1. Bingo Example: Design (Random Distinct Numbers Algorithm)
J. Michael Moore
CSCE 121

2. Random Number Generators
Really pseudorandom number generator
Given the same seed, it will always produce the same sequence of “random” values
If you don’t know the seed, the choices seem random.
Can give the same value two times in a row
In C++, take remainder to get values in a specified range

3. Goal Fill container with n distinct numbers.
If they didn’t have to be distinct, we’d just use the built in random number generator directly.

4. Build a Model
Let’s do one based on how the random number generator works…

5. Brute Force Potential Infinite Loop!
What if you always get the same random number?
Possible if not Probable
Assume
maxVal contains the maximum value
numToGet contains the number of values to get
Start with empty container, vals
While size of vals < numToGet
candidate ← randomvalue from 1 to maxVal
newVal ← true (assume candidate will work)
For each item in vals
if item equals candidate
newVal ← false (candidate is already there)
If newVal
add candidate to vals
Potential Infinite Loop!

6. Build a Different Model
How do we actually draw numbers?

7. Alternative
Idea
Fill another container with all of the possible values to use
Draw value and put in our container of distinct values
gets removed from possible values
Since we removed the value, the next time we get one, we won’t get a duplicate!

8. Draw No Infinite Loop! Assume Start with empty container, vals
maxVal contains the maximum value
numToGet contains the number of values to get
Start with empty container, vals
Load bag with values from 1 to maxVal
While size of vals < numToGet
index ← randomvalue that is any valid index for bag
value ← bag[index]
Remove value from bag
Add value to vals
No Infinite Loop!