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/frame= s)
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 =3D number
end
def primeFactors
@factors =3D Prime.prime_division(@number,
Prime::TrialDivisionGenerator.new).flatten.uniq.sort.join(', ')
end
end
#prime =3D PrimeFactor.new(13195)
prime =3D 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 =3D number
end
def primeFactors
factors =3D Array.new
(2..@number).each do |num|
if ( @number % num =3D=3D 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 =3D=3D factor
next if candidate.nil?
factors
=3D nil if candidate % factor =3D=3D 0
end
end
factors.compact.join(',')
end
end
class PrimeFactor_Shrt
def initialize(number)
@number =3D number
end
def primeFactors
factors =3D Prime.prime_division(@number,
Prime::TrialDivisionGenerator.new).flatten.uniq.sort.join(', ')
end
end
lots =3D 13195
bunches_of =3D 1_000
Benchmark.bmbm do |x|
x.report('Long') do
bunches_of.times do
long =3D PrimeFactor_Long.new lots
long.primeFactors
end
end
x.report('Shrt') do
bunches_of.times do
shrt =3D PrimeFactor_Shrt.new lots
shrt.primeFactors
end
end
end
puts
long =3D PrimeFactor_Long.new lots
puts "long factors: #{long.primeFactors}"
shrt =3D 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 projec= t
for a class or something, use the included Prime class!
hex
2011/3/23 Jes=FAs Gabriel y Gal=E1n said:
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 =3D number
end
def primeFactors
factors =3D Array.new
(2..Math.sqrt(@number).ceil).each do |num|
if ( @number % num =3D=3D 0 )
factors.insert(factors.length,num)
end
end
(0...factors.length).each do |i|
prime =3D factors
factors =3D factors.select { |x| x =3D=3D prime || x % prime != =3D0 }
end
factors.compact.join(',')
end
end
prime =3D 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 =3D factors
factors =3D factors.select { |x| x =3D=3D prime || x % prime !=3D0 }
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 =3D=3D factor
factors =3D nil if candidate % factor =3D=3D 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 =3D number
end
def primeFactors
factors =3D Array.new
(2..@number).each do |num|
if ( @number % num =3D=3D 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 =3D=3D factor
next if candidate.nil?
factors =3D nil if candidate % factor=
=3D=3D
0
end
end
factors.compact.join(',')
end
end
#prime =3D PrimeFactor.new(13195)
prime =3D 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.
