# How to implement 20 - point and point - 20 using coerce()? **URL:** \ **Category:** ruby-talk\ **Created:** [10 May 2010 08:23 UTC](https://rubytalk.org/t/how-to-implement-20-point-and-point-20-using-coerce/58464 "2010-05-10T08:23:45Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![Jian\_Lin](https://yyz1.discourse-cdn.com/flex029/user_avatar/rubytalk.org/jian_lin/32/1836_2.png) [@Jian\_Lin](https://rubytalk.org/u/Jian_Lin)\ **Post date:** [10 May 2010 08:23 UTC](https://rubytalk.org/t/how-to-implement-20-point-and-point-20-using-coerce/58464/1 "2010-05-10T08:23:45Z")
In Ruby, the operation of     point - 20     20 - point are to be implemented. But the following code:     class Point       attr\_accessor :x, :y       def initialize(x,y)         @x, @y = x, y       end       def -(q)       if (q.is\_a? Fixnum)         return Point.new(@x - q, @y - q)       end         Point.new(@x - q.x, @y - q.y)       end       def -@         Point.new(-@x, -@y)       end       def \*(c)         Point.new(@x \* c, @y \* c)       end       def coerce(something)         [self, something]       end     end     p = Point.new(100,100)     q = Point.new(80,80)     p (-p)     p p - q     p q - p     p p \* 3     p 5 \* p     p p - 30     p 30 - p Output:     #\     #\     #\     #\     #\     #\     #\ "30 - p" will actually be taken as "p - 30" by the coerce function. Can it be made to work? I am actually surprise that the "-" method won't coerce the argument this way:     class Fixnum       def -(something)         if (/\* something is unknown class \*/)           a, b = something.coerce(self)           return -(a - b) # because we are doing a - b but we wanted b - a, so it is negated         end       end     end that is, the function returns a "negated version of a - b" instead of just returning "a - b". > **···** > > -- > Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/). ---
**Author:** ![Colin\_Bartlett1](https://avatars.discourse-cdn.com/v4/letter/c/dbc845/32.png) [@Colin\_Bartlett1](https://rubytalk.org/u/Colin_Bartlett1)\ **Post date:** [10 May 2010 10:39 UTC](https://rubytalk.org/t/how-to-implement-20-point-and-point-20-using-coerce/58464/2 "2010-05-10T10:39:42Z")
My apologies for the brevity and lack of explanation (I'm about to try to catch a train), but this might do the sort of thing that you want? (I think for coerce to work as you'd expect you need the operands(?) in the coerce array to be in the same order as in the calculation. Sorry if that doesn't make any sense! A fuller explanation will follow quite a bit later if you need one.) class Point   attr\_accessor :x, :y   def initialize(x,y); @x, @y = x, y; end   def -(q)     return Point.new(@x - q.x, @y - q.y) if q.kind\_of? Point     Point.new(@x - q, @y - q)   end   def \*(c); Point.new(@x \* c, @y \* c); end   def -@(); Point.new(-@x, -@y); end   def coerce(something); [Point::Coerce.new(something), self]; end end class Point::Coerce   attr\_reader :value   def initialize(v); @value = v; end   def \*(p)     puts "Point::Coerce #{@value} \* #{p.inspect}"     Point.new(@value \* p.x, @value \* p.y)   end   def -(p)     puts "Point::Coerce #{@value} - #{p.inspect}"     Point.new(@value - p.x, @value - p.y)   end end p = Point.new(200,100) q = Point.new(80,70) puts p p, q puts p (-p) puts p p - q p q - p puts p p \* 3 p 5 \* p puts p p - 30 p 30 - p ---
**Author:** ![Rick\_DeNatale1](https://avatars.discourse-cdn.com/v4/letter/r/bbce88/32.png) [@Rick\_DeNatale1](https://rubytalk.org/u/Rick_DeNatale1)\ **Post date:** [10 May 2010 11:49 UTC](https://rubytalk.org/t/how-to-implement-20-point-and-point-20-using-coerce/58464/3 "2010-05-10T11:49:56Z")
While this explains the usage of the coerce method well, I'm not sure that the answer to the original poster's question is mu\* I may be wrong here, it's early and I haven't yet had a full cup of coffee, but while multiplication of a point (vector) and a scalar makes sense, I'm not sure that there is a conventional meaning to subtraction (or addition) of a scalar and a vector. Colin's solution treats a scalar in this case as a unit point multiplied by the scalar. If this is what's wanted it can be done without the separate Point::Coerce class def coerce(something)    [Point.new(something, something), self] end \* [http://en.wikipedia.org/wiki/Mu\_(negative)](http://en.wikipedia.org/wiki/Mu_(negative)) > **···** > > On Mon, May 10, 2010 at 6:39 AM, Colin Bartlett \ wrote: > > > My apologies for the brevity and lack of explanation (I'm about to try > > to catch a train), but this might do the sort of thing that you want? > > (I think for coerce to work as you'd expect you need the operands(?) > > in the coerce array to be in the same order as in the calculation. > > Sorry if that doesn't make any sense! A fuller explanation will follow > > quite a bit later if you need one.) > > > > class Point > > attr\_accessor :x, :y > > def initialize(x,y); @x, @y = x, y; end > > > > def -(q) > > return Point.new(@x - q.x, @y - q.y) if q.kind\_of? Point > > Point.new(@x - q, @y - q) > > end > > > > def \*(c); Point.new(@x \* c, @y \* c); end > > > > def -@(); Point.new(-@x, -@y); end > > > > def coerce(something); [Point::Coerce.new(something), self]; end > > end > > > > class Point::Coerce > > attr\_reader :value > > > > def initialize(v); @value = v; end > > > > def \*(p) > > puts "Point::Coerce #{@value} \* #{p.inspect}" > > Point.new(@value \* p.x, @value \* p.y) > > end > > > > def -(p) > > puts "Point::Coerce #{@value} - #{p.inspect}" > > Point.new(@value - p.x, @value - p.y) > > end > > end > > > > p = Point.new(200,100) > > q = Point.new(80,70) > > puts > > p p, q > > puts > > p (-p) > > puts > > p p - q > > p q - p > > puts > > p p \* 3 > > p 5 \* p > > puts > > p p - 30 > > p 30 - p > > -- > Rick DeNatale > > Blog: [http://talklikeaduck.denhaven2.com/](http://talklikeaduck.denhaven2.com/) > Github: [rubyredrick (Rick DeNatale) · GitHub](http://github.com/rubyredrick) > Twitter: @RickDeNatale > WWR: [http://www.workingwithrails.com/person/9021-rick-denatale](http://www.workingwithrails.com/person/9021-rick-denatale) > LinkedIn: [http://www.linkedin.com/in/rickdenatale](http://www.linkedin.com/in/rickdenatale)