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Lecture 3

CPSC 319 Lecture 3: 03 Searching

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Computer Science
CPSC 319
Leonard Manzara

Searching and Sorting Terminology - Data: independent facts, observations, or events - Record: the data pertaining to a unique object o Sometimes called an element o Consists of one or more fields - Field: a constituent part of a record, usually consisting of a single data element o Has a specified type and size - File: a collection of records - Key: the data field used to select or order records o May or may not be unique for a set of records - Primary Key: the field used first for selecting or sorting records o E.g. Last names in a telephone book - Secondary Key: the field used if 2 or more records have equal primary keys o E.g. First names in a telephone book - Satellite Data: data in the non-key fields, not used when sorting or selecting records o E.g. Phone numbers in a telephone book - Search: an operation that returns a pointer to a record that matches a key value or nil - Sorting: arranges the items of a list into ascending (or descending) order comparing keys Sequential Search - Basic idea: starting at the beginning of a list, compare each successive item to a query key until we find a match or reach end of the list - Works on both sorted and unsorted lists - Can be used on arrays and linked lists - Likely to be the fastest algorithm on small lists (up to about 20 records) - Java code to search an integer array: int sequentialSearch(int[] array, int key) { for (int i = 0; i < array.length; i++) if (array[i] == key) return i; // success return -1; // failure: key not found } - In the best case we find a match immediately (perform 1 comparison) - In the worst case (an unsuccessful search/match found at the end), n comparisons occur - On average, (# + 1) / 2 comparisons are performed - Is an O(n) algorithm Binary Search - Works only on a sorted array of records - Basic idea: Divide the array in half by locating the middle term o If the query key matches this item, return o If the key is less than the middle item, divide the left sub-array in half o If the key is greater than the middle item, divide the rig
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