Let's say I want to combine several massive files into one and then uniq! the one (THAT alone might take a hot second)
It's my understanding that File.readlines() loads ALL the lines into memory. Is there a way to read it line by line, sort of like how node.js pipe() system works?
One of the great things about Ruby is that you can do file IO in a block:
File.open("test.txt", "r").each_line do |row|
puts row
end # file closed here
so things get cleaned up automatically. Maybe it doesn't matter on a little script but it's always nice to know you can get it for free.
you aren't operating on the entire file contents at once, and you don't need to store the entirety of each line either if you use readline.
file = File.open("sample.txt", 'r')
while !file.eof?
line = file.readline
puts line
end
Large files are best read by streaming methods like each_line as shown in the other answer or with foreach which opens the file and reads line by line. So if the process doesn't request to have the whole file in memory you should use the streaming methods. While using streaming the required memory won't increase even if the file size increases opposing to non-streaming methods like readlines.
File.foreach("name.txt") { |line| puts line }
uniq! is defined on Array, so you'll have to read the files into an Array anyway. You cannot process the file line-by-line because you don't want to process a file, you want to process an Array, and an Array is a strict in-memory data structure.
Related
Edit (I adjusted the title): I am currently using CSV.foreach but that starts at the first row. I'd like to start reading a file at an arbitrary line without loading the file into memory. CSV.foreach works well for retrieving data at the beginning of a file but not for data I need towards the end of a file.
This answer is similar to what I am looking to do but it loads the entire file into memory; which is what I don't want to do.
I have a 10gb file and the key column is sorted in ascending order:
# example 10gb file rows
key,state,name
1,NY,Jessica
1,NY,Frank
1,NY,Matt
2,NM,Jesse
2,NM,Saul
2,NM,Walt
etc..
I find the line I want to start with this way ...
file = File.expand_path('~/path/10gb_file.csv')
File.open(file, 'rb').each do |line|
if line[/^2,/]
puts "#{$.}: #{line}" # 5: 2,NM,Jesse
row_number = $. # 5
break
end
end
... and I'd like to take row_number and do something like this but not load the 10gb file into memory:
CSV.foreach(file, headers: true).drop(row_number) { |row| "..load data..." }
Lastly, I'm currently handling it like the next snippet; It works fine when the rows are towards the front of the file but not when they're near the end.
CSV.foreach(file, headers: true) do |row|
next if row['key'].to_i < row_number.to_i
break if row['key'].to_i > row_number.to_i
"..load data..."
end
I am trying to use CSV.foreach but I'm open to suggestions. An alternative approach I am considering but does not seem to be efficient for numbers towards the middle of a file:
Use IO or File and read the file line by line
Get the header row and build the hash manually
Read the file from the bottom for numbers near the max key value
I think you have the right idea. Since you've said you're not worried about fields spanning multiple lines, you can seek to a certain line in the file using IO methods and start parsing there. Here's how you might do it:
begin
file = File.open(FILENAME)
# Get the headers from the first line
headers = CSV.parse_line(file.gets)
# Seek in the file until we find a matching line
match = "2,"
while line = file.gets
break if line.start_with?(match)
end
# Rewind the cursor to the beginning of the line
file.seek(-line.size, IO::SEEK_CUR)
csv = CSV.new(file, headers: headers)
# ...do whatever you want...
ensure
# Don't forget the close the file
file.close
end
The result of the above is that csv will be a CSV object whose first row is the row that starts with 2,.
I benchmarked this with an 8MB (170k rows) CSV file (from Lahman's Baseball Database) and found that it was much, much faster than using CSV.foreach alone. For a record in the middle of the file it was about 110x faster, and for a record toward the end about 66x faster. If you want, you can take a look at the benchmark here: https://gist.github.com/jrunning/229f8c2348fee4ba1d88d0dffa58edb7
Obviously 8MB is nothing like 10GB, so regardless this is going to take you a long time. But I'm pretty sure this will be quite a bit faster for you while also accomplishing your goal of not reading all of the data into the file at once.
Foreach will do everything you need. It streams, so it works well with big files.
CSV.foreach('~/path/10gb_file.csv') do |line|
# Only one line will be read into memory at a time.
line
end
Fastest way to skip data that we’re not interested in is to use read to advance through a portion of the file.
File.open("/path/10gb_file.csv") do |f|
f.seek(107) # skip 107 bytes eg. one line. (constant time)
f.read(50) # read first 50 on second line
end
Probably a simple question, but I need to delete the contents of a file after a specific line number? So I wan't to keep the first e.g 5 lines and delete the rest of the contents of a file. I have been searching for a while and can't find a way to do this, I am an iOS developer so Ruby is not a language I am very familiar with.
That is called truncate. The truncate method needs the byte position after which everything gets cut off - and the File.pos method delivers just that:
File.open("test.csv", "r+") do |f|
f.each_line.take(5)
f.truncate( f.pos )
end
The "r+" mode from File.open is read and write, without truncating existing files to zero size, like "w+" would.
