Reference no: EM131565851

Question: Searching an unsorted array

This problem examines three algorithms for searching for a value x in an unsorted array A consisting of n elements. Consider the following randomized strategy: pick a random index i into A. If A[i] = x, then we terminate; otherwise, we continue the search by picking a new random index into A. We continue picking random indices into A until we find an index j such that A[j] = x or until we have checked every element of A. Note that we pick from the whole set of indices each time, so that we may examine a given element more than once.

a. Write pseudo code for a procedure RANDOM-SEARCH to implement the strategy above. Be sure that your algorithm terminates when all indices into A have been picked.

b. Suppose that there is exactly one index i such that A[i] = x. What is the expected number of indices into A that we must pick before we find x and RANDOM-SEARCH terminates?

c. Generalizing your solution to part (b), suppose that there are k _ 1 indices I such that A[i] = x. What is the expected number of indices into A that wemust pick before we find x and RANDOM-SEARCH terminates? Your answershould be a function of n and k.

d. Suppose that there are no indices i such that A[i] = x. What is the expected number of indices into A that we must pick before we have checked all elements of A and RANDOM-SEARCH terminates?

Now consider a deterministic linear search algorithm, which we refer to as DETERMINISTIC-SEARCH. Specifically, the algorithm searches A for x in order, considering A[1],A[2],A[3], ...,A[n] until either it finds A[i] = x or it reaches the end of the array. Assume that all possible permutations of the input array are equally likely.

e. Suppose that there is exactly one index i such that A[i] = x. What is the average-case running time of DETERMINISTIC-SEARCH? What is the worst case running time of DETERMINISTIC-SEARCH?

f. Generalizing your solution to part (e), suppose that there are k _ 1 indices I such that A[i] = x. What is the average-case running time of DETERMINISTICSEARCH?What is the worst-case running time of DETERMINISTIC-SEARCH?Your answer should be a function of n and k.

g. Suppose that there are no indices i such that A[i] = x. What is the average-case running time of DETERMINISTIC-SEARCH? What is the worst-case running time of DETERMINISTIC-SEARCH? Finally, consider a randomized algorithm SCRAMBLE-SEARCH that works by first randomly permuting the input array and then running the deterministic linear search given above on the resulting permuted array.

h. Letting k be the number of indices i such that A[i] = x, give the worst-case and expected running times of SCRAMBLE-SEARCH for the cases in which k = 0 and k = 1. Generalize your solution to handle the case in which k = 1.

i. Which of the three searching algorithms would you use? Explain your answer.