# Probabilistic (Average-Case) Analysis and Randomized Algorithms Two different approaches –Probabilistic analysis of a deterministic algorithm –Randomized

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• Slide 1
• Probabilistic (Average-Case) Analysis and Randomized Algorithms Two different approaches Probabilistic analysis of a deterministic algorithm Randomized algorithm Example Problems/Algorithms Hiring problem (Chapter 5) Sorting/Quicksort (Chapter 7) Tools from probability theory Indicator variables Linearity of expectation
• Slide 2
• Probabilistic (Average-Case) Analysis Algorithm is deterministic; for a fixed input, it will run the same every time Analysis Technique Assume a probability distribution for your inputs Analyze item of interest over probability distribution Caveats Specific inputs may have much worse performance If distribution is wrong, analysis may give misleading picture
• Slide 3
• Randomized Algorithm Randomize the algorithm; for a fixed input, it will run differently depending on the result of random coin tosses Randomization examples/techniques Randomize the order that candidates arrive Randomly select a pivot element Randomly select from a collection of deterministic algorithms Key points Works well with high probability on every inputs May fail on every input with low probability
• Slide 4
• Common Tools Indicator variables Suppose we want to study random variable X that represents a composite of many random events Define a collection of indicator variables X i that focus on individual events; typically X = X i Linearity of expectations Let X, Y, and Z be random variables s.t. X = Y + Z Then E[X] = E[Y+Z] = E[Y] + E[Z] Recurrence Relations
• Slide 5
• Hiring Problem Input A sequence of n candidates for a position Each has a distinct quality rating that we can determine in an interview Algorithm Current = 0; For k = 1 to n If candidate(k) is better than Current, hire(k) and Current = k; Cost: Number of hires Worst-case cost is n
• Slide 6
• Analyze Hiring Problem Assume a probability distribution Each of the n! permutations is equally likely Analyze item of interest over probability distribution Define random variables Let X = random variable corresponding to # of hires Let X i = indicator variable that i th interviewed candidate is hired Value 0 if not hired, 1 if hired X = i = 1 to n X i E[X i ] = ? Explain why: E[X] = E[ i = 1 to n X i ] = i = 1 to n E[X i ] = ? Key observation: linearity of expectations
• Slide 7
• Alternative analysis Analyze item of interest over probability distribution Let X i = indicator random variable that the i th best candidate is hired 0 if not hired, 1 if hired Questions Relationship of X to X i ? E[X i ] = ? i = 1 to n E[X i ] = ?
• Slide 8
• Questions What is the probability you will hire n times? What is the probability you will hire exactly twice? Biased Coin Suppose you want to output 0 with probability and 1 with probability You have a coin that outputs 1 with probability p and 0 with probability 1-p for some unknown 0 < p < 1 Can you use this coin to output 0 and 1 fairly? What is the expected running time to produce the fair output as a function of p?
• Slide 9
• Quicksort Algorithm Overview Choose a pivot element Partition elements to be sorted based on partition element Recursively sort smaller and larger elements
• Slide 10
• Quicksort Walkthrough 17 12 6 23 19 8 5 10 6 8 5 10 17 12 23 19 5 6 8 17 12 19 23 6 8 12 17 23 6 17 5 6 8 10 12 17 19 23
• Slide 11
• Pseudocode Sort(A) { Quicksort(A,1,n); } Quicksort(A, low, high) { if (low < high) { pivotLocation = Partition(A,low,high); Quicksort(A,low, pivotLocation - 1); Quicksort(A, pivotLocation+1, high); }
• Slide 12
• Pseudocode int Partition(A,low,high) { pivot = A[high]; leftwall = low-1; for i = low to high-1 { if (A[i] < pivot) then { leftwall = leftwall+1; swap(A[i],A[leftwall]); } swap(A[high],A[leftwall+1]); } return leftwall+1; }
• Slide 13
• Worst Case for Quicksort
• Slide 14
• Average Case for Quicksort?
• Slide 15
• Intuitive Average Case Analysis 0 n/4 n/2 3n/4 n Anywhere in the middle half is a decent partition (3/4) h n = 1 => n = (4/3) h log(n) = h log(4/3) h = log(n) / log(4/3) < 2 log(n)
• Slide 16
• How many steps? At most 2log(n) decent partitions suffices to sort an array of n elements. But if we just take arbitrary pivot points, how often will they, in fact, be decent? Since any number ranked between n/4 and 3n/4 would make a decent pivot, half the pivots on average are decent. Therefore, on average, we will need 2 x 2log(n) = 4log(n) partitions to guarantee sorting.
• Slide 17
• Formal Average-case Analysis Let X denote the random variable that represents the total number of comparisons performed Let indicator variable X ij = the event that the ith smallest element and jth smallest element are compared 0 if not compared, 1 if compared X = i=1 to n-1 j=i+1 to n X ij E[X] = i=1 to n-1 j=i+1 to n E[X ij ]
• Slide 18
• Computing E[X ij ] E[X ij ] = probability that i and j are compared Observation All comparisons are between a pivot element and another element If an item k is chosen as pivot where i < k < j, then items i and j will not be compared E[X ij ] = Items i or j must be chosen as a pivot before any items in interval (i..j)
• Slide 19
• Computing E[X] E[X] = i=1 to n-1 j=i+1 to n E[X ij ] = i=1 to n-1 j=i+1 to n 2/(j-i+1) = i=1 to n-1 k=1 to n-i 2/(k+1)
• Computing S B i = 1/n O i = k/(i-1) S i = k/(n(i-1)) S = i>k S i = k/n i>k 1/(i-1) is probability of success k/n (H n H k ): roughly k/n (ln n ln k) Maximized when k = n/e Leads to probability of success of 1/e