The block form of File.open ensures that the file is closed when the block ends.
I'm not aware of any methods to delete from a file so my first thought was to read the file and then write back to it. Something like this:
path = '/path/to/thefile'
start_line = 0
end_line = 4
File.write(path, File.readlines(path)[start_line..end_line].join)
File#readlines reads the file and returns an array of strings, where each element is one line of the file. You can then use the subscript operator with a range for the lines you want
This isn't going to be very memory efficient for large files, so you may want to optimise if that's something you'll be doing.
I am following Wicked cool ruby scripts book.
here,
there are two files, file_output = file_list.txt and oldfile_output = file_list.old. These two files contain list of all files the program went through and going to go through.
Now, the file is renamed as old file if a 'file_list.txt' file exists .
then, I am not able to understand the code.
Apparently every line of the file is read and the line is stored in oldfile hash.
Can some one explain from 4 the line?
And also, why is gets used here? why cant a .each method be used to read through every line?
if File.exists?(file_output)
File.rename(file_output, oldfile_output)
File.open(oldfile_output, 'rb') do |infile|
while (temp = infile.gets)
line = /(.+)\s{5,5}(\w{32,32})/.match(temp)
puts "#{line[1]} ---> #{line[2]}"
oldfile_hash[line[1]] = line[2]
end
end
end
Judging from the redundant use of quantifiers ({5,5} and {32,32}) in the regex (which would be better written as {5}, {32}), it looks like the person who wrote that code is not a professional Ruby programmer. So you can assume that the choice taken in the code is not necessarily the best.
As you pointed out, the code could have used each instead of while with gets. The latter approach is sort of an old-school Ruby way of doing it. There is nothing wrong in using it. Until the end of file is reached, gets will return a string, and when it does reach the end of file, gets will return nil, so the while loop works as the same when you use each; in each iteration, it reads the next line.
It looks like each line is supposed to represent a key-value pair. The regex assumes that the key is not an empty string, and that the key and the value are separated by exactly five spaces, and the the value consists of exactly thirty-two letters. Each key-value pair is printed (perhaps for monitoring the progress), and is stored in oldfile_hash, which is most likely a hash.
So the point of using .gets is to tell when the file is finished being read. Essentially, it's tied to the
while (condition)
....
end
block. So gets serves as a little method that will keep giving ruby the next line of the file until there is no more lines to give.
I am working on little problem and would have some advice on how to solve it:
Given a csv file with an unknown number of columns and rows, output a list of columns with values and the number of times each value was repeated. without using any library.
if the file is small this shouldn't be a problem, but when it is a few Gigs, i get NoMemoryError: failed to allocate memory. is there a way to create a hash and read from the disk instead of loading the file to Memory? you can do that in perl with tied Hashes
EDIT: will IO#foreach load the file into memory? how about File.open(filename).each?
Read the file one line at a time, discarding each line as you go:
open("big.csv") do |csv|
csv.each_line do |line|
values = line.split(",")
# process the values
end
end
Using this method, you should never run out of memory.
Do you read the whole file at once? Reading it on a per-line basis, i.e. using ruby -pe, ruby -ne or $stdin.each should reduce the memory usage by garbage collecting lines which were processed.
data = {}
$stdin.each do |line|
# Process line, store results in the data hash.
end
Save it as script.rb and pipe the huge CSV file into this script's standard input:
ruby script.rb < data.csv
If you don't feel like reading from the standard input we'll need a small change.
data = {}
File.open("data.csv").each do |line|
# Process line, store results in the data hash.
end
For future reference, in such cases you want to use CSV.foreach('big_file.csv', headers: true) do |row|
This will read the file line by line from the IO object with minimal memory footprint (should be below 1MB regardless of file size).
If I want to append the contents of a src file into the end of a dest file in Ruby, is it better to use:
while line = src.gets do
or
while buffer = src.read( 1024 )
I have seen both used and was wondering when should I use each method and why?
One is for reading "lines", one is for reading n bytes.
While byte buffering might be faster, a lot of that may disappear into the OS which likely does buffering anyway. IMO it has more to do with the context of the read--do you want lines, or are you just shuffling chunks of data around?
That said, a performance test in your specific environment may be helpful when deciding.
You have a number of options when reading a file that are tailored to different situations.
Read in the file line-by-line, but only store one line at a time:
while (line = file.gets) do
# ...
end
Read in all lines of a file at once:
file.readlines.each do |line|
# ...
end
Read the file in as a series of blocks:
while (data = file.read(block_size))
# ...
end
Read in the whole file at once:
data = file.read
It really depends on what kind of data you're working with. Generally read is better suited towards binary files, or those where you want it as one big string. gets and readlines are similar, but readlines is more convenient if you're confident the file will fit in memory. Don't do this on multi-gigabyte log files or you'll be in for a world of hurt as your system starts swapping. Use gets for situations like that.
gets will read until the end of the line based on a separator
read will read n bytes at a time
It all depends on what you are trying to read.
It may be more efficient to use read if your src file has unpredictable line lengths.