[Tutor] read a random line from a file (fwd)

[Danny Yoo]

Hi Kirk,

Let me forward this to the other people here.  Your function looks
good; there's just one line that might need a fix:

###
x = int(1+random.random()*l)
return db[x]
###

Is it possible for x to be 0?  The reason this is important is because
db[0] is the first line in our file --- if we don't allow the possibility
of returning the first line in a file, it's less random than it should be.  
*grin*

This is an example of a "boundary" case: a "boundary" case just means to
tread carefully around the edges of our problem.  In this case, the edges
we should think about are the first and last lines of a file --- make sure
that the function can deal with them well.

Good luck!

---------- Forwarded message ----------
Date: Mon, 17 Dec 2001 14:14:04 -0500
From: Kirk Bailey <deliberatus@my995internet.com>
To: Danny Yoo <dyoo@hkn.eecs.berkeley.edu>
Subject: Re: [Tutor] read a random line from a file

This works:
def fileoneline(filename):
	f1=open(filename,'r')
	db=f1.readlines()
	f1.close()
	l=len(db)
	x=int(1+random.random()*l)
	return db[x]

Can it be improved?

Danny Yoo wrote:
> 
> On Mon, 17 Dec 2001, Andrei Kulakov wrote:
> 
> > On Sun, Dec 16, 2001 at 11:20:29PM -0500, Kirk Bailey wrote:
> > > OK, I know there is a simple way to do it, and I cannot locate the
> > > correct ommand. I want to open a flat text file, read ONE randomly
> > > selected line into a string, and close the file. I need the command for
> > > reading a line at random, and what module it is in. I understand there
> > > is such a command already defined, and it is simple to use, but canot
> > > find it again.
> 
> Actually, I'm not sure if it's built in at all.  However, we can cook up a
> few definitions so that it becomes a tool we can use.
> 
> To get a random line from a file, it might be useful to write something
> that tells us how many lines a file contains.  Here's one way to do it:
> 
> ###
> def countLines(file):
>     """Given a file, returns the number of lines it contains.
> 
>     The current file position should be preserved as long as the file
>     supports tell() and seek()."""
>     old_position = file.tell()
>     count = 0
>     while file.readline() != '':
>         count = count + 1
>     file.seek(old_position)
>     return count
> ###
> 
> If we had a way to choose a random number between 0 and the countLines()
> of a file, we'd be in business.  Thankfully, there is something in the
> library that can help us: random.randrange():
> 
> ###
> >>> random.randrange(10)
> 4
> >>> random.randrange(10)
> 7
> >>> random.randrange(10)
> 4
> ###
> 
> There is documentation on random.randrange() here:
> 
>     http://www.python.org/doc/lib/module-random.html
> 
> if you're interested in playing around with random stuff.  *grin*
> 
> Also, it would be great if, when we knew which line we wanted to pull out,
> we could get get that particular line.  There's a module called
> 'linecache' that can do this for us:
> 
>     http://www.python.org/doc/lib/module-linecache.html
> 
> But if 'linecache' wasn't there, it still wouldn't be too hard to cook
> something up like it.
> 
> With these tools, it becomes possible to write a simple random line
> grabber:
> 
> ###
> def getRandomLine(filename):
>     """Given a filename, returns a random line."""
>     linecount = countLines(open(filename))
>     chosen_line_number = random.randrange(linecount)
>     return linecache.getline(filename, chosen_line_number)
> ###
> 
> Writing function definitions can be a lot of fun.  Hope this helps!
> 
> _______________________________________________
> Tutor maillist  -  Tutor@python.org
> http://mail.python.org/mailman/listinfo/tutor

-- 
Respectfully,
             -Kirk D Bailey (C)2001
              Addme! icq #27840081
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