02-background-review.ppt

02-background-review - CS4102 Algorithms Spring 2010 • Properties of algorithms • Counting basic operations • Time and space complexity •

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Unformatted text preview: CS4102, Algorithms, Spring 2010 • Properties of algorithms • Counting basic operations • Time and space complexity • Worst-case and average-case • Lower Bounds and Optimality • …and one slide of summations First Principles Analyzing Algorithms and Problems • We analyze algorithms with the intention of improving them, if possible, and for choosing among several available for a problem. • Correctness • Simplicity • Amount of work done, and space used • Optimality Correctness can be proved! • An algorithm consists of sequences of steps (operations, instructions, statements) for transforming inputs (preconditions) to outputs (postconditions) • Proving • if the preconditions are satisfied, • then the postconditions will be true, • when the algorithm terminates. • Good news for you! • This course does not emphasize proving correctness. Simplicity • Simplicity in an algorithm is a virtue. • Understandability matters • Especially for long-lived software • Easier to understand, more difficult to break it when making changes later We said: Analyzing Algorithms and Problems • Some terms from page 51 in text about problems : • Feasible, tractable problems • Intractable problems • The class of NP-complete problems • Unsolvable problems • The Halting Problem • Is a problem solvable? If so, is it possible to find a reasonably efficient solution? Levels for Talking about Problem Solving • Defining the problem • Describing an overall strategy • Describing an algorithm : • Inputs and outputs • Describing the processing steps to transform input to output • Analysis • Correctness; Time & Space • Is it an optimal algorithm? • Implementation issues • Verification : Is it guaranteed correct? Example: Search in an unordered array • Problem: • Let list be an array containing n entries, list[0], …, list[n-1], in no particular order. • Find an index of a specified key target , if it’s in the array; • return –1 as the answer if target is not in the array. • Strategy: • Compare target to each entry in turn until a match is found or the array is exhausted....
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This note was uploaded on 03/21/2010 for the course CS 445 taught by Professor Bloomfield,a during the Spring '08 term at UVA.

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02-background-review - CS4102 Algorithms Spring 2010 • Properties of algorithms • Counting basic operations • Time and space complexity •

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