Discrete Structures
Sept 26 2011
Assignment 5:
Due at beginning of class Monday, Oct 3rd
Prof. Hopcroft
*******Note: this homework is subject to the style guide on the website.
Points will be
deducted for homeworks not following the guidelines.********
*******Please print out and staple the grade sheet to the back of your homework!******
**********Remember Midterm this Friday!
We consider this homework to be a good
review.**********
1. Describe a method to store a set whose elements can be any 20digit integer. Describe how someone
using your solution can check whether or not a new element is in the set in constant time (see definitions
on course website for explanation of constant time).
Hint: your solution will need to deal with the
possibility of collisions.
Solution:
The solution is to use hashing with linear probing or with linked lists. A description of hashing can be
found:
http://www.cs.cornell.edu/courses/CS2800/2011fa/Lectures/lec
sept
21.pdf
Special attention was paid to the size of the hashing array, as that is what makes hashing work.
Without a correct or manageble array size, the rest of hashing does not work.
2. I wish to calculate
mod 9 for very large numbers (e.g. 4756213912
mod 9). Prove that I can get the
correct answer by adding the digits of the large number and taking the result mod 9. That is to say,
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 '07
 SELMAN
 Rational number, Irrational number

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