How to get a particular line from a file

Question

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"?

Solution 1

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

Solution 2

You could get it by index from readlines.

line = IO.readlines("file.txt")[42]

Only use this if it's a small file.

Solution 3

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.

Solution 4

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

Solution 5

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

View more solutions

Comments

[Ulysse BN]

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.

[Bushra Khan]

Hehe, that's what you get from fellow Rubyists when you try to write optimized code eh? :)

[Nakilon]

In different languages you must optimize different things.

[Bushra Khan]

I'd argue that reading an entire 20MB file into memory to get one line from it is bad practice in any language.

[Bushra Khan]

..I'd also argue that get_line_from_file is a terrible method name. And that the implementation itself could be rewritten to be more ruby-like. But I digress..

[Nakilon]

@August Lilleaas, i'd argue, that here you don't need result = nil, return and write C-style loops, when you have times.

[Mark Thomas]

Is solution #2 getting line n+1?

[Nakilon]

@Mark Thomas, in #1 too. I supposed indexing from 0.

[Nakilon]

Nice. That's when global $-variables become useful.

[steenslag]

This keeps on reading the file when the line is already found.

[tig]

Thanks for [*File.open('…')], did not know that to_a for File instance can give me its lines

[tig]

@Nakilon: file lines are indexed from 1 (all editors and even cat -n do so)

[ghostdog74]

Doesn't matter. If the line number is last 2nd line for example, it has to read until that line as well...

[Mark Thomas]

You don't really need lineno(). You could replace the 'until' line with (lineno-1).times {f.gets}.

[Bushra Khan]

It's a matter of preference. I prefer to explicitly have a return for longer methods where you're supposed to use the return value. I think it increases clarity by making the usage of the method clearer.

[Maxim Kulkin]

Regarding #2: I think unsplat is costly operation. Also, syntax is confusing.

[Nakilon]

@August Lilleaas, I think, Ruby is not your language of life ..)

[Bushra Khan]

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 :)

[Joseph Ravenwolfe]

Thanks for this answer, exactly what I was looking for.

[PJP]

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!

[Joshua Grosso Reinstate CMs]

@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.