# How to implement 20 - point and point - 20 using coerce()?

**URL:** <https://www.ruby-forum.com/t/how-to-implement-20-point-and-point-20-using-coerce/188322>\
**Category:** Ruby\
**Created:** [May 10, 2010, 10:23am UTC](https://www.ruby-forum.com/t/how-to-implement-20-point-and-point-20-using-coerce/188322 "2010-05-10T10:23:43Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![winterheat](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/7993a0/32.png) [@winterheat](https://www.ruby-forum.com/u/winterheat)\
**Post date:** [May 10, 2010, 10:23am UTC](https://www.ruby-forum.com/t/how-to-implement-20-point-and-point-20-using-coerce/188322/1 "2010-05-10T10:23:43Z")

</div>

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:

```
#<Point:0x2424e54 @x=-100, @y=-100>
#<Point:0x2424dc8 @x=20, @y=20>
#<Point:0x2424d3c @x=-20, @y=-20>
#<Point:0x2424cc4 @x=300, @y=300>
#<Point:0x2424c38 @x=500, @y=500>
#<Point:0x2424bc0 @x=70, @y=70>
#<Point:0x2424b20 @x=70, @y=70>

```

“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”.

---

<div class="post-metadata">

**Author:** ![winterheat](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/7993a0/32.png) [@winterheat](https://www.ruby-forum.com/u/winterheat)\
**Post date:** [May 10, 2010, 12:40pm UTC](https://www.ruby-forum.com/t/how-to-implement-20-point-and-point-20-using-coerce/188322/2 "2010-05-10T12:40:12Z")

</div>

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 \*©; 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 \*§  
puts “Point::Coerce #{@value} \* #{p.inspect}”  
Point.new(@value \* p.x, @value \* p.y)  
end

def -§  
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

---

<div class="post-metadata">

**Author:** ![winterheat](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/7993a0/32.png) [@winterheat](https://www.ruby-forum.com/u/winterheat)\
**Post date:** [May 10, 2010, 1:50pm UTC](https://www.ruby-forum.com/t/how-to-implement-20-point-and-point-20-using-coerce/188322/3 "2010-05-10T13:50:27Z")

</div>

On Mon, May 10, 2010 at 6:39 AM, Colin B.  
[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> end
> 
> puts  
> p (-p)  
> puts  
> p p - q  
> p q - p  
> puts  
> p p \* 3  
> p 5 \* p  
> puts  
> p p - 30  
> p 30 - p

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

- [Mu (negative) - Wikipedia](http://en.wikipedia.org/wiki/Mu_(negative))

–  
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)