Hi Dirk,
I have translated your code a bit. You can make it much shorter if you
use blocks:
This method selects all factors of a number. Here, I first created a
list of values from 1 to n (1…n). Then only the factors are selected
(the non-factors are filtered).
def find_factors(n)
(1…n).select{|factor| n % factor == 0}
end
This method creates a list of numbers from 1 to max (1…max). Then it
replaces each number n in the list with find_factors(n). That is what
map does:
def find_prime_numbers(max)
(1…max).map{|n| find_factors(n)}
end
The output method is still very tricky, and not elegant. I first changed
factor_list.length.to_s.length to Math.log10(list.length).ceil. Then I
moved it out of the loop, because it is more efficient (it will only be
computed once, and not again in every iteration). I think you know what
log10 is? Well, log10 and then ceil (round up) returns the number of
numbers in a number. So 23 has 2 numbers, and 1234 has 4.
Then I replaced the output variable with inject, and I made some minor
changes like:
factors.length == 2 ? output << “*” : output << " "
to:
output + (factors.length == 2 ? ‘*’ : ’ ')
And
factors.each do | factor |
output << "#{factor} "
end
to:
factors.join(’ ')
and in non-detail mode, I used select and join again.
def output_factor_list(list, detail = false)
if detail
spaces = Math.log10(list.length).ceil
number = 0
list.inject(’’) do |output, factors|
number += 1
output + (factors.length == 2 ? ‘*’ : ’ ‘) +
number.to_s + (’ ’ * (spaces - Math.log10(number + 1).ceil))
+
‘{ ’ + factors.join(’ ') + " } \n"
end
else
‘{ ’ + list.select{|factors| factors.length == 2}.join(’ ') + ’
} ’
end
end
Everything combined:
def find_factors(n)
(1…n).select{|factor| n % factor == 0}
end
def find_prime_numbers(max)
(1…max).map{|n| find_factors(n)}
end
def output_factor_list(list, detail = false)
if detail
spaces = Math.log10(list.length).ceil
number = 0
list.inject(’’) do |output, factors|
number += 1
output + (factors.length == 2 ? ‘*’ : ’ ‘) +
number.to_s + (’ ’ * (spaces - Math.log10(number + 1).ceil))
+
‘{ ’ + factors.join(’ ') + " } \n"
end
else
‘{ ’ + list.select{|factors| factors.length == 2}.join(’ ') + ’
} ’
end
end
print output_factor_list(find_prime_numbers(200), false)
I think most of the changes are from imperative to functional.
Inject/map/select /join really make things simpler, but they are hard to
understand if you haven’t used them.
I hope this was helpful,
Jules
