Randomly selecting lines from files

Question

I have bunch of files and very file has a header of 5 lines. In the rest of the file, pair of line form an entry. I need to randomly select entry from these files. How can

Answer 1

Two other means to do so: 1- by generators (may still require a lot of memory): http://www.usrsb.in/Picking-Random-Items--Take-Two--Hacking-Python-s-Generators-.html

2- by a clever seeking (best method actually): http://www.regexprn.com/2008/11/read-random-line-in-large-file-in.html

I here copy the code of the clever Jonathan Kupferman:

#!/usr/bin/python

import os,random

filename="averylargefile"
file = open(filename,'r')

#Get the total file size
file_size = os.stat(filename)[6]

while 1:
      #Seek to a place in the file which is a random distance away
      #Mod by file size so that it wraps around to the beginning
      file.seek((file.tell()+random.randint(0,file_size-1))%file_size)

      #dont use the first readline since it may fall in the middle of a line
      file.readline()
      #this will return the next (complete) line from the file
      line = file.readline()

      #here is your random line in the file
      print line