Compsci 201, O-Notation and Maps (Interfaces too) Owen Astrachan ola@cs.duke.edu http://bit.ly/201spring19 February 8, 2019 2/8/19 Compsci 201, Spring 2019: O-Notation + Maps

Compsci 201, Spring 2019: O-Notation + Maps I is for … Interface LinkedList implements List Inheritance LinkedList extends AbstractSequentialList 2/8/19 Compsci 201, Spring 2019: O-Notation + Maps

Compsci 201, Spring 2019: O-Notation + Maps PFmMaBBD Interfaces: List, Set, and Map When it makes sense to use general type Empirical and Analytical measures of efficiency Maps: API and Problem Solving Keys and Values Big-Oh and O-Notation Building a mathematical formalism with intuition 2/8/19 Compsci 201, Spring 2019: O-Notation + Maps

Problems and Solutions String that occurs most in a list of strings? CountingStringsBenchmark.java, two ideas See also CountingStringsFile for same ideas https://coursework.cs.duke.edu/201spring19/classwork-spring19 Parallel arrays: word[k] occurs count[k] times Use ArrayLists: 2 “the”, 3 “fat”, 4 “fox” the fox cried fat tears 2 4 1 3 5 0 1 2 3 4 2/8/19 Compsci 201, Spring 2019: O-Notation + Maps

Compsci 201, Spring 2019: O-Notation + Maps How does the code work? Process each string s First time words.add(s),counter.add(1) Otherwise, increment count corresponding to s c[x] += 1 ? 2/8/19 Compsci 201, Spring 2019: O-Notation + Maps

What is complexity of this code? Search for each word and … if occurs at k +1 to counter.get(k), else add at end Search complexity? O(M) when M different words One search is O(M) – what about all searches? Tracking all words. First time zero, then one, … Avoid analyzing duplicates for the moment Will take longer if we have multiple occurrences of some of M words 2/8/19 Compsci 201, Spring 2019: O-Notation + Maps

Compsci 201, Spring 2019: O-Notation + Maps Tracking N strings Complexity of search? O(M) for M different words Complexity of words.indexOf(..) is O(M) what about all calls? 1 + 2 + … N is N(N+1)/2 O(n2) 2/8/19 Compsci 201, Spring 2019: O-Notation + Maps

Should we be more precise? Adding M different words will be O(M2) 1 + 2 + … + M = M(M+1)/2 Adding duplicates: we need to be precise about adding N total words. Sometimes word will be found, still O(M) for M different words We have both M and N here, but treat M == N for easier analysis. 2/8/19 Compsci 201, Spring 2019: O-Notation + Maps

Understanding O-notation This is an upper bound and in the limit Coefficients don’t matter, order of growth N + N + N + N = 4N is O(N) --- why? N*N is O(N2) – why? O(1) means independent of N, constant time In analyzing code and code fragments Account for each statement How many teams is each statement executed? 2/8/19 Compsci 201, Spring 2019: O-Notation + Maps

Running times in seconds machine: 109 instructions/sec O(log N) O(N) O(N log N) O(N2) 10 3E-9 1E-8 3.3E-8 0.0000001 100 7E-9 1E-7 6.64E-7 0.0001 1,000 1E-6 0.00001 0.001 10,000 1.3E-8 0.0001329 0.102 100,000 1.7E-8 0.001661 10.008 1,000,000 0.00000002 0.0199 16.7 min 1,000,000,000 0.00000003 1.002 65.8 31.8 years This slide has errors, replaced by new version in slides 10 2/8/19 Compsci 201, Spring 2019: O-Notation + Maps

Compsci 201, Spring 2019: O-Notation + Maps Just Say No.. When you can O(n2) Quadratic is too slow. We can do better. Our goal is linear or better. We cannot always do this, but we can with HashSet 2/8/19 Compsci 201, Spring 2019: O-Notation + Maps

Example: Analyze using big-Oh What is runtime of stuff(N) How to reason about this What is return value of stuff(N) What if code changes to sum += k 2/8/19 Compsci 201, Spring 2019: O-Notation + Maps

Counting for O-notation Why is O(1) complexity of sum += n Is this O(1) for any x += y ? Loop executes N times, doing O(1) per iteration Total runtime for method? O(n) 2/8/19 Compsci 201, Spring 2019: O-Notation + Maps

Practice with O-Notation What is big-Oh of runtime of call calc(N) ? Number of statements executed, O(1) for 146? Use calc(16) and generalize What is big-Oh of value returned by calc(N) ? Table? N = 1, 2, 4, 8, 16 2/8/19 Compsci 201, Spring 2019: O-Notation + Maps

Compsci 201, Spring 2019: O-Notation + Maps WOTO http://bit.ly/201spring19-feb8-1 2/8/19 Compsci 201, Spring 2019: O-Notation + Maps

Compsci 201, Spring 2019: O-Notation + Maps Luis von Ahn Duke 2000, Math Duke Honorary Degree 2017 CEO Duolingo Macarthur Award, 2006 MIT-Lemelson Prize, 2018 Shafi is a serious theoretical computer scientist. Not much to do here but read the bullets and the quote “It’s amazing how motivating it is to sit with somebody and say, ‘What you’re doing is really important.’ I use that a lot.” 2/8/19 Compsci 201, Spring 2019: O-Notation + Maps