On Mon, 19 Feb 2001, Bret Hughes wrote:
> Johannes Eriksson wrote:
> >
> > * Bret Hughes [Mon, Feb 19, 2001 at 12:39:22AM -0600]:
> > > i believe you. I just can't figure out what it is doing? I have been
> > > working with perl lately and always looking to learn. Can you explain
> > > what it is doing please?
>
> > Here's the same script in a less compact but easier-to-understand
> > form:
> >
> > 01 #!/usr/bin/perl
> > 02
> > 03 srand;
> > 04 while (<>)
> > 05 {
> > 06 if (rand($.) < 1)
> > 07 {
> > 08 $line = $_;
> > 09 }
> > 10 }
> > 11 print $line;
> >
> > As you can see, I've rewritten the '(EXPR1) while (EXPR2)' statement
> > into a more readable 'while (EXPR) BLOCK' (line 4), and the
> > '(EXPR1) && (EXPR2)' statement into an 'if (EXPR) BLOCK'. Otherwise
> > it's pretty much the same.
> >
> > For each loop iteration, a new input line is read and stored into
> > the $_ variable (this is what <> does). Line 6 is where the magic
> > occurs: $. holds the current line number (starting from 1, call it N),
> > so the call to rand will always produce a random fractional number in
> > the range 0..N. As you can see, the probability of selecting a line is
> > always 1/lines_read and thus the final probability for each line becomes
> > 1/N.
> >
> > The selected line is then printed on line 11.
>
>
> Thanks that helps alot.I now understand the implied if using the logical
> "and" and it becomes clear. I also see that $_ may get written over
> several times as the loop cruises along but only the last case where
> rand returns < 1 will be used. It seems that this would more tend to
> return the earlier lines in the file rather than the later since it does
> not actually use the number chosen as $. = the last line, but will
> select it only if on the last iteration rand happens to return <1.
actually, if you examine the algorithm, the probability that line n
will be chosen as the "new" random line as you read through the file
is precisely 1/n, so all lines are equally likely.
rday
--
Robert P. J. Day
Eno River Technologies, Durham NC
Unix, Linux and Open Source training
"This is Microsoft technical support. How may I misinform you?"
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