y=x,y
  end
end

2)

With inject, aren't you iterating over the entire array every time you add a new point, such that series.add(p) becomes O n^2?
Daniel Schierbeck wrote:

···

Robert Mela wrote:

def add_point( p )
  @points.push(p)
   if !@minx || @minx > p.x
       @minx=p.x
   end
   if ....

@points is an array of arrays, right?

  def add_point(p)
    @points.push(p)
    @max_x = @points.inject{|max, cur| cur[0] > max[0] ? cur : max}[0]
    @min_x = @points.inject{|min, cur| cur[0] < min[0] ? cur : max}[0]
    @max_y = @points.inject{|max, cur| cur[1] > max[1] ? cur : max}[1]
    @min_y = @points.inject{|min, cur| cur[1] < min[1] ? cur : max}[1]
  end

Depending on how often you update the array, and how often you want the min/max values, you ought to consider putting each #inject in its own method instead:

  def max_x
    @points.inject{|max, cur| cur[0] > max[0] ? cur : max}[0]
  end

Cheers,
Daniel

6

But doesn't that result in an iteration over the points array every time we want to get min/max values?

Farrel Lifson wrote:

···

You could have functions that query the points in the system instead
of keeping track as you add them.

def min_x
(@points.min{|a,b| a.x <=> b.x}).x
end

def min_y
(@points.min{|a,b| a.y <=> b.y}).y
end

def max_x
(@points.max{|a,b| a.x <=> b.x}.x
end

def max_y
(@points.min{|a,b| a.y <=> b.y}).y
end

7

Yes, which is slow, but also secure. Is there a way to
remove items from your list? what happens to the minx, etc.
if you remove the smallest one?

suggestion:

def min_x
  @minx ||= (@points.min{|a,b| a.x <=> b.x}).x
end

...

def add_point( p )
   @points.push(p)
   @minx = @maxx = @miny = @maxy = nil
end

def del_point( p )
   @points.delete(p)
   @minx = @maxx = @miny = @maxy = nil
end

cheers

Simon

···

-----Original Message-----
From: Robert Mela [mailto:rmela@rcn.com]
Sent: Thursday, June 08, 2006 4:06 PM
To: ruby-talk ML
Subject: Re: Easy question

But doesn't that result in an iteration over the points array
every time
we want to get min/max values?

8

It does and if that's a concern due to the number of points then it's
probably best to keep track of the min/max then.

9

Robert Mela wrote:

With inject, aren't you iterating over the entire array every time you add a new point, such that series.add(p) becomes O n^2?

Yes, I didn't see that until after i posted :frowning:
I actually did the #(max|min)_(x|y) thing first, and then just copied the code after re-reading your post. My bad.

Robert Mela wrote:
> @points is an array of point objects
>
> class Point
> :attr_reader :x, :y
> def initialize( x, y )
> @x, @y=x,y
> end
> end

   def max_x
     @points.inject{|max, cur| cur.x > max.x ? cur : max}.x
   end

I'd personally use that approach -- but if you're accessing the method often, it's rather inefficient.

Daniel