| Algorithms are a familiar idea. Our goal is to learn to specify them right so someone or something else does the work |
| Algorithm, a precise, systematic method to produce a specified result | |||
| We have seen algorithms already... | |||
| Placeholder technique is an algorithm with an easy specification: | |||
| longStringWithShortStringInIt ¬ placeholder | |||
| ShortString ¬ e | |||
| placeholder ¬ longStringWithShortStringInIt | |||
| For an algorithm to be well specified it must have … | |||
| Inputs specified | |||
| Outputs specified | |||
| Definiteness | |||
| Effectiveness | |||
| Finiteness | |||
| A program is an algorithm specialized to a particular situation | |||
| Algorithm: | |||
| longStringWithShortStringInIt ¬ placeholder | |||
| ShortString ¬ e | |||
| placeholder ¬ longStringWithShortStringInIt | |||
| Program: ¿¿ ¬ # | |||
| ¿ ¬ e | |||
| # ¬ ¿¿ | |||
| 1. Def Artist_of Use Artist_of to refer to the group name | |||
| 2. Pick Alpha Decide which end of rack is to be start of alphabetic sequence, and call the first slot alpha | |||
| 3. Pick Beta Call the slot next to alpha, beta | |||
| 4. Exchange If Artist_of the CD in the alpha slot is later in the alphabet than the Artist_of the CD in the beta slot, interchange the CDs, otherwise continue on | |||
| 5. More Betas? If a slot follows beta slot, begin calling it the beta slot and go to step 4, otherwise continue on | |||
| 6. More Alphas? If two slots follow the alpha slot, begin calling the next one the alpha slot and the one following it the beta slot; go to step 4; otherwise stop | |||
| Abstraction means removing an idea or process form a situation | |||
| Beta sweep -- while alpha points to a fixed slot, beta sweeps through slots following alpha, interchanging as necessary | |||
| The beta sweep is a concept removed based on our understanding of the operation of the algorithm | |||
| By abstracting we can analyze parts of an algorithm … | |||
| The beta sweep has 4 properties: | |||
| Exhaustive -- it considers all CDs after alpha | |||
| Non-redundant -- no slot pair is checked twice | |||
| Progressive -- the alphabetically earliest CD considered so far is always in the alpha slot | |||
| Effective -- at completion, the alphabetically earliest CD from alpha to end is in alpha slot | |||
| The alpha sweep... | |||
| Process of sweeping through all of the CDs (but the last) performing the beta sweep | |||
| Exhausitve -- considers all but last CD | |||
| Non-redundant -- a slot is alpha only once | |||
| Progressive -- when beta sweep completes the alphabetically next CD in alpha | |||
| Complete -- when last beta sweep is done the last slot’s CD is later than next to last slot | |||
| Effective -- the alpha sweep alphabetizes | |||
| We figure out most algorithms on our own, abstracting from specific cases | ||
| Also we abstract parts of an algorithm or program to understand them | ||
| Thinking of how the program works and reasoning about its properties allows us to know why an algorithm works … and then we can let the computer do it | ||
| Google Bombing: To sabotage Google’s page-rank system |
| Many pages make their anchor text ‘miserable failure’ and make the anchor link to the Bush biography |