Finding palindromic numbers in Ruby - ruby

Question

So, I'm doing Project Euler to solidify my Ruby skills. I'm on problem #4, which reads:
A palindromic number reads the same
both ways. The largest palindrome made
from the product of two 2-digit
numbers is 9009 = 91 * 99.
Find the largest palindrome made from
the product of two 3-digit numbers.
First, I'm trying to verify my code using the information from the first paragraph. I've defined a palindrome function as so:
def palindrome?(blah)
string = blah.to_s
string.reverse == string
end
My code looks like:
array = (90..99).to_a
array = array.map{|u| array.map{|y| u*y}}
array = array.sort
array = array.select{|u| palindrome?(u)}
puts array
The program doesn't output anything. If I do the following:
array = (90..99).to_a
array = array.map{|u| array.map{|y| u*y}}
array = array.sort
#array = array.select{|u| palindrome?(u)}
puts array
I get a long series of unsorted four-digit numbers, so I guess it's ignoring the sort. Finally, if I simply do:
#array = (90..99).to_a
#array = array.map{|u| array.map{|y| u*y}}
#array = array.sort
array = [7447, 9009, 3551, 2419]
array = array.select{|u| palindrome?(u)}
puts array
I get 7447 and 9009, like I should. Why is this happening?
I'm using 1.8.6, because that's the only version available on this Windows machine.

Answer 1

This line of yours
array = array.map{|u| array.map{|y| u*y}}
returns a nested array, you should unwrap.
Hints: (but I'm not going to tell you how)
You probably don't need sort on the intermediary value
You need to remove duplicates
Tips:
next time, run your code in the interactive interpreter, that way you'll see the result of each line of code.

Answer 2

You can use something like this
new_arr = array.inject([]) { |a,u| a += array.map { |y| u*y } }
instead of
array = array.map{|u| array.map{|y| u*y}}
it returns a nested "[ [8100,..],[],[] ] " type of array. So thats the reason why your code doesn't work

Answer 3

This looks more understandable:-
#Steps
# Define a method for palindrome
# List out all 3-digit numbers
# Multiply each numbers by each numbers
# List out all palindrome numbers
# Choose the largest (max) palindrome number
def is_a_palindrome?(n)
n == n.to_s.reverse.to_i
end
def problem_four
palindrome = [ ]
array = 111.upto(999)
array.each do |x|
array.each do |y|
multiply = x * y
if is_a_palindrome?(multiply)
palindrome << multiply
end
end
end
palindrome.max
end
puts problem_four
#$ ruby problem_four.rb
#906609

Answer 4

Find the largest palindrome made from the product of two 3-digit numbers.
I have made this solution, I hope this will help some newbie
to=999
from=100
palindromes=[]
for i in from..to do
for j in 1..to do
k=i*j
palindromes << k if k.to_s==k.to_s.reverse
end
end
palindromes.max #this will return 906609