Large File Reading and Writing

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 [email protected] 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

Great Thanx Joel. That clears it up for me.

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