cartesian product

Hello,

How would I create a function that will take list (array) of integers and 
return their cartesian product?

Thank You,

Walter Kehowski
nuby 

> How would I create a function that will take list (array) of integers and

You could do something like:

a = [1, 2, 3]

b = []

a.each do |elementone|
 a.each do |elementtwo|
    b << [elementone, elementtwo]
 end
end

That should do the trick (I haven't tested it though)

Giovanni,

Yes, that works. Another nuby question: How to make it a function? Such as 
cartprod(a,b) returning c?

Walter

Remember that in Ruby you seldom have standalone functions. You should
probably create a class that has the cartprod method.

Giovanni,

Here's what I have so far:

def cartprod(a,b)

 c=[]

 a.each do |ae|
  b.each do |be|
    c << [ae, be]
  end
 end

return c

end

a=[1,2,3]

b=[4,5,6]

c=cartprod(a,b)

c.each { |ce| print ce,"\n" }

and this does print all the elements of c. Great. Now how do I do a class? 
Can I create a new method for Array?

Walter Kehowski

Remember that in Ruby you seldom have standalone functions. You should
probably create a class that has the cartprod method.

class Array
  def cartprod(b)
    c=[]
    each do |ae|
      b.each do |be|
        c << [ae, be]
      end
    end
    c
  end
end

a=[1,2,3]
b=[4,5,6]

c = a.cartprod(b)

c.each { |ce| p ce }


daz

if you want to create a new method for array just do

class Array
  def cartprod(ary)
    result = []
    self.each do |ae|
      ary.each do |be|
        result << [ae, be]
      end
    end
    result
  end
end

this way you can have an array and do new_array =
source_array.cartprod(other_array)

Note that it might be more memory efficient to write a iteration method:

def cartprod(a,b)
  a.each {|ae| b.each {|be| yield ae, be}}
end

Then you can also do this if needed

c=[]
cartprod(a,b) {|x,y| c << [x,y]}

If you just need every combination once the iteration approach is more 
efficient.

Kind regards

    robert

Or even better, 

if block_given?
  yield ae, be
else
  (c ||= []) << ae, be
end

martin

I'm sure you know what you're talking about, but I wonder how many others 
will, even with comments.  (Maybe I just need to go back to Pascal?)

Randy Kramer

Haha... if it's this line you have trouble with

   (c ||= []) << ae, be

it's not that hard.

The ||= is a well-known idiom -- same as c = c || []  here. Basically
"assign c this value, unless it is already non-nil."

So then you have either an existing array or an empty one.
Then you can append (<<) onto it.

If it's the yield that bothers you, just study a bit more
Ruby. Pascal was wonderful in 1980 -- unless you already knew
something better -- but I'm content to let it go now. ;)


Hal

Hal,

Thanks--that line was the most confusing. (Yield I at least have a clue about, 
though I would need to think about it in this context a little more.)

regards,

Randy Kramer

Hello,

Here's what I have so far, combining the previous suggestions of Robert 
Klemme and Martin DeMello:

class Array

  def cartprod(a,b)
     a.each {|ae|
     b.each {|be|
      if block_given?
        yield ae,be
      else
        (c ||= []) << ae,be
      end
  end

end #Array

However,

c=cartprod([1,2,3],[4,5,6])

c.each { |ce| print ce,"\n" }

gives the following syntax error:

cartprod.rb:24: syntax error
        (c ||= []) << ae,be
                         ^
cartprod.rb:26: syntax error
cartprod.rb:132: syntax error

and I would like to take the cartesian product of an arbitrary number of 
sets in the most elegant way.

Hello,

The following is not in the spirit of previous comments

class Array

  def cartprod(b)

    c=[]
    each {|ae| b.each {|be| c << [ae, be] } }
    c.collect! {|ce| ce.flatten}

  end #cartprod

end #Array

a=[[1,2,3],[4,5,6],[7,8,9]]

c=a[0]
as=a.slice(1..a.length-1)
as.each {|ase| c=c.cartprod(ase) }
c.each  {|ce| ce.each {|x| print x," " }; print "\n" }

Would the following be desirable? If cartprod has two or more arguments, do 
the usual thing, but if it has just one, it takes the cartesian product with 
that argument, i.e, cartprod(a) gives cartprod(a,a) or a.cartprod(a), etc.

And thanks for all the suggestion so far. I usually just play around with 
Perl but got $#@% fatigue. It was a toss-up between Python and Ruby so here 
I am.

Walter Kehowski

walter a kehowski wrote...

class Array
  def cartprod(b)
    each do |ae|
      b.each do |be|
        if block_given?
          yield ae, be  # <-- you can yield multiple objects
        else
          (c ||= []) << [ae, be]   # <-- you need to put (ae, be) in a 
single
                                   # object (i.e. an array) to put in an 
array
        end
      end   # <-- you had missed 2 closing braces (line 132 error)
    end
    c  # <-- you want to return this if no block was given
  end
end

Cheers,
Dave