Simon Brunning wrote:
> I couldn't resist. ;-)
Me neither...
> import random
>
> def randomLines(filename, lines=1):
> selected_lines = list(None for line_no in xrange(lines))
>
> for line_index, line in enumerate(open(filename)):
> for selected_line_index in xrange(lines):
> if random.uniform(0, line_index) < 1:
> selected_lines[selected_line_index] = line
>
> return selected_lines
>
> This has the advantage that every line had the same chance of being
> picked regardless of its length. There is the chance that it'll pick
> the same line more than once, though.
The idea is simple: have a chain of "pickers" that pass all items that are
not picked on to the next picker. The current implementation, however, may
hit the recursion limit for large samples.
import random
class PickOne(object):
"""Pick a random item from an iterable and store it in the 'picked'
attribute.
As an iterator, yield all items that are not picked.
Helper for the pick() function.
"""
def __init__(self, iterable):
self.iterable = iterable
def __iter__(self):
it = enumerate(self.iterable)
index, self.picked = it.next()
for index, item in it:
if random.uniform(0, index) < 1:
yield self.picked
self.picked = item
else:
yield item
def pick(iterable, N, accept_less=False):
"""Pick N random items from an iterable.
"""
# Set up the PickOne(PickOne( ... PickOne(iterable) ... )) chain
keepers = []
for i in xrange(N):
iterable = PickOne(iterable)
keepers.append(iterable)
# We are interested in the side effects, i. e. the PickOne instances
# picking random items from the initial iterable
for item in iterable:
pass
# The following could be
# return [k.picked for k in keepers]
# but we want to handle the case len(iterable) < N, too
result = []
for keeper in keepers:
try:
picked = keeper.picked
except AttributeError:
if accept_less:
return result
raise ValueError("sequence contains less items than shall be
picked")
result.append(picked)
return result
def demo():
"""Pick random lines from a set of files.
"""
import optparse
from itertools import chain, izip, repeat, count
parser = optparse.OptionParser()
parser.usage = ("%prog [nl] file1 file2 ... fileN\n\n"
"Choose random lines from files given on the command line.")
parser.add_option("-n", "--count", type="int", default=10,
help="number of lines that shall be picked (default=10)")
parser.add_option("-l", "--less", action="store_true",
help="accept less picked lines than specified, if there are less
total lines in the file(s)")
options, args = parser.parse_args()
index_name_line = chain(*[izip(count(), repeat(fn), file(fn)) for fn in
args])
try:
picked = pick(index_name_line, options.count, options.less)
except ValueError:
raise SystemExit("Cannot pick more lines than found in the file(s)")
print "".join([("%3d <%s> %s" % ifl) for ifl in picked])
if __name__ == "__main__":
demo()
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