CSCE 2100: Computing Foundations 1 Probability Theory Tamara Schneider Summer 2013

Why Probability Theory in CS? Estimation of Running Time – Very large worst-case running time – Better average-case running time Decisions in algorithms in presence of uncertainty – Medical diagnosis – Allocation of resource based on future needs 2

Probability Space Finite set of points, whereby each of them represents a possible outcome of a specific experiment Each point (outcome) has a probability associated with it – Probabilities are always positive!!! – The sum of all probabilities is always 1 – Assume an equal probability distribution if not otherwise stated e.g. 1/6 for a specific number of a die throw (unless die is not fair) 3

Example: Probability Space · Event E · Probability space Event E that you roll a 2, 4, or 5 Imagine you throw a dart randomly at the box. You will hit the area of E 50% of the time. P(E) = 0.5 4

Probability and Combinatorics We need to count the number of possible outcomes (probability space) We need to count the number of points in the probability space for a specific event 5

Example: Craps Throw 2 dice and calculate the probability of obtaining a total of 7 or possible outcomes The event contains 8 points. p = 8/36 ≈ 22% 6

Example: Keno Randomly select 20 numbers in the range of 1 and 80 (not repeated) Players guess 5 numbers and are rewarded if they have guessed 3, 4, or 5 correct numbers Probability space: Number of selections of twenty numbers out of eighty. 7

Example: Keno p ≈ = 8.4% 8 Ways of choosing 20 of 80: Ways of picking 3 winners out of 5 and 17 losers out of 75:

Conditional Probability Definition: If E and F are two events in a probability space, the conditional probability of F given E is the sum of the probabilities of the points that are both in E and F (region A) divided by the sum of the probabilities of the points in E. Notation: P(F/E) “ the probability of F given E E F A B A: in E and F B: in E but not in F P(F/E) = A/E 9

Example 1: Conditional Probability Toss of 2 dice Probability space has 36 elements with equal probability 1/36 E: First comes out 1 (E 1 ) F: Second comes out 1 (E 2 ) P(E) = 6/36 = 1/6 P(F) = 6/36 = 1/6 P(F/E) = 1/6 The experiments are independent, since P(F) = P(F/E). It does not matter if E occurred or not; the probability of F stays the same. 10

Deal of 2 cards from a 52 card deck Number of points in experiment (probability space): Π(52,2) = 52 × 51 = 2,652 E: First card is an ace: 4 × 51 = 204 (4 choices for ace, 51 choices for second card) P(E) = 204/2,652 = 1/13 F: Second card is an ace: 4 × 51 = 204 (4 choices for ace, 51 choices for first card) P(F) = 204/2,652 = 1/13 P(F/E) = 12/204 = 1/17 (= 3/51) since there are 4×3 = 12 combinations for aces. Example 2: Conditional Probability [1] 11

Example 2: Conditional Probability [1] Probability Space: 52 × 51 = 2,652 E: first card is an ace F: second card is an ace 2 aces none of the cards is an ace 4 × 51 = 204 4×3=12 P(E) = 204/2,652 P(F) = 204/2,652 P(F/E) = 12/204 The experiments are not independent, since P(F) ≠ P(F/E). It does matter if E occurred or not; the probability of F changes. 12

Product Rule for Independent Experiments For a sequence of outcomes of k independent experiments, we can multiply the probabilities Example E: The last 4 digits of a phone# are 1234 – E 1 = E 2 = E 3 = E 4 = 0.1 – P(E) = (0.1) 4 = = 0.01% 13

Programming Applications - Example [1] Find out if x is in array A If x is not found, running time: O(n) If x is found, running time: O(n) Is the average case running time better? Assume that all points are equally likely 14 bool find(int x, int A[], int n){ for(int i=0; i<n; i++) if(A[i] == x) return true; return false } //find

Programming Applications - Example [2] Probability space has n points: from 0 to n-1 If x is in A[k], then the loop iterates k times Assume we find it in the i-th iteration with cost c per iteration – cost d needed for initialization and return statement – running time: ci + d O(n) 15 bool find(int x, int A[], int n){ for(int i=0; i<n; i++) if(A[i] == x) return true; return false } //find

Monte Carlo Algorithms Deterministic algorithm: For the same data and same input we will always receive the same output. Monte-Carlo algorithm: Makes a random selection at each iteration. 16

Monte Carlo Example Computer chip factory Probability of a bad chip in untested box is 0.1 Testing a box of n chips takes O(n) time Monte Carlo: Select k chips from each box – If all k tested chips are OK, declare box OK – 1/10 of the chips are bad if box is untested – Probability of saying OK after testing 1 chip : 0.9 – Probability of error: (0.9) k For k=131 tests (0.9) k ≈ (if the box is good, we should find a bad chip with a probability ≈ none) ⇒ So we find a bad chip with a probability = % 17

Summary Probability Space Conditional Probabilities Independent Experiments Monte Carlo Algorithms 18