From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2011-03-23T19:31:25+09:00
Subject: Re: Nil can't be coerced into Fixnum (TypeError)

On Wed, Mar 23, 2011 at 10:57 AM, Mayank K. <mayank.kohaley@gmail.com> wrote:
> I am trying to execute the following code and it seems like the array is
> going out of bounds. Let me know where am I going wrong.
>
> class PrimeFactor
>  def initialize(number)
>    @number = number
>  end
>
>  def primeFactors
>    factors = Array.new
>    (2..Math.sqrt(@number).ceil).each do |num|
>      if ( @number % num == 0 )
>        factors.insert(factors.length,num)
>      end
>    end
>    (0...factors.length).each do |i|
>      prime = factors[i]
>      factors = factors.select { |x| x == prime || x % prime !=0 }
>    end
>
>    factors.compact.join(',')
>  end
> end
>
> prime = PrimeFactor.new(13195)
> puts "#{prime.primeFactors}"
>
> C:\Temp\Study\Ruby>ruby --version
> ruby 1.8.7 (2011-02-18 patchlevel 334) [i386-mingw32]
>
> C:\Temp\Study\Ruby>ruby prime.rb
> prime.rb:16:in `%': nil can't be coerced into Fixnum (TypeError)
>        from prime.rb:16:in `primeFactors'
>        from prime.rb:16:in `select'
>        from prime.rb:16:in `primeFactors'
>        from prime.rb:14:in `each'
>        from prime.rb:14:in `primeFactors'
>        from prime.rb:24

The problem with your code is that you are modifying the factors array
inside a precalculated iteration. Try adding some print statements in
the last loop you'll see what's going on:

[... snip...]
p factors.length
(0...factors.length).each do |i|
  p factors
  p i
  prime = factors[i]
  factors = factors.select { |x| x == prime || x % prime !=0 }
end

As you'll see, you are modifying the array, but the each loop is still
going from 0 to 7. Another approach could be to set to nil the
multiples of each factor, and then compact:

factors.each do |factor|
  next if factor.nil?
  factors.each_with_index do |candidate,i|
    next if candidate == factor
    factors[i] = nil if candidate % factor == 0
  end
end

BTW, your logic about the Math.sqrt being the top possible factor is
wrong (that's used to know if a number is prime). For example, for 15,
Math.sqrt(15) is less than 4, and 5 is a factor of 15 which your logic
will skip. All in all, this works:

class PrimeFactor
        def initialize(number)
                @number = number
        end

        def primeFactors
                factors = Array.new
                (2..@number).each do |num|
                        if ( @number % num == 0 )
                                factors.insert(factors.length,num)
                        end
                end
                p factors
                factors.each do |factor|
                        next if factor.nil?
                        factors.each_with_index do |candidate,i|
                                next if candidate == factor
                                next if candidate.nil?
                                factors[i] = nil if candidate % factor == 0
                        end
                end
                factors.compact.join(',')
        end
end

#prime = PrimeFactor.new(13195)
prime = PrimeFactor.new(ARGV.shift.to_i)
puts "#{prime.primeFactors}"

Maybe it can be further optimized, but you can start from here. This
is the ouput for 13195:

$ ruby prime_factors.rb 13195
[5, 7, 13, 29, 35, 65, 91, 145, 203, 377, 455, 1015, 1885, 2639, 13195]
5,7,13,29

Jesus.