Classnote04-1x2

Classnote04-1x2 - Algorithm Efficiency Analysis CSE 250...

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Unformatted text preview: Algorithm Efficiency Analysis CSE 250 Lecture Note 04 Algorithm Efficiency Analysis Adapted from the Koffman-Wolfgang C++ Primer, from the Instructor Materials slides by Elliot Koffman and Paul Wolfgang. This material is used by permission of John Wiley & Sons, Inc For private use only by students in CSE250 Re-distribution or public posting in any form is expressly forbidden. c Xin He (University at Buffalo) CSE 250 Data Structures Classnote 04 1 / 46 Algorithm Analysis It means: estimating the resources required. The resources of algorithms: time and space . We mainly consider time: harder to estimate; often more critical. The efficiency of an algorithm is measured by a runtime function T ( n ) . n is the size of the input. Commonly, n is the # of items in the input, if each item is of fixed size . c Xin He (University at Buffalo) CSE 250 Data Structures Classnote 04 3 / 46 Whats T ( n ) ? Defining T ( n ) as the real run time is meaningless, because the real run time depends on many factors, such as the machine speed, the programming language used, the quality of compilers etc . These are not the properties of the algorithm. T ( n ) def = the number of basic instructions performed by the algorithm. Basic instructions: + ,- , * , / , read from/write into a memory location, comparison, branching to another instruction ... These are not basic instructions: input/output statement, sin ( x ) , exp ( x ) . . . . These actions are done by function calls, not by a single machine instruction. Knowing T ( n ) and the machine speed , we can estimate the real runtime . Example 1: The machine speed is 10 8 ins/sec. T ( n ) = 10 6 . The real runtime would be about 10- 8 10 6 = . 01 sec. c Xin He (University at Buffalo) CSE 250 Data Structures Classnote 04 4 / 46 Example Example 2: Consider this simple program: 1: s = 2: for i = 1 to n do 3: for j = 1 to n do 4: s = s + i + j 5: end for 6: end for T ( n ) = ? Its hard to get the exact expression of T ( n ) even for this very simple program. Also, the exact value of T ( n ) depends on factors such as prog language, compiler. These are not the properties of the loop, so not our concern. We can see: the loop iterates n 2 times , and loop body takes constant number of instructions. So T ( n ) = an 2 + bn + c for some constants a , b , c . We say the growth rate of T ( n ) is n 2 . This is the sole property of the algorithm and is our main concern. c Xin He (University at Buffalo) CSE 250 Data Structures Classnote 04 5 / 46 The Order of Performance: (Big) O Basic idea: 1 Ignore constant factor: computer and language implementation details affect that: go for fundamental rate of increase with problem size. 2 Consider fastest growing term: Eventually, for large problems, it will dominate....
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Classnote04-1x2 - Algorithm Efficiency Analysis CSE 250...

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