[Ruby] file rename

filetype = ".jpg"

path = "."
prefix="slide_"

dir=Dir.open(path)
a=Array.new(dir.collect.length)
num=0
dir.each do |file|
    if(file.length>filetype.length)
        if (file[file.length-4..file.length]==filetype)
            a[num]=file
            num=num+1
        end
    end
end
puts num

count=0
for i in 0..num-1
    fnew=prefix+"%05d"%count+a[i][a[i].length-4..a[i].length]
    count=count+1
    old_file = path + "/" + a[i]
    new_file = path + "/" + fnew
    puts "old   "+old_file
    puts "new   "+new_file
    puts "--------------"
    File.rename(old_file, new_file) 
end