Dimsum is like head or tail, but pulls randomly from the entire file or stream

http://blog.noblemail.ca/2012/11/how-to-get-random-lines-out-of-file-or.html

by snoble • 14 years ago

29 29 14 years ago

blog.noblemail.ca

andrewcooke2• 14 years ago

uses reservoir sampling -http://en.wikipedia.org/wiki/Reservoir_sampling

(so it presumably consumes the entire stream before giving any results; any alternative i can think of would not be "really random" unless you knew the length of the stream in advance).

avibryant• 14 years ago

Yep, though a feature request I've put in is to respond to a ctl-c by producing the results from the stream so far... that way if it's taking a while on a large file you can interrupt and still get something useful.

cgs10192• 14 years ago

This breaks ctl-c in my opinion. When I ctl-c I want shit to stop, not dump (potentially large quantities of) output into my terminal.

snoble6• 14 years ago

the magic of reservoir sampling is the memory footprint is the size of the output while being completely random. In this particular implementation the order of the output is slightly biased but each row has an equal chance of being in the output.

snoble6• 14 years ago

but yes. it has to wait until the stream is done before producing any output.

cgs10192• 14 years ago

Maybe I'm overlooking something obvious but wouldn't piping through awk 'rand()<.1{print}' give a decent streaming random sample of roughly 10% of input?

minimax• 14 years ago

Reservoir sampling allows you to return a specific number of samples regardless of the size of the input rather than a specific fraction of samples (which is what your awk script does).

taltman13• 14 years ago

As minimax stated, your awk code won't provide exactly k samples. Here's a bit of awk code that implements reservoir sampling (apologies in advance for any bugs) and prints the current sample to stdout, without needing to first process the entire stream. It simply prints the set of samples to stdout whenever the sample updates, with sets of samples separated by a distinct string. It is called as follows (of course, replace dmesg with any stream of your choosing):

  dmesg | gawk -f reservoir-sample.awk k=5 record_separator='==='


  #!/usr/bin/gawk -f

  ### reservoir-sample.awk
  ###
  ### Sample k random lines from a stream, without knowing the size of the stream.
  ###
  ### (Tomer Altman)

  ### Parameters: (set from command-line)
  ##
  ## k: number of lines to sample
  ## record_separator: string used to separate iterations of the sample in stdout.

  ## Define a function for returning a number between 1 and n:
  function random_int (n) { return 1 + int(rand() * n) }

  ## Initialize the PRNG seed:
  BEGIN { srand() }

  ## For the first k lines, initialize the sample array:

  NR <= k { sample[NR] = $0 }

  ## If we've initialized the sample array, and we pick an integer between one and the current record number that is less than or equal to k, update the sample array and print it to stdout:
  NR > k && (current_random_int = random_int(NR)) <= k {

    sample[current_random_int] = $0

    for (i in sample) print sample[i]
    
    print (record_separator != "") ? record_separator : "--"
  }

nullc• 14 years ago

uhhh. You mean like shuf -n NNNN ?

bo1024• 14 years ago

I wonder if the implementation of shuf would handle very large input efficiently? Reservoir sampling wouldn't need to keep the whole input in memory, which could be an advantage. But I don't know how shuf works.

teraflop• 14 years ago

Doesn't look like it. I just tried running "yes | shuf -n 1" (using the latest version of GNU coreutils, 8.20) and its memory consumption increased steadily until I killed it.

It seems like this would be a really useful improvement, and I'm surprised that it doesn't already seem to have been requested on the coreutils issue tracker.

nburger• 14 years ago

or "sort -R"...

gwern• 14 years ago

sort -R isn't actually a random permutation or shuffle like one would ordinarily expect from a 'random' output: if you read the man page very carefully, you'll notice it only speaks of 'random hash' which means that 2 lines which are identical (quite possible and common) will be hashed to the same value and placed consecutively in the output.

(This bit me once. Filed a bug about maybe clarifying the man page, but nope, apparently we're all supposed to recognize instantly the implication of a random hash.)

neilk3• 14 years ago

This doesn't come with an elaborate test suite, but it does pretty much everything that does in a few dozen lines of Perl.

https://github.com/neilk/misc/blob/master/randline

I've had this script (or versions of it) around for more than a decade. I didn't know the technique had a name.

andrewcooke2• 14 years ago

it was an example in the original camel book.

[edit: i was going to delete this, but since you replied i'll leave it - it does appear (too?) in the camel book, on p 246 of my copy, but like you say, it's for a single line. hadn't opened that book in years, took me some time to find it...]

neilk3• 14 years ago

I believe it was an example in the Perl Cookbook, but for picking a single line only. (Ancient UseNet thread: http://bit.ly/Thd4eE)

snoble6• 14 years ago

the code isn't special. but it's easy for anyone to install and use

patrick_grant• 14 years ago

I really don't like how this behaves for populating the array initially, and how it behaves for small inputs...

  $ seq 15 | dimsum  -n 10 
  14
  12
  3
  4
  5
  6
  7
  8
  9
  10

snoble6• 14 years ago

yup, the order is biased. definitely worth a couple of minutes to fix that

taltman13• 14 years ago

Looks like a valid sample to me. Are you bothered by the ordering of the sample members? Then I'd continue the pipeline to include a call to shuf.

Dimsum is like head or tail, but pulls randomly from the entire file or stream

Related Stories