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Counting. Often in CS we need to Enumerate/account for all possibilities of something, such as: generating passwords exhaustive testing of an algorithm Calculate a probability In general, there are two ways to count:

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counting
Counting
  • Often in CS we need to
    • Enumerate/account for all possibilities of something, such as:
      • generating passwords
      • exhaustive testing of an algorithm
    • Calculate a probability
  • In general, there are two ways to count:
    • Chicken method  We’ll get an answer eventually, but we won’t necessarily understand why
    • Analytically  but we need to learn some rules
counting outline
Counting outline
  • Some types of problems to address
  • Definition of probability
    • Based on some “experiment” or “event” that has a set of outcomes called our sample space.
    • P = (# favorable outcomes)/ (total in sample space)
  • Multiplication rule, i.e. fundamental counting principle
    • Number of possible phone numbers or license plate numbers
  • Permutations
  • Combinations
  • *** See handout for details
  • Common technique required: break a problem down into distinct cases.
multiplication rule
Multiplication rule
  • If you need to make consecutive (or independent) choices, just multiply.
    • If you have x ways of making one decision, and y ways of making a second decision, the total number of outcomes is xy.
  • We can generalize to a “lining up” situation. If you have x items that need to be arranged in some order, this can be done in x! ways.
  • Sometimes we have different cases to add.
    • E.g. At 3:00, you may choose from either the lunch or dinner menu. Do lunch and dinner separately, then add.
distinguishable
Distinguishable
  • The number of ways to line up n objects, but a of them are identical, b of them are identical, etc.

= n! / (a! b! …)

  • Think of 3 people sitting in a row of 3 chairs versus 5 chairs.
  • Examples: How many ways can we arrange the letters in the following words?
    • NEEDED 6! / (3! 2!)
    • DIVISIBILITY 12! / 5!
    • AAABBB 6! / (3! 3!)
handling restriction
Handling restriction
  • (This is a permutation problem with a twist.)
  • We need to select a president, vice president and secretary from 10 people. However, A can’t be president and B cannot be secretary. How many ways can this be done?
  • Focus on cases that are legal:
    • B is president, A is VP, anyone else can by sec’y
    • B is president, A is secretary, anyone else can be VP
    • B is VP, A is sec’y, anyone else can be president
    • Any more?
combinations
Combinations
  • Definition
  • Contrast with permutations
  • Evaluating
  • Applications
  • Practice
when to use
When to use
  • Questions dealing with making some selection, membership, inclusion, presence vs. absence…. In which order does not matter
  • Combinations answer this type of question: A set has n elements. How many subsets contain r elements?
    • Try examples when n = 4.
  • Contrast with permutations:
    • How many ways can we select 3 letters from the alphabet?
    • How many 3 letter words can we make?
points to remember
Points to remember
  • Order doesn’t matter
    • Subsets, committees
    • What matters is whether something is chosen or not.
  • Evaluation:
    • The number of ways to choose r items out of a set of n is n! / (r!(n – r)!).
    • Shorthand notation: C(n, r) or
    • Shortcut to evaluate: C(10, 3) = 10 * 9 * 8 / (3 * 2 * 1)
    • Incidentally: choosing r is the same as (not) choosing n – r.
examples
Examples
  • How many 8-bit representations have exactly three 0’s?
    • Choose which positions get a 0: choose 3 out of 8: C(8, 3).
  • Six people need to be divided into two teams A and B. How many ways can this be done?
    • Choose 3 to go to team A: C(6, 3).
    • Choose 3 remaining people for B: C(3, 3) = 1.
  • What is the probability that exactly 4 out of 7 coin tosses will be heads?
    • Choose which 4 occurrences are heads: C(7, 4).
    • Divide by the total size of sample space: 27.
watch the wording
Watch the wording
  • Sometimes we want “at least” or “at most”
  • Both
  • Neither
  • Be careful with “or”. There could be overlap.
    • King or club
    • Number or red
  • Are objects/categories distinguishable?
    • “two teams” may be indistinguishable
    • Selecting cards from a deck: almost always the order that we deal cards does not matter
indirect probability
Indirect probability
  • Sometimes it’s easier to find the probability that an even will NOT happen, and then subtract from 1.
  • Ex. Suppose we roll 5 dice, as in Yahtzee.
    • P(all sixes) = 1/7776
    • P(didn’t get all sixes) = 1 – 1/7776
    • P(1000 tries, didn’t get all sixes) = (1 – 1/7776)1000
    • P(1000 tries, got all sixes at least once) = 1 – (1 – 1/7776)1000
  • Ex. The “birthday problem.” In a group of n people, what is the probability that at least two of them have the same birthday? What is n if p > ½?
poker hands
Poker hands
  • How many straight flushes are there?
    • The high card can be anything from 5 up to Ace.
    • All cards of the same suit, so select a suit.
  • How many 4-of-a-kind hands?
    • Choose which denomination you want 4 of.
    • Choose those 4 cards.
    • Choose the 5th card.
  • Full house?
    • Choose two denominations.
    • Choose three of one and two of the other.
more counting problems
More counting problems
  • Indistinguishable subsets
  • Ball-in-urn: the objects being selected are identical
  • Pascal’s triangle, a combinatorical argument
  • Stirling numbers
indistinguishable subsets
Indistinguishable subsets
  • How many ways can 4 people divide into 2 teams of 2, as in a doubles match?
    • The teams are the same size and don’t have inherent name, place, uniform, etc.
    • AB/CD, AC/BD, AD/BC
    • Is the answer 3 or 6?
  • How many ways to divide … ?
    • 20 people into 4 teams of 5
    • 20 people into teams of 3, 4, 5, 8
    • 20 people into teams of 4, 5, 5, 6
ball in urn
Ball in urn
  • The number of ways to distribute
    • n distinct objects into r categories = rn
    • n identical objects into r categories = C(n + r – 1, n)
  • Give 4 cards to 2 people.
    • If face up, they are distinct. Each card has 2 possible destinations  24.
    • If face down, then the only thing that matters is how many cards you have.

