Chapter 5 Algorithms (2) Introduction to CS 1 st Semester, 2015 Sanghyun Park

Outline  Informal Definition of an Algorithm(previous file)  FindLargest(previous file)  Three Basic Constructs(previous file)  Sorting Algorithms(previous file)  Searching Algorithms(previous file)  Recursion(previous file)  Algorithm Performance  Time Complexity  Asymptotic Notation  Growth Rate

Algorithm Performance  Algorithm ____________ is the amount of computer memory and time needed to run an algorithm  How is it determined?  Analytically: performance ________  Experimentally: performance ____________  Space complexity is defined as the amount of ________ an algorithm needs to run to completion  Time complexity is defined as the amount of computer _____ an algorithm needs to run to completion

Time Complexity  How to measure?  Count a particular operation(________ counts)  Count the number of steps(_____ counts)  ___________ complexity  Running Example: _________ Sort for (int i = 1; i < n; i++)// n is the number of elements in an array { // insert a[i] into a[0:i−1] int t = a[i]; int j; for (j = i−1; j ≥ 0 && t < a[j]; j--) a[j+1] = a[j]; a[j+1] = t; }

Operation Count  Pick an ________ characteristic … n, n = the number of elements in case of insertion sort  Determine count as a ________ of this instance characteristic

Comparison Count  How many comparisons are made?  The number of comparisons depends on ___ and __ as well as on __  ______ case count = maximum count  ______ case count = minimum count  ______ case count for (int i = 1; i < n; i++) for (j = i−1; j ≥ 0 && t < a[j]; j--) a[j+1] = a[j];

Worst Case Comparison Count for (j = i−1; j ≥ 0 && t < a[j]; j--) a[j+1] = a[j];  a = [1,2,3,4] and t = 0→ __ comparisons  a = [1,2,3,4,…,i] and t = 0→ __ comparisons for (int i = 1; i < n; i++) for (j = i−1; j ≥ 0 && t < a[j]; j--) a[j+1] = a[j];  Total comparisons = …+(n−1) = ________

Asymptotic Complexity of Insertion Sort  O(__)  What does this mean?  Time or number of operations does not ________ c  n 2 on any input of size n  So, the worst case time is expected to ________ each time n is doubled

Growth Rates  It is often necessary to __________ the number of steps required to execute an algorithm  We can approximate one function with another function using a _______ rate  We will examine the asymptotic behavior of two functions f and g by comparing f(n) and g(n) for _____ positive values of n

Asymptotic Notation  Big O is an example of asymptotic notation  Bio O provides an ______ bound for an algorithm’s time performance  Other asymptotic notation  Big Omega notation provides a ______-bound for an algorithm’s time performance  Big Theta notation is used when an algorithm can be bounded _____ from above and below by the ______ function

Big Oh Notation  This is written f(n) = O(g(n)) and is stated, “f(n) is Big Oh g(n)”  A more mathematical definition of Big Oh is: f(n) = O(g(n)), if there are positive numbers c and m such that ____________ for all n ≥ m  Since 5n 3 +4n 2 <= 7n 3 for all n ≥ 2, 5n 3 +4n 2 = O(__)  Big Oh is often called an _______ bound

Big Omega Notation  “f has an order greater than or equal to g” if there are positive numbers c and m such that _____________ for all n ≥ m  This is written f(n) =  (g(n)) and is stated as “f(n) is Big Omega g(n)”  Big Omega is often called a ______ bound

Big Theta Notation  “f has same growth rate as g” if we can find a number m and two nonzero numbers c and d such that _________________ for all n ≥ m  Big Theta ( θ ) notation is used when an algorithm can be bounded both from _______ and _______ by the same function

Common Growth Rate Functions (1/2)  1 (constant) growth is ______________ of the problem size N  log 2 N (__________) growth increases slowly compared to the problem size (binary search)  N (_______) growth is directly proportional to the size of the problem  N * log 2 N typical of some divide and conquer approaches (_______ sort)

Common Growth Rate Functions (2/2)  N 2 (quadratic) typical in _______ loops  N 3 (______) more nested loops  2 N (___________) growth is extremely rapid and possibly __________  N!

Growth Rate log 2 N

Growth Rate N 2

Practical Complexities logNNNlogNN2N2 N3N3 2N2N