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CS583.Fall'06: Introduction to Analysis of Algorithms

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  1. Introduction CS 583 Fall 2006 Analysis of Algorithms CS583 Fall'06: Introduction

  2. Outline • Class discussion • Algorithms • Definition • Examples • Efficiency • Insertion sort • Pseudocode • Proof of correctness • Performance analysis CS583 Fall'06: Introduction

  3. Syllabus: General Information • Text book • Introduction to Algorithms, Second Edition by T. Cormen, C. Leiserson, R. Rivest, and C. Stein, MIT Press, 2001. • Course • This course covers concepts associated with design and analysis of algorithms. • Algorithms capture fundamental ideas of computational systems and often determine the quality and efficiency of software systems. • Designing efficient algorithms in many cases depends on choosing proper data structures; hence the attention will be paid to both concepts. • The main topics include: complexity analysis, data structures, sorting, and graph algorithms. CS583 Fall'06: Introduction

  4. Syllabus: Grading • 3 homeworks – 20% • Midterm exam – 30% • Final exam – 40% • Class participation – 10% • Both exams are comprehensive, i.e. based on all material covered to date. • Class participation grade will be based on being active during the class: answering questions, suggesting solutions, etc. • Active participation is encouraged, but no participation will not be penalized. CS583 Fall'06: Introduction

  5. Syllabus: Schedule CS583 Fall'06: Introduction

  6. Schedule (cont.) CS583 Fall'06: Introduction

  7. Algorithms • Definitions • An algorithm is a well-defined computational procedure that takes a set of values as input and produces a set of values as output. • An algorithm can also be thought of as a tool for solving a computationalproblem that specifies in general terms the desired input/output relationship. For example, the sorting problem: • Input: A sequence of n numbers <a1, ... , an>. • Output: Reordering of the input sequence <a1', ... , an'> such that: • a1' <= a2' <= ... <= an' CS583 Fall'06: Introduction

  8. Algorithms (cont.) • Definitions • An algorithm is correct if for every input instance, it finishes with the correct output. We say that a correct algorithm solves the given computational problem. • A data structure is a way to store and organize data in order to facilitate access and modifications. • There are problems for which no efficient solution is known. An interesting subset of these problems is called NP-complete. These problems have a remarkable property that if an efficient algorithm exists for any one of them, then efficient algorithms exist for all of them. CS583 Fall'06: Introduction

  9. Examples of Problems • Internet related problems: • Finding good routs on which data will travel. • Implementing search engines to quickly find pages on which particular information resides. • Financial problems. • Find a price of a complex financial instrument using simulated environments. • Value a large portfolio of securities. • Information processing. • Data base related problems such as records search, insertion, deletion. • XML based processing. CS583 Fall'06: Introduction

  10. Algorithm Efficiency • Two algorithms that are designed to solve the same problem may differ dramatically in their efficiency. • For example, compare an insertion sort algorithm with merge sort algorithm. • While the difference may be small for small input size, it becomes very pronounced for large size inputs, where time may really matter. CS583 Fall'06: Introduction

  11. Insertion Sort • The procedure takes as a parameter an array A[1..n] containing a sequence of n elements to be sorted. The input numbers are sorted in place. • The algorithm works by finding a correct position for each number among the numbers sorted so far. The number slides into the right position by comparing with the sorted numbers. There are two approaches: • Compare from the beginning of the array. In this case all array would need to be shifted forward. • Compare from the end of an array. This requires shifting one number at a time. CS583 Fall'06: Introduction

  12. Insertion Sort: Pseudocode INSERTION-SORT (A, n) 1 for j=2 to n 2 key=A[j] 3 // insert A[j] into sorted A[1..j-1] 4 i=j-1 5 while i>0 and A[i]>key 6 A[i+1]=A[i] 7 i=i-1 8 A[i+1] = key CS583 Fall'06: Introduction

  13. Insertion Sort: Correctness • Loop invariants • At the start of the for loop of lines 1-8, the subarray A[1..j-1] consists of the elements originally in A[1..j-1] but in sorted order. Need to show the following: • Initialization: It is true prior to the first iteration of the loop. • One element is always sorted. • Maintenance: It remains true before the next iteration • An array is moved to find the right place for A[j], hence it remains sorted. • Termination: When the loop terminates, the invariant gives us a useful property that help to show that the algorithm is correct. • When the loop ends, A[1..n] array is sorted. CS583 Fall'06: Introduction

  14. Performance Analysis • Analyzing an algorithm means predicting the resources that the algorithm requires. Most often we want to measure computational time. • We assume a generic one-processor random-access machine (RAM) model of computation. In the RAM model, instructions are executed one after another, with no concurrent operations. • The RAM model contains arithmetic, data movement, and control instructions. Each such instruction takes a constant amount of time. • The data types in the RAM model are integer and floating point. CS583 Fall'06: Introduction

  15. Insertion Sort Analysis • Note that the loop statement is executed one extra time to exit from it. • Let t_j be the number of time the while loop is executed for that value of j. INSERTION-SORT (A, n) costtimes 1 for j=2 to n c1 n 2 key=A[j] c2 n-1 3 // insert A[j] into sorted A[1..j-1] 4 i=j-1 c4 n-1 5 while i>0 and A[i]>key c5 j=2..n tj 6 A[i+1] = A[i] c6 j=2..n (tj-1) 7 i = i-1 c7 j=2..n (tj-1) 8 A[i+1] = key c8 n-1 CS583 Fall'06: Introduction

  16. Insertion Sort Analysis (Cont.) Total cost T(n) is: T(n) = c1*n +c2*(n-1) + c4*(n-1) + c5*j=2..n tj +c6*j=2..n(tj-1) + c7*j=2..n(tj-1) + c8(n-1) The best case occurs when the array is already sorted: T(n) = c1*n +c2*(n-1) + c4*(n-1) + c8(n-1) = (c1+c2+c4+c5+c8)n – (c2+c4+c5+c8) The running time can be expressed as an+b, i.e. a linear function of n. CS583 Fall'06: Introduction

  17. Insertion Sort Analysis (Cont.) If the array is in reverse order – the worst case results. In this case tj = j, which means j=2..n tj = n(n+1)/2 -1 j=2..n (tj-1) = n(n-1)/2 T(n) = (c5/2+c6/2+c7/2)n2 + (c1+c2+c4+c5/2-c6/2-c7/2+c8)n – (c2+c4+c5+c8) The worst-case running time can be expressed as an^2+bn+c; it is thus a quadraticfunction of n. CS583 Fall'06: Introduction

  18. Order of Growth • It is the rate of growth of the running time that really interests us. We therefore consider only a leading term of a formula in the running time and ignore the constant. • We write the insertion sort worst-case running time as (n2). CS583 Fall'06: Introduction

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