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What makes a good language

What makes a good language. Does the task you want Keeps you from making mistakes Supports debugging when you need it Has a strong tool kit. Big number bug.

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What makes a good language

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  1. What makes a good language • Does the task you want • Keeps you from making mistakes • Supports debugging when you need it • Has a strong tool kit

  2. Big number bug On June 4, 1996 an unmanned Ariane 5 rocket launched by the European Space Agency exploded just forty seconds after its lift-off from Kourou, French Guiana. The rocket was on its first voyage, after a decade of development costing $7 billion. The destroyed rocket and its cargo were valued at $500 million. A board of inquiry investigated the causes of the explosion and in two weeks issued a report. It turned out that the cause of the failure was a software error in the inertial reference system. Specifically, a 64 bit floating point number relating to the horizontal velocity of the rocket with respect to the platform was converted to a 16 bit signed integer. The number was larger than 32,768, the largest integer that could be stored in a 16 bit signed integer, and so the conversion failed.

  3. Pentium II bug • Software bug encoded in hardware • Division algorithm uses a lookup table of 1066 entries • Only 1061 of the entries are downloaded to the PLA (programmed logic array from which the data are used) • Intel had to recall all versions of the chip

  4. Syntax “typo” bugs • NASA Mariner 1 , Venus probe (1992) • Intended to be the first US spacecraft to visit another planet, it was destroyed by a range officer on 22 July 1962 when it behaved erratically four minutes after launch. • Essentially a period instead of a comma in a FORTRAN DO-Loop

  5. Control flow bug • AT&T long distance service fails for nine hours(Wrong BREAK statement in C code) • January 15, 1990: • 70 million of 138 million long distance customers in the US lost long distance service. • Cost to ATT was between $ 75 Million and $100 Million (plus the loss of good will).

  6. Data structure management bug • E-mail buffer overflow (1998) • Several E-mail systems suffer from a "buffer overflow error", when extremely long e-mail addresses are received.  The internal buffers receiving the addresses do not check for length and allow their buffers to overflow causing the applications to crash.  Hostile hackers use this fault to trick the computer into running a malicious program in its place.

  7. Summary • Programming is difficult • Have to thoroughly understand the task • Have to anticipate all possibilities • Code is written at a fairly primitive level • Impossible to anticipate what users might do • Programming languages allow the user to use tools to build code • But everything still has bugs • The cost of a bug can be very large • There is no Moore’s Law for software.

  8. The big picture • We built a computer • We talked about languages and compilers to make programming the computer easier • Next we talk about algorithms, which are implemented by programs

  9. Algorithms • Recipes for doing computations • The underpinnings of programming • Think out your algorithm • Show that it works • Determine it’s efficiency • Write it as a program

  10. What is an algorithm • Algorithm is a recipe • Has • Inputs • Rules • Evaluation Criteria • Output

  11. When Do We Use Algorithms • When we have a problem to solve • Examples of problems • Baking cookies • Putting things in alphabetical order • Searching the Web

  12. Chocolate chip cookies

  13. Chocolate chip cookies • Input • flour (2 ¼ c) • baking soda (1t) • salt (1t) • butter (1c) • granulated sugar (3/4 c) • brown sugar(3/4c) • vanilla(1t) • eggs (2) • chocolate chip morsels (2c) • chopped nuts (1c) • Output • 5 dozen cookies

  14. Chocolate chip cookies: Steps • Combine flour, baking soda, and salt in small bowl. • Beat butter, granulated sugar, brown sugar and vanilla in large bowl • Add eggs one at a time Beating after adding each egg • Gradually beat in flour mixture • Stir in morsels and nuts • Drop by rounded tablespoons onto ungreased baking sheets • Bake 9-11 minutes • Let stand for 2 minutes

  15. Chocolate chip cookie algorithm • Primitives • Inputs • Flour, baking soda, salt, butter, brown sugar, granulated sugar, vanilla, egg, morsels, nuts • Alternatively, chocolate chip cookie mix • Alternatively, wheat, sugar cane, … • Operators • Combine, Beat, Gradually beat, Stir, Drop, Bake, Let stand

  16. Chocolate chip cookie algorithm • Execution • First 2 steps can be done in parallel? • Parbegin (Combine(),Beat()) Parend • Machine dependencies • Ovens vary (Bake 9-11 minutes) • Ingredients vary and so need to be handled differently

  17. Chocolate chip cookie algorithm • Algorithm testing • Proof of the pudding is in the eating • How do we mechanize this?

  18. Chocolate chip cookie algorithm • Comparing different algorithms • Quality of results • User time • Machine (oven) time

  19. Putting things in alphabetical order • Data set sizes • Course list for COS 111 10-15 students • PU directory assistance 10,000 people • Manhattan phone book 1 million people • Social Security database 1 billion records • Long distance call billing records 100 billion/year • Different methods for different tasks • Fast for large • Simple for small

  20. A simple method for sorting • Find smallest value -- put it first in list • Find second smallest value -- put it second • … • Find next smallest value – put it next • … • When no more values, you’re done

  21. How it works

  22. How it works Find smallest value -- put it first in list

  23. How it works Find second smallest value -- put it second

  24. How it works Finish the sorting

  25. A simple method for sorting • To sort array x = {x[1],x[2], … , x[n]} ForI= 1 to n ForJ=I+1 to n If (x[I] > x[J]) Then swap their values next next

  26. Another sorting algorithm • Sorting by Merging • Key idea  It’s easy to merge 2 sorted lists

  27. Merging 2 sorted lists

  28. Merging 2 sorted lists Start at the top of each list

  29. Merging 2 sorted lists 190 is bigger than 155

  30. Merging 2 sorted lists Record 155 and move the arrow

  31. Merging 2 sorted lists 190 is less than 255

  32. Merging 2 sorted lists 219 is less than 255

  33. Merging 2 sorted lists 255 is less than 463

  34. Merging 2 sorted lists Finished when at the end of each list

  35. Sorting by Merging • Key idea  It’s easy to merge 2 sorted lists • Sort larger lists by • Sorting smaller lists • Merging the results • How do we sort smaller lists?