Your cards and my cards are separated by a divider

CARD CARDCARD / CARD

0 0 0 1 0

2 categories  1 divider that can go anywhere:

answer = C(5, 1) or C(5, 4).

ball in urn 2
Ball in urn (2)
  • Give 5 cards to 3 people.
    • If distinct, ask each card where it’s going: 35.
    • If identical:

CARD CARD / CARD CARD / CARD

0 0 1 0 0 1 0

3 categories  2 dividers that can go anywhere

Total = C(7, 2)

  • 3 divers find 5 gold coins. How many ways to share?
  • A case of 12 bottles of wine to distribute among 4 tables.
ball in urn 3
Ball in urn (3)
  • What if we must give each table at least 1 bottle of wine?
    • We have less freedom to choose.
    • Only 8 bottles can be freely distributed.
  • Let’s buy 7 cans of soup. 3 varieties are available.
ride tickets
Ride tickets
  • Buy 10 ride tickets at the fair. There are 3 possible rides.
    • If the issue is how many times you want to go on each, this is a ball in urn problem  C(10 + 3 – 1, 2)
    • If order matters  310
    • If the tickets say we must go in haunted house 4 times, roller coaster 3 times and ferris wheel 3 times, and order matters. It’s a combination question. Choose when you do each type of ride  C(10, 4) * C(6, 3) * C(3, 3).
    • If the tickets are bought 4x, 3x and 3x, and order doesn’t matter  1.
  • *** More examples from book and handout.
understanding c n r
Understanding C(n, r)
  • Let’s justify: C(5, 2) = C(4, 1) + C(4, 2)
  • We want to select two elements from { A, B, C, D, E }.
    • Is E part of the selection?
    • If so, then we pick one of the others: C(4, 1)
    • If not, then we pick both from the others: C(4, 2).
  • This line of reasoning works in general for C(n, r).
  • Now that we understand Pascal’s triangle, let’s look at another one. 
what good is pascal s
What good is Pascal’s Δ
  • Combinatorical arguments
    • C(n, r)  numbers in Pascal’s triangle 
    • Stirling numbers
  • Recap important ideas
    • When to use formulas: n!, P(n, r), C(n, r), C(n + r – 1, n)
    • Go over practice handout
stirling numbers
Stirling numbers
  • S(n, r) = the number of ways to break up a set of n elements into r groups.
  • If you study these numbers in detail, you can solve more complex counting problems, and determine coefficients of more Bernoulli-type summations!
starting simple
Starting simple
  • Let’s consider S(3, 2). How many ways can we split up a 3-element set { A, B, C } into two groups?

