Programming for Engineers in Python

Transcription

1 Programming for Engineers in Python Lecture 9: Sorting, Searching and Time Complexity Analysis Autumn

2 Lecture 8: Highlights Design a recursive algorithm by 1. Solving big instances using the solution to smaller instances 2. Solving directly the base cases Recursive algorithms have 1. Stopping criteria 2. Recursive case(s) 3. Construction of a solution using solution to smaller instances 2

3 Information Today Importance of quick access to information How can it be done? Preprocessing the data enables fast access to what we are interested in Example: dictionary The most basic data structure in Python: list Sorting preprocessing Searching fast access Time complexity 3

4 Information There are about 20,000,000,000 web pages in the internet 4

5 Sorting A sorted array is an array whose values are in ascending/descending order Very useful Sorted array example: Super easy in Python the sorted function 5

6 Why is it Important to Sort Information? To find a value, and fast! Finding M values in a list of size N Naive solution: given a query, traverse the list and find the value Not efficient, average of N/2 operations per query Better: sort the array once and than perform each query much faster 6

7 Why not Use Dictionaries? Good idea! Not appropriate for all applications: Find the 5 most similar results to the query Query's percentile We will refer to array elements of the form (key, value) 7

8 Naïve Search in a General Array Find location of a value in a given array 8

9 Binary Search (requires a sorted array) Input: sorted array A, query k Output: corresponding value / not found Algorithm: Check the middle element in A If the corresponding key equals k return corresponding value If k < middle find k in A[0:middle-1] If k > middle find k in A[middle+1:end] 9

10 Example Searching for 56 index value

11 Example Searching for 4 index value

12 Code Binary Search 12

13 Binary Search 2 nd Try 13

14 Time Complexity Worst case: Array size decreases with every recursive call Every step is extremely fast (constant number of operations - c) There are at most log 2 (n) steps Total of approximately c*log 2 (n) For n = 1,000,000 binary search will take 20 steps - much faster than the naive search 14

15 15 Time Complexity על רגל אחת Algorithms complexity is measured by run time and space (memory) Ignoring quick operations that execute constant number of times (independent of input size) Approximate time complexity in order of magnitude, denoted with O ( Example: n = 1,000,000 O(n 2 ) = constant * trillion (Tera) O(n) = constant * million (Mega) O(log 2 (n)) = constant * 20

16 Order of Magnitude n log 2 n n log 2 n n ,096 65,536 1,048,576 16,777, ,048 65, ,152 16,777, ,048,565 4,294,967, ,971,520 1,099,511,627, ,653, ,474,976,710,656 16

17 17 Graphical Comparison

18 Code Iterative Binary Search 18

19 Testing Efficiency Preparations (Tutorial for timeit: 19

20 Testing Efficiency 20

21 Results 21

22 Until now we assumed that the array is sorted How to sort an array efficiently? 22

23 Bubble Sort מיון בועות נסרוק את המערך ונשווה כל זוג ערכים שכנים נחליף ביניהם אם הם בסדר הפוך נחזור על התהליך עד שלא צריך לבצע יותר החלפות )המערך ממויין( למה בועות? האלגוריתם "מבעבע" בכל סריקה את האיבר הגדול ביותר למקומו הנכון בסוף המערך. 23

24 Bubble Sort Example (done)

25 Code Bubble Sort n iterations i iterations constant (n-1 + n-2 + n ) * const ~ ½ * n 2 25

26 דוגמאות לחישוב סיבוכיות זמן ריצה מצא ערך מקסימלי במערך לא ממויין מצא ערך מקסימלי במערך ממויין מצא את הערך החמישי הכי גדול במערך ממויין מצא ערך מסויים במערך לא ממויין מצא ערך מסויים במערך ממויין ענה על n "שאלות פיבונאצ'י" שאלת פיבונאצ'י: מהו הערך ה- K נניח ש- K מוגבל להיות קטן מ- MAX בסדרת פיבונאצ'י? 26

27 We showed that it is possible to sort in O(n 2 ) Can we do it faster? Yes! Merge Sort 27

28 Comparing Bubble Sort with sorted 28

29 The Idea Behind Merge Sort Sorting a short array is much faster than a long array Two sorted arrays can be merged to a combined sorted array quite fast (O(n)) 29

30 Generic Sorting We would like to sort all kinds of data types Ascending / descending order What is the difference between the different cases? Same algorithm! Are we required to duplicate the same algorithm for each data type? 30

31 The Idea Behind Generic Sorting Write a single function that will be able to sort all types in ascending and descending order What are the parameters? The list Ascending/descending order A comparative function 31