That's certainly One Way To Do It, but it's certainly not the fastest.

The process goes something like this:

• read a line from the file
• stick it in the array
• repeat the previous steps for the second file
• loop through each line of the first array
• check against *every* line of the second array, with a case insensitive match

As either array grows, the number of necessary checks grows. With 2 lines in each file, you'll do four checks. With 10 lines in each file, you'll do 100 checks. (At least, if my math unit is working today.)

With the hash solution, you only loop through each file once. You don't have to check each element in one file against each element of the other file -- if it already exists in the hash, no problem. Besides that, you only have to run lc() on each line once, instead of having to build a case-insensitive version of each element in the second file for each line of the first file.

If you have 2 lines in each file, you have 4 hashings, 4 lc calls, and 4 hash assignments. No big win there. If you have 10 lines in each file, you have 20 hash assignments. You do the math for 100 elements in each file.

Besides that, it's less work. I'd say the hash is the clear winner in this case, and hopefully more people will understand why. (Apologies to the literati here for boring them. :)