hey all,
so while Jesus's implementation works, unless you want to implement a
prime number generator yourself you're probably better off using Rubys
built-in Prime class (doc here: http://rdoc.info/stdlib/prime/1.9.2/frames\)
to do this. not only does it give you cleaner code, it is SIGNIFICANTLY
faster.
# the spiffified code
require 'prime' # you're gonna need this
require 'mathn' # you're gonna want this
class PrimeFactor
def initialize(number)
@number = number
end
def primeFactors
@factors = Prime.prime_division(@number,
Prime::TrialDivisionGenerator.new).flatten.uniq.sort.join(', ')
end
end
#prime = PrimeFactor.new(13195)
prime = PrimeFactor.new(ARGV.shift.to_i)
puts "#{prime.primeFactors}"
in fact, benchmarking the two thusly:
# the benchmark code:
require 'prime' # you're gonna need this
require 'mathn' # you're gonna want this
require 'benchmark'
class PrimeFactor_Long
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
class PrimeFactor_Shrt
def initialize(number)
@number = number
end
def primeFactors
factors = Prime.prime_division(@number,
Prime::TrialDivisionGenerator.new).flatten.uniq.sort.join(', ')
end
end
lots = 13195
bunches_of = 1_000
Benchmark.bmbm do |x|
x.report('Long') do
bunches_of.times do
long = PrimeFactor_Long.new lots
long.primeFactors
end
end
x.report('Shrt') do
bunches_of.times do
shrt = PrimeFactor_Shrt.new lots
shrt.primeFactors
end
end
end
puts
long = PrimeFactor_Long.new lots
puts "long factors: #{long.primeFactors}"
shrt = PrimeFactor_Shrt.new lots
puts "shrt factors: #{long.primeFactors}"
gives this:
serialhex@livecd:~/src/test> ruby bench_prime.rb
Rehearsal ----------------------------------------
Long 3.160000 0.070000 3.230000 ( 3.523138)
Shrt 0.050000 0.000000 0.050000 ( 0.070737)
------------------------------- total: 3.280000sec
user system total real
Long 3.010000 0.030000 3.040000 ( 3.227173)
Shrt 0.060000 0.000000 0.060000 ( 0.070470)
long factors: 5,7,13,29
shrt factors: 5,7,13,29
no joke... it's more than 50x faster 
so yeah, unless this is a project
for a class or something, use the included Prime class!
hex
2011/3/23 Jesús Gabriel y Galán <jgabrielygalan@gmail.com>
> 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.
>
>