Note: their sizes must be 1 and 2.

    • A / BC
    • B / AC
    • C / AB
  • Consider S(3, 3). Splitting { A, B, C } into 3 groups.
    • A / B / C
  • Next, let’s work out various cases of S(5, r) for 1  r  5.
s 5 r
S(5, r)
  • S(5, 1): We need to “split up” a set of 5 elements into a single group. Degenerate case: we want 1 group of size 5. Answer = 1.
  • S(5, 2): Split into 2 groups
    • Their sizes could be 2 and 3. In this case, we have groups like A B / C D E. Select which 2 go in one group: C(5, 2).
    • Their sizes could be 1 and 4. In this case, we have groups like A / B C D E. Select 1 to go into the first group: C(5, 1).
    • Answer is the sum: 10 + 5 = 15.
s 5 r continued
S(5, r) continued
  • S(5, 3): We need to split into 3 groups
    • The sizes could be 1, 1, 3. For example A / B / CDE. Think of it in terms of distinguishable permutations. The 5 elements can be ordered in 5! ways. But the 3 elements in the size-3 group have no inherent order, so we have to divide by 3!. Also, the two groups of size 1 are indistinguishable, so we divide by another 2!. 5!/(3!2!) = 10
    • The sizes could be 1, 2, 2. For example A / BC / DE. The total number of permutations is 5!. But the 2 groups of 2 are indistinguishable. Also, the pairs of values inside each group of 2 are indistinguishable. 5!/(2!2!2!) = 15.
    • Total = 25.
s 5 r continued1
S(5, r) continued
  • S(5, 4): We need to split into 4 groups
    • The sizes could only be 1, 1, 1, 2. The two positions inside the size-2 group are not distinguishable, and the 3 groups of size 1 are not distinguishable. 5!/(2!3!) = 10.
  • S(5, 5): pulverize the set of 5 into singleton sets. Just one way to do it. (You can think of all 5 size-1 groups as being indistinguishable, and 5!/5! = 1)
  • So, row 5 of Stirling’s triangle is: 1, 15, 25, 10, 1
general approach
General approach
  • It’s a bit tedious to keep evaluating each S(n, r) from scratch in terms of every possible case of group size.
  • Let’s make use of earlier/lower values of S.
  • Consider S(6, 3). Split { A, B, C, D, E, F } into 3 parts.
  • When we partition into groups, we may have F by itself.
    • F / (2 more groups of A-E). Answer = S(5, 2).
    • After partitioning A-E, just put F into any current group. There are 3 to choose from. Answer = 3 * S(5, 3)
    • Add it up: S(5, 2) + 3 * S(5, 3)
    • In general: S(n, r) = S(n – 1, r – 1) + r * S(n – 1, r)
using the formula
Using the formula
  • How can we use the Stirling formula to quickly determine S(n, r) for higher values of n?
  • Cell value = NW + N * (column number)
    • Observe 90 = 15 + 25*3 and 65 = 25 + 10*4
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