Again, we skip the proof of correctness of this algorithm. (d) What is the worst-case running time of Find-Index-2(A[1 : n)? What about its worst-case expected running time? Remember to prove your answer formally.

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Again, we skip the proof of correctness of this algorithm.
(d) What is the worst-case running time of Find-Index-2(A[1 n])? What about its worst-case
expected running time? Remember to prove your answer formally.
Transcribed Image Text:Again, we skip the proof of correctness of this algorithm. (d) What is the worst-case running time of Find-Index-2(A[1 n])? What about its worst-case expected running time? Remember to prove your answer formally.
Problem 2. Suppose we have an array A[1 : n] which consists of numbers {1,...,n} written in some
arbitrary order (this means that A is a permutation of the set {1,...,n}). Our goal in this problem is to
design a very fast randomized algorithm that can find an index i in this array such that A[i] mod 8 € {1,2},
i.e., the reminder of dividing A[i] by 8 is either 1 or 2. For simplicity, in the following, we assume that n
itself is a multiple of 8 and is at least 8 (so a correct answer always exist).
For instance, if n = 8 and the array is A = [8,7, 2,5, 4, 6,3, 1], we want to output either of indices 3 or 8.
(a) Suppose we sample an index i from {1,...,n} uniformly at random. What is the probability that i is
a correct answer, i.e., A[i] mod 8 E {1,2}?
(b) Suppose we sample m indices from {1,...,n} uniformly at random and with repetition. What is the
probability that none of these indices is a correct answer?
Now, consider the following simple algorithm for this problem:
Find-Index-1(A[1: n]):
• Let i = 1. While A[i] mod 8 ¢ {1,2}, sample i e {1,...,n} uniformly at random. Output i.
The proof of correctness of this algorithm is straightforward and we skip it in this question.
(c) What is the worse-case expected running time of Find-Index-1(A[1 : n])? Remember to prove your
answer formally.
The problem with Find-Index-1 is that in the worst-case (and not in expectation), it may actually never
terminate! For this reason, let us consider a simple modification to this algorithm as follows.
Find-Index-2(A[1 : n]):
• For j = 1 to n:
- Sample i e {1,...,n} uniformly at random and if A[i] mod 8 € {1,2}, output i and terminate;
otherwise, continue.
• If the for-loop never terminated, go over the array A one element at a time to find an index i with A[i]
mod 8 € {1,2} and output it as the answer.
Again, we skip the proof of correctness of this algorithm.
Transcribed Image Text:Problem 2. Suppose we have an array A[1 : n] which consists of numbers {1,...,n} written in some arbitrary order (this means that A is a permutation of the set {1,...,n}). Our goal in this problem is to design a very fast randomized algorithm that can find an index i in this array such that A[i] mod 8 € {1,2}, i.e., the reminder of dividing A[i] by 8 is either 1 or 2. For simplicity, in the following, we assume that n itself is a multiple of 8 and is at least 8 (so a correct answer always exist). For instance, if n = 8 and the array is A = [8,7, 2,5, 4, 6,3, 1], we want to output either of indices 3 or 8. (a) Suppose we sample an index i from {1,...,n} uniformly at random. What is the probability that i is a correct answer, i.e., A[i] mod 8 E {1,2}? (b) Suppose we sample m indices from {1,...,n} uniformly at random and with repetition. What is the probability that none of these indices is a correct answer? Now, consider the following simple algorithm for this problem: Find-Index-1(A[1: n]): • Let i = 1. While A[i] mod 8 ¢ {1,2}, sample i e {1,...,n} uniformly at random. Output i. The proof of correctness of this algorithm is straightforward and we skip it in this question. (c) What is the worse-case expected running time of Find-Index-1(A[1 : n])? Remember to prove your answer formally. The problem with Find-Index-1 is that in the worst-case (and not in expectation), it may actually never terminate! For this reason, let us consider a simple modification to this algorithm as follows. Find-Index-2(A[1 : n]): • For j = 1 to n: - Sample i e {1,...,n} uniformly at random and if A[i] mod 8 € {1,2}, output i and terminate; otherwise, continue. • If the for-loop never terminated, go over the array A one element at a time to find an index i with A[i] mod 8 € {1,2} and output it as the answer. Again, we skip the proof of correctness of this algorithm.
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