Read random lines from huge CSV file

Question

I have this quite big CSV file (15 Gb) and I need to read about 1 million random lines from it. As far as I can see - and implement - the CSV utility in Python only allows to iterate sequentially in the file.

It's very memory consuming to read the all file into memory to use some random choosing and it's very time consuming to go trough all the file and discard some values and choose others, so is there any way to choose some random line from the CSV file and read only that line?

I tried without success:

import csv  with open('linear_e_LAN2A_F_0_435keV.csv') as file:     reader = csv.reader(file)     print reader[someRandomInteger] 

A sample of the CSV file:

331.093,329.735 251.188,249.994 374.468,373.782 295.643,295.159 83.9058,0 380.709,116.221 352.238,351.891 183.809,182.615 257.277,201.302 61.4598,40.7106 

Answer 1

import random  filesize = 1500                 #size of the really big file offset = random.randrange(filesize)  f = open('really_big_file') f.seek(offset)                  #go to random position f.readline()                    # discard - bound to be partial line random_line = f.readline()      # bingo!  # extra to handle last/first line edge cases if len(random_line) == 0:       # we have hit the end     f.seek(0)     random_line = f.readline()  # so we'll grab the first line instead 

As @AndreBoos pointed out, this approach will lead to biased selection. If you know min and max length of line you can remove this bias by doing the following:

Let's assume (in this case) we have min=3 and max=15

1) Find the length (Lp) of the previous line.

Then if Lp = 3, the line is most biased against. Hence we should take it 100% of the time If Lp = 15, the line is most biased towards. We should only take it 20% of the time as it is 5* more likely selected.

We accomplish this by randomly keeping the line X% of the time where:

X = min / Lp

If we don't keep the line, we do another random pick until our dice roll comes good. :-)