[Tutor] Simple Question...

[Bill Mill]

To do this without loading the file into memory, and without relying
on wc (which ought to be very fast even with large files, if you need
that), you could do:

import random

def linecount(f):
    """count the number of lines in the file then rewind it"""
    l = ' '
    c = 0
    while l:
        l = f.readline()
        if l[-1:] == '\n': c +=1    #I don't think the 'if' is necessary, 
                                         #but for safety's sake we'll leave it
    f.seek(0)
    return c

def getrandline(f):
    """get a random line from f (assumes file pointer is at beginning)"""
    lines = linecount(f)
    r = random.randint(0,int(lines))
    for i in range(1, r): f.readline()
    return f.readline()

anybody have a faster implementation?

Peace
Bill Mill
bill.mill at gmail.com

On Sat, 16 Oct 2004 16:52:16 -0400, R. Alan Monroe
<amonroe at columbus.rr.com> wrote:
> >         This will work perfectly if your file is small enough to fit in your
> > computer's memory. If you want a function that does this on large
> > files, you'll have to use something in those lines:
> 
> > import random
> 
> > def randomLineFromBigFile(fileName, numLines):
> >         whatLine = random.randint(1, numLines)  # choose a random line number
> >         source = open(fileName, 'r')
> >         i = 0
> >         for line in source:
> >                 i += 1
> >                 if i == whatLine: return line
> >         return None
> 
> >         This function uses very little (and a constant amount of) memory. The
> > downside is that you have to know the total number of lines in the file
> > (that's the numLines argument) before calling it. It's not a very hard
> > thing to do.
> 
> Wouldn't it be much quicker to do something like this?
> 
> import os.path
> import random
> 
> size = os.path.getsize('test.txt')
> print size
> 
> randline = random.randint(1, size)
> print randline
> 
> testfile = open('test.txt', 'r')
> testfile.seek(randline)
> print testfile.readline()  #read what is likely half a line
> print testfile.readline()  #read the next whole line
> testfile.close()
> 
> You'd just need to add some exception handling in the event you tried
> to read off the end of the file.
> 
> Alan
> 
> 
> 
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