# Large File Reading and Writing **URL:** \ **Category:** Ruby\ **Created:** [October 18, 2006, 10:40am UTC](https://www.ruby-forum.com/t/large-file-reading-and-writing/72659 "2006-10-18T10:40:04Z")\ **Posts on this page:** 5\ **Page:** 1

**Author:** ![liquid](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/47e85d/32.png) [@liquid](https://www.ruby-forum.com/u/liquid)\ **Post date:** [October 18, 2006, 10:40am UTC](https://www.ruby-forum.com/t/large-file-reading-and-writing/72659/1 "2006-10-18T10:40:04Z")

Hi, A project that I’ve currently got at work consists of reading large files and writing another larger file. Unfortunately the machine I’m on is pretty lame and it runs out of RAM pretty quick and so takes a long time. (This is currently in perl and I’m not keen to play with it in perl.) I have had a look around the docs and searched the mailing list ( a quick search ) for how to do buffered reading and writing but I’ve come up with nothing. I have a bunch of text files. I want to read some lines, and then write a line, the move on to the next line etc but I only want to have a few lines in memory at once. Any clues as to how to achive this? Cheers Daniel ---

Daniel N wrote: > search ) for how to do buffered reading and writing but I’ve come up with > nothing. > > I have a bunch of text files. I want to read some lines, and then write a > line, the move on to the next line etc but I only want to have a few lines > in memory at once. Any clues as to how to achive this? > > Cheers > Daniel Something like this maybe… history = [] ARGF.each do |line| history \<\< line if enough\_history\_to\_generate\_some\_output write\_output history.clear end end The ARGF thingy is explained in ‘ri IO’: ``` The global constant ARGF (also accessible as $<) provides an IO-like stream which allows access to all files mentioned on the command line (or STDIN if no files are mentioned). ARGF provides the methods #path and #filename to access the name of the file currently being read. ``` You can of course open a file by name from your code. ---

Thanx. I’m not sure that my head is really around it though… Pls see inline On 10/13/06, Joel VanderWerf [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > end > Does the output file not stay in memory? In my case the output file is > almost a concatenation of large files so once I’ve written a line I > don’t > really want to keep that line in memory. The ARGF thingy is explained in ‘ri IO’: > ``` > The global constant ARGF (also accessible as $<) provides an > IO-like stream which allows access to all files mentioned on the > command line (or STDIN if no files are mentioned). ARGF provides > the methods #path and #filename to access the name of the file > currently being read. > > ``` > > You can of course open a file by name from your code. If I have a named file and I open it. I think it will be easier if I open specify the files from withing ruby by feeding it only a directory. Once I have my list of files if I open a file File.open( “my\_file”, “r” ) Is there a way to buffer this input so that the entire file is not read? Sorry if this is implicit in your last reply but I don’t understand it if it is. Cheers ---

Daniel N wrote: > > ARGF.each do |line| > > really want to keep that line in memory. > > Instead of the line ``` write_output ``` let’s say you have something like ``` output_line = ... puts output_line ``` Each time these lines are executed, you have a variable that refers to the _current_ line of output, but there is no reference to the string that was printed last time around. This means that the garbage collector can reclaim that space if it needs to. So the whole output file need not be kept in memory. > If I have a named file and I open it. I think it will be easier if I open > specify the files from withing ruby by feeding it only a directory. Once I > have my list of files if I open a file > > File.open( “my\_file”, “r” ) > > Is there a way to buffer this input so that the entire file is not read? Sure. If you use IO.gets (or IO.each, as above), then only one line at a time is read. File.open( “my\_file”, “r” ) do |f| f.each do |line| … # do something with line end end ---

**Author:** ![liquid](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/47e85d/32.png) [@liquid](https://www.ruby-forum.com/u/liquid)\ **Post date:** [October 18, 2006, 10:44am UTC](https://www.ruby-forum.com/t/large-file-reading-and-writing/72659/5 "2006-10-18T10:44:06Z")

Great Thanx Joel. That clears it up for me.