How to start reading a file(CSV) from a given line n in Ruby

Question

I have a large CSV file. I want to start reading from line n. Currently I have the following code

CSV.foreach(path) do |row|
  #process
end

I need to start reading from file n.

have the lines (or columns) always the same length or may it differ? And remember, with UTF-8 the bytes may differ, even if there a similar numbers of characters. — knut
– knut, Commented Nov 15, 2014 at 21:02

Tom Gasaner · Accepted Answer · 2014-11-15 21:25:08Z

3

You can read specific rows using .readlines method:

require 'csv'

p CSV.readlines(path)[15..20] # array returned

# Benchmark 
#     user     system      total        real
# 0.020000   0.000000   0.020000 (  0.015769)

Other way(which, I believe, should not load entire file in memory):

from = 15
to = 20
csv = CSV.open(file, 'r')

# skipping rows before one we need
from.times { csv.readline }

# reading rows we need
(to - from).times { p csv.readline }

# Benchmark 
#     user     system      total        real
# 0.000000   0.000000   0.000000 (  0.000737)

edited Nov 15, 2014 at 21:25

answered Nov 15, 2014 at 20:46

Tom Gasaner

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2 Comments

yatish mehta Over a year ago

It is large file >10gb wouldnt readlines hog the entire memory?

Tom Gasaner Over a year ago

@yatishmehta do you know specific lines which you want to read? I mean from i to j?

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