Lecture5

Lecture5 - Computational Optimization ISE 407 Lecture 5 Dr...

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Computational Optimization ISE 407 Lecture 5 Dr. Ted Ralphs

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ISE 407 Lecture 5 1 Reading for this Lecture Aho, Hopcroft, and Ullman, Chapter 1 Miller and Boxer, Chapters 1 and 5 Fountain, Chapter 4
ISE 407 Lecture 5 2 Problems and Instances Roughly, a problem specifies what output is desired for each given input. In more rigorous mathematical terms, solving a problem can be equated to evaluating a (mathematical) function . A problem instance is to evaluate the given function for a specific input (argument). An algorithm is a procedure for converting the inputs to an output of the desired form (called a solution ).

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ISE 407 Lecture 5 3 Simple Example Consider the following simple problem: Input : x R and n Z . Output : x n . What is the most straightforward algorithm for solving this problem? Is there a better algorithm?
ISE 407 Lecture 5 4 A More Efficient Algorithm Let’s assume that n = 2 m for m Z . We can use repeated squaring: def pow(x, n): for i in range(log(n, 2)): x *= x Questions : Is this algorithm correct? How much more efficient is it? What do we mean by efficiency ? Why don’t we call it speed ? How do we modify it for the general case?

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ISE 407 Lecture 5 5 Algorithms An algorithm is a procedure for evaluating the function associated with a given problem. In general, a problem will have more than one algorithm. How do we compare? Correctness (accuracy) Efficiency An algorithm that is guaranteed to result in a solution for every instance is said to be correct . In some cases, it is not possible to produce the solution exactly (problems with irrational solutions). In such cases, we generally want know how close the approximate solution is to the true solution. This is the domain of numerical analysis , which we will later in the course.
ISE 407 Lecture 5 6 Example: Log Function in Python >>> math.log10(1000) 3.0 >>> math.log(1000, 10) 2.9999999999999996 Here, we are using two different algorithms for calculating the logarithm base 10 of 1000. One is more accurate than the other. We will look at issues of floating point error later in the course.

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ISE 407 Lecture 5 7 Analyzing Algorithms The goal of analyzing algorithms is to determine the resources required to execute the algorithm in practice. Generally speaking, time and space are the two most important resources to consider. A simple approach to evaluating performance in practice would be to do an empirical analysis . Rigorous empirical analyses are more difficult than they appear because of factors that are difficult to take into account. What instances should we use to do the testing? How should we measure performance (CPU time, wallclock time)? How should we take into account performance variability?
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• Fall '13
• TedRalphs
• Optimization, Analysis of algorithms, Computational complexity theory, ISE

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