Is it possible to extract a particular line from a file knowing its line number? For example, just get the contents of line N as a string from file "text.txt"?
- I felt really misleaded by a lot of answers below. IMHO, the best one is the comment below this answer: no memory issue, and straight to the point.Ulysse BN– Ulysse BN2019-09-18 16:35:51 +00:00Commented Sep 18, 2019 at 16:35
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7 Answers
You could get it by index from readlines.
line = IO.readlines("file.txt")[42] Only use this if it's a small file.
2 Comments
the Tin Man
This is only the right answer if the file is small, less than a couple MB. Otherwise it'll force Ruby to load the entire file at once, which, in the case of a large file is slower than using a
foreach or gets based solution. See stackoverflow.com/questions/25189262/why-is-slurping-a-file-bad which contains benchmarks.
Jonas Elfström
I was surprised to learn that it takes as much as a couple of MB!
Try one of these two solutions:
file = File.open "file.txt" #1 solution would eat a lot of RAM p [*file][n-1] #2 solution would not n.times{ file.gets } p $_ file.close 5 Comments
Mark Thomas
Is solution #2 getting line n+1?
Nakilon
@Mark Thomas, in #1 too. I supposed indexing from 0.
tig
Thanks for
[*File.open('…')], did not know that to_a for File instance can give me its linestig
@Nakilon: file lines are indexed from 1 (all editors and even
cat -n do so)
Maxim Kulkin
Regarding #2: I think unsplat is costly operation. Also, syntax is confusing.
def get_line_from_file(path, line) result = nil File.open(path, "r") do |f| while line > 0 line -= 1 result = f.gets end end return result end get_line_from_file("/tmp/foo.txt", 20) This is a good solution because:
- You don't use
File.read, thus you don't read the entire file into memory. Doing so could become a problem if the file is 20MB large and you read often enough so GC doesn't keep up. - You only read from the file until the line you want. If your file has 1000 lines, getting line 20 will only read the 20 first lines into Ruby.
You can replace gets with readline if you want to raise an error (EOFError) instead of returning nil when passing an out-of-bounds line.
answered Oct 25, 2010 at 12:21
August Lilleaas
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9 Comments
August Lilleaas
Hehe, that's what you get from fellow Rubyists when you try to write optimized code eh? :)
August Lilleaas
I'd argue that reading an entire 20MB file into memory to get one line from it is bad practice in any language.
Nakilon
@August Lilleaas, i'd argue, that here you don't need
result = nil, return and write C-style loops, when you have times.August Lilleaas
Thankfully, I can write as much ruby as I want, the way I want, and I don't have to listen to what cargo culters want me to do :)
Josh Grosso
@AugustLilleaas when you use a language, however, you should write it idiomatically, if only for the reason that languages often optimize idiomatic constructs more than other ones. Also, if anyone's ever going to maintain your code, it's a disservice to not learn the right way to do it.
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File has a nice lineno method.
def get_line(filename, lineno) File.open(filename,'r') do |f| f.gets until f.lineno == lineno - 1 f.gets end end 
1 Comment
Mark Thomas
You don't really need lineno(). You could replace the 'until' line with
(lineno-1).times {f.gets}.linenumber=5 open("file").each_with_index{|line,ind| if ind+1==linenumber save=line # break or exit if needed. end } or
linenumber=5 f=open("file") while line=f.gets if $. == linenumber # $. is line number print "#{f.lineno} #{line}" # another way # break # break or exit if needed end end f.close If you just want to get the line and do nothing else, you can use this one liner
ruby -ne '(print $_ and exit) if $.==5' file 
3 Comments
Nakilon
Nice. That's when global $-variables become useful.
steenslag
This keeps on reading the file when the line is already found.
ghostdog74
Doesn't matter. If the line number is last 2nd line for example, it has to read until that line as well...
If you want one liner and do not care about memory usage, use (assuming lines are numbered from 1)
lineN = IO.readlines('text.txt')[n-1] or
lineN = f.readlines[n-1] if you already have file opened.
Otherwise it would be better to do like this:
lineN = File.open('text.txt') do |f| (n-1).times { f.gets } # skip lines preceeding line N f.gets # read line N contents end 
Comments
These solutions work if you want only one line from a file, or if you want multiple lines from a file small enough to be read repeatedly. Large files (for example, 10 million lines) take much longer to search for a specific line so it's better to get the necessary lines sequentially in a single read so the large file doesn't get read multiple times.
Create a large file:
File.open('foo', 'a') { |f| f.write((0..10_000_000).to_a.join("\n")) } Pick which lines will be read from it and make sure they're sorted:
lines = [9_999_999, 3_333_333, 6_666_666].sort Print out those lines:
File.open('foo') do |f| lines.each_with_index do |line, index| (line - (index.zero? ? 0 : lines[index - 1]) - 1).times { f.gets } puts f.gets end end This solution works for any number of lines, does not load the entire file into memory, reads as few lines as possible, and only reads the file one time.
