# What Computers Can’t Do For You Catherine C. McGeoch Amherst College February 2005.

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What Computers Can’t Do For You Catherine C. McGeoch Amherst College February 2005

OR/MS and Computer Science: Connections Theory Systems and Languages Problem Domains Management Science Operations Research Finance Marketing Softwar e

Three Themes (with variations) Theory: You can’t always get what you want. (Jagger) Software Engineering: Everything Put Together Falls Apart. (Simon) Policy: Oops!..I did it again. (Spears) We’re going wrong. (Cream)

Theory and Practice and OR... Theory Practice My Research

What is experimental algorithmics?

Hockey Science: Predict the in- game puck velocities generated by new stick technologies.

Isolate Components Attach Probes Control Key Factors

Complexity Theory World View Unsolvable problems Intractable problems Tractable problems Interesting non-problems

An Unsolvable Problem The Halting Problem: Take a program P and an input I for P. Will P go into an infinite loop, or will it eventually halt? Program H: Reads P and I, computes, always correctly answers yes or no, and never itself goes into an infinite loop. Theorem: Program H cannot exist.

Intractable: Can be solved, but not efficiently Input Size Algorithm Operations

Algorithm time as a function of input size:

NP-Hard and NP-Complete Problems Unsolvable Intractable Tractable Hundreds of problems, including many from Operations Research Which?

NP-Completeness ``I can’t find an efficient algorithm, I guess I’m just too dumb.’’

NP-Completeness `` I can’t find an efficient algorithm, because no such algorithm is possible!’’

NP-Completeness ``I can’t find an efficient algorithm, but neither can all these famous people.’’

NP-Completeness Humbug! Try harder! Now get out of my office!

Lessons from Experimental Algorithmics NP-Completeness Theory is... Pessimistic. Worst-case assumptions about input; your input may be fine. Usually about finding optimal solutions. Almost-optimal might be tractable. Not the final word.... Constructive heuristics Approximation Algorithms Heuristic search...Solutions to your problems. With luck and compromise, algorithm research and experimental algorithmics can give you...

Example: Traveling Salesperson Problem What is the biggest problem you can solve in one day of computing? Theory: about n=20 cities, worst case. Practice: Some instances of size n>10,000 have been solved.

Theme from Theoretical Computer Science You can’t always get what you want ~ but if you try sometimes ~ you might find ~ you get what you need. Oh yeah, hey hey hey, oh...

Software Engineering Methods for efficient development of large software systems. Ensure reliability, robustness, safety. Predict development cycles and times.

Software Engineering But accidents happen!

Royal Australian Navy Frigate backs onto rocks after the computer overrode manual command. Long-standing errors found in the national urology medical student match system; code was redone this year. New kinds of spoofing possible with DNS policy change. Serious security flaws found in Windows Eudora. In-car computers corrupted by virus via cell phones. catless.ncl.ac.uk/Risks Friendly fire from Patriot missile. Records database (35k records) stolen; at least 50 cases of identity fraud so far. German TollCollect system charges trucks not on the toll road. 1200 gallons of gas sold at.19/gallon.

Royal Australian Navy Frigate backs onto rocks after the computer overrode manual command. Long-standing errors found in the national urology medical student match system; code was redone this year. New kinds of spoofing possible with DNS policy change. Serious security flaws found in Windows Eudora. In-car computers corrupted by virus via cell phones. catless.ncl.ac.uk/Risks Friendly fire from Patriot missile. Records database (35k records) stolen; at least 50 cases of identity fraud so far. German TollCollect system charges trucks not on the toll road. 1200 gallons of gas sold at.19/gallon. And that’s just since Sunday...

Worse things have happened... Industrial robots kill workers. Woman kills children, attempts suicide, after erroneous HIV notification. Therac-14. Six people die from radiation overdoses from cancer therapy machine. Flight from New York to Brazil crashes into mountain due to human/computer navigation error. U.S. Navy mistakenly shoots down Iranian passenger plane.

Lessons Software fails when ported inappropriately to new applications. Some error-detection problems are unsolvable. Many are intractable. Humans should not override computer controls. Computers should not override human controls.

Theme from Software Engineering Uh, huh, spare your heart ~ everything put together sooner or later falls apart ~ there’s nothing to it, nothing to it ~ you can cry ~ you can lie ~ for all the good it’ll do you.

Policy: Laws with Unintended Consequences CAN-SPAM Act is blamed for increase in spam from 50 to 80 percent of emails. Patriot Act allows unprecedented electronic intrusions into privacy. DMCA prohibits actions that are legal with non-electronic media (like books and papers). Electronic spyware laws are different from standard spying laws.

Theme for Policy-Makers Please open your mind ~ see what you can find ~ I found out today ~ we’re going wrong ~ we’re going wrong ~ we’re going wrong.

Conclusions Not all problems can be solved by computers. Even when they can be, software is brittle. Computers can’t make policy. Neither can policy-makers. Have a nice day.

Information Technology has the potential to make our lives safer, healthier, and more enjoyable and to enrich our understanding of nature. A future that includes improvements in our quality of life through applications of IT looks very bright. --William Wulf Human Genome Project Gutenberg Project iPOD World Wide Web Assistive Technologies But things are not all bad. The future is what we make of it... Online Communities Drug discovery cycle Safer highways Safer air travel

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