CSCI 1900 Discrete Structures. Combinations Reading: Kolman, Section 3.2. Order Doesn’t Matter. In the previous section, we looked at two cases where order matters: Multiplication Principle – duplicates allowed Permutations – duplicates not allowed.
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CombinationsReading: Kolman, Section 3.2
In the previous section, we looked at two cases where order matters:
Notice the example given on the previous slide of the possible sequences involving the elements 6, 5, and 2. The number of arrangements of 6, 5, and 2 equals the number of ways three elements can be ordered, i.e., 3P3.
3P3 = 3!/(3-3)! = 6/1 = 6
10P3 = 10!/(10-3)! = 10!/7! = 720
nCr = n!/[r! (n – r)!]
52C5 = 52!/(5! (52-5)!)
Assume you are walking with your grocery cart past the 2 liter sodas in Walmart. You need to pick up 10 bottles out of:
Last zero is unnecessary
Moved to Sprites
Moved to Pepsi
Moved to A&W
1 1 0 0 1 1 1 0 1 1 0 1 1 1
2 Cokes were picked
3 Dr. Peppers were picked
3 A&W’s were picked
No Sprites were picked
2 Pepsis were pickedBuying Sodas (continued)
14C10 = 14!/(10! (14 - 10)!) = 1001
The general formula for order doesn’t matter and duplicates allowed for a selection of r items from a set of n items is:
(n + r – 1)Cr