Binary Search Manolis Koubarakis Data Structures and Programming Techniques 1.

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Presentation transcript:

Binary Search Manolis Koubarakis Data Structures and Programming Techniques 1

Tables as Sorted Arrays It is interesting to see whether an efficient searching algorithm can be devised for tables implemented as sorted arrays of structs. Binary search is the algorithm of choice in this case. Data Structures and Programming Techniques 2

Binary Search The problem to be addressed in binary searching is to find the position of a search key K in an ordered array A[0:n- 1] of distinct keys arranged in ascending order: A[0] < A[1] < … < A[n-1]. The algorithm chooses the key in the middle of A[0:n- 1], which is located at A[Middle], where Middle=(0+(n-1))/2, and compares the search key K and A[Middle]. If K==A[Middle], the search terminates successfully. If K < A[Middle] then further search is conducted among the keys to the left of A[Middle]. If K > A[Middle] then further search is conducted among the keys to the right of A[Middle]. Data Structures and Programming Techniques 3

Iterative Binary Search int BinarySearch(Key K) { int L, R, Midpoint; /* Initializations */ L=0; R=n-1; /* While the interval L:R is non-empty, test key K against the middle key */ while (L<=R){ Midpoint=(L+R)/2; if (K==A[Midpoint]){ return Midpoint; } else if (K > Midpoint) { L=Midpoint+1; } else { R=Midpoint-1; } /* If the search interval became empty, key K was not found */ return -1; } Data Structures and Programming Techniques 4

Recursive Binary Search int BinarySearch (Key K, int L, int R) { /* To find the position of the search key K in the subarray A[L:R]. Note: To search for K in A[0:n-1], the initial call is BinarySearch(K, O, n-1) */ int Midpoint; Midpoint=(L+R)/2; if (L>R){ return -1; } else if (K==A[Midpoint]){ return Midpoint; } else if (K > A[Midpoint]){ return BinarySearch(K, Midpoint+1, R); } else { return BinarySearch(K, L, Midpoint-1); } Data Structures and Programming Techniques 5

Complexity Data Structures and Programming Techniques 6

Complexity (cont’d) Data Structures and Programming Techniques 7

Complexity (cont’d) Data Structures and Programming Techniques 8

Complexity (cont’d) Data Structures and Programming Techniques 9

Readings T. A. Standish. Data Structures, Algorithms and Software Principles in C. – Sections 5.6 and 6.5 M.T. Goodrich, R. Tamassia and D. Mount. Data Structures and Algorithms in C++. 2 nd edition. – Section 9.3 Data Structures and Programming Techniques 10