Question

In ruby, I am trying to compare a list of URLs with a previous list of URLs, and get only the new ones.

I put the old list in a text file with one URL per line. I am reading the text file into an array like so:

oldLines = File.open('logfile.txt', 'r').readlines

I have an array of new values populated using the exact same method as the old list, and will probably have some overlap with the old list called ‘newLines’. I am trying to get only values that don’t match with the old list. Let’s say ‘newList’.length = 100 and ‘oldlist’.length = 95, and I know through visual inspection that something like 90 elements overlap between them.
Things I have tried:

newList = newList - oldList
#(newList | oldList) returns 195
#(newList & oldList) returns 0


newList.delete_if { |x| oldList.include?(x) }

In both scenarios, nothing gets deleted from newList. I know I am missing something here. Thanks.

Answer 1

I did the following:

a.txt

http://yahoo.com
http://google.com
http://bing.com

b.txt

http://bing.com
http://yahoo.com

test.rb

a = File.open('a.txt', 'r').readlines.map!(&:chomp)
b = File.open('b.txt', 'r').readlines.map!(&:chomp)
p a-b #=> ["http://google.com"]

Without the chomp it fails because in a.txt I have http://yahoo.com\n while on b.txt I simply have http://yahoo.com without the \n at the end.