Solved

Algoriothm/puzzle

Posted on 2008-10-03
5
427 Views
Last Modified: 2010-10-05
Assume your computer is reading characters one by one from a stream (you don't know the length of the stream before ending). Note that you have only one character of storage space (so you cann't save the characters you've read to a something like a strong). When you've finished reading you should return a character out of the stream with equal probability.
0
Comment
Question by:shwetasingh206
  • 2
5 Comments
 
LVL 84

Accepted Solution

by:
ozo earned 168 total points
ID: 22632035
perldoc -q "How do I select a random line from a file"
       How do I select a random line from a file?

       Here's an algorithm from the Camel Book:

               rand($.) < 1 && ($line = $_) while <>;

       This has a significant advantage in space over reading the whole file
       in.  You can find a proof of this method in The Art of Computer
       Programming, Volume 2, Section 3.4.2, by Donald E. Knuth.
0
 
LVL 53

Assisted Solution

by:Infinity08
Infinity08 earned 166 total points
ID: 22632041
>> a character out of the stream with equal probability.

with equal probability ? What do you mean ? You want a uniformly random character from the stream without knowing the size of the stream ?

If so, simply do something like this :

1st character : save it in the buffer
2nd character : save it in the buffer with 1/2 probability
3rd character : save it in the buffer with 1/3 probability
...
nth character : save it in the buffer with 1/n probability
...
0
 
LVL 84

Expert Comment

by:ozo
ID: 22632078
So for single characters
stream | perl -ne 'BEGIN{$/=\1}$char = $_ if rand($.) < 1;END{print $char}'
0
 
LVL 22

Assisted Solution

by:NovaDenizen
NovaDenizen earned 166 total points
ID: 22636477
This is the way to do it, but it requires additional storage to hold the count of the number of characters you've read.  There is a counter behind $., or perl is using ftell() and the counter is hidden in the runtime libraries.

So really you need something like ceil(lg(n)) + 8 bits of storage to handle up to n characters of input.
0

Featured Post

Is Your Active Directory as Secure as You Think?

More than 75% of all records are compromised because of the loss or theft of a privileged credential. Experts have been exploring Active Directory infrastructure to identify key threats and establish best practices for keeping data safe. Attend this month’s webinar to learn more.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

This algorithm (in C#) will resize any image down to a given size while maintaining the original aspect ratio. The maximum width and max height are both optional but if neither are given, the original image is returned. This example is designed t…
The greatest common divisor (gcd) of two positive integers is their largest common divisor. Let's consider two numbers 12 and 20. The divisors of 12 are 1, 2, 3, 4, 6, 12 The divisors of 20 are 1, 2, 4, 5, 10 20 The highest number among the c…
This Micro Tutorial demonstrates using Microsoft Excel pivot tables, how to reverse engineer competitors' marketing strategies through backlinks.
With the power of JIRA, there's an unlimited number of ways you can customize it, use it and benefit from it. With that in mind, there's bound to be things that I wasn't able to cover in this course. With this summary we'll look at some places to go…

920 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

16 Experts available now in Live!

Get 1:1 Help Now