lec12 - CS240 Programming in C Lecture 12 Polymorphic...

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Unformatted text preview: CS240: Programming in C Lecture 12: Polymorphic Sorting Monday, March 7, 2011 Sorting Given a collection of items and a total order over them, sort the collection under this order. Total order: every item is ordered with respect to every other time in the collection Given an array of numbers, sort the numbers in ascending order sort in descending order sort odd before even, with each sub-collection respecting an ordering Given an array of strings, sort according to lexiographic ordering Given an array of structures, sort the structures based on the value of the field named “index” Given an array of arrays, sort the lists based on the value of their least element Monday, March 7, 2011 Key Observation The code necessary to perform the sort swapping elements, rearranging the array as necessary works independently of the type of the elements being sorted. As long as the elements have a comparator that understands ordering, the same sorting function can be used to sort different kinds of elements Polymorphism: poly : many morphism : form 3 Monday, March 7, 2011 The Swap function 4 void swap(void *v, int i, int j) { void *temp; temp = v[i]; v[i] = v[j]; v[j] = temp; } A void type specifies an empty set of values. An expression that has a type denotes an expression where the value of that type is not required. Any pointer to an object can be converted to void * without loss of information, and can be recovered to the original pointer type. Thus, void is C’s mechanism to support a generic or polymorphic type A function with void * arguments thus accepts pointers of any type Monday, March 7, 2011 Quicksort 5 Start with: 06 34 69 33 75 64 04 74 25 95 15 58 78 36 51 73 13 27 End with: 04 06 13 15 25 27 33 34 36 51 58 64 69 73 74 75 78 95 Basic idea: 06 34 69 33 75 64 04 74 __ 95 15 58 78 36 51 73 13 27 pivot = 25 Pick an element (hopefully close to the middle) and sort around it __ 34 69 33 75 64 04 74 06 95 15 58 78 36 51 73 13 27 pivot = 25 Start examining elements to the right of the hole until we find an element smaller than the pivot +----------------------+ | | V | 04 34 69 33 75 64 __ 74 06 95 15 58 78 36 51 73 13 27 +--------------------+ | | | V 04 __ 69 33 75 64 34 74 06 95 15 58 78 36 51 73 13 27 Monday, March 7, 2011 Quicksort 6 After processing 06, 04 06 __ 33 75 64 34 74 69 95 15 58 78 36 51 73 13 27 pivot = 25 At the end of the process, we have: 04 06 15 13 __ 64 34 74 69 95 33 58 78 36 51 73 75 27 pivot = 25 Repeat the process for each of the two subarrays Recursion ends with a partition has only two elements Monday, March 7, 2011 In C .... 7 void qsort(void *v, int left, int right, int (*comp)(void *, void *)) { int i, last; void swap(void *v, int, int); if (left >=right) return; swap(v, left, (left + right)/2); last = left; for (i = left+1; i <= right; i++) if ((*comp)(v[i], v[left])) < 0 swap(v, ++last,i); swap(v, left, last); qsort(v, left, last-1, comp); qsort(v, last+1, right, comp)...
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This note was uploaded on 02/23/2012 for the course CS 240 taught by Professor Rego during the Spring '08 term at Purdue.

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lec12 - CS240 Programming in C Lecture 12 Polymorphic...

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