Reputation: 287
For some reason, I can't find any tutorial mentioning how to do this... So, how do I read the first n lines from a file?
I've come up with:
while File.open('file.txt') and count <= 3 do |f|
...
count += 1
end
end
but it is not working and it also doesn't look very nice to me.
Just out of curiosity, I've tried things like:
File.open('file.txt').10.times do |f|
but that didn't really work either.
So, is there a simple way to read just the first n lines without having to load the whole file? Thank you very much!
Upvotes: 12
Views: 12228
Reputation: 923
File inherits from IO and IO mixes in Enumerable methods which include #first
Passing an integer to first(n)
will return the first n items in the enumerable collection. For a File object, each item is a line in the file.
File.open('filename.txt', 'r').first(10)
This returns an array of the lines including the \n line breaks.
You may want to #join
them to create a single whole string.
File.open('filename.txt', 'r').first(10).join
Upvotes: 1
Reputation: 20106
You could try the following:
`head -n 10 file`.split
It's not really "pure ruby" but that's rarely a requirement these days.
Upvotes: -1
Reputation: 87541
Here is a one-line solution:
lines = File.foreach('file.txt').first(10)
I was worried that it might not close the file in a prompt manner (it might only close the file after the garbage collector deletes the Enumerator returned by File.foreach). However, I used strace
and I found out that if you call File.foreach
without a block, it returns an enumerator, and each time you call the first
method on that enumerator it will open up the file, read as much as it needs, and then close the file. That's nice, because it means you can use the line of code above and Ruby will not keep the file open any longer than it needs to.
Upvotes: 32
Reputation: 24367
There are many ways you can approach this problem in Ruby. Here's one way:
File.open('Gemfile') do |f|
lines = 10.times.map { f.readline }
end
Upvotes: 5
Reputation: 172
File.foreach('file.txt').with_index do |line, i|
break if i >= 10
puts line
end
Upvotes: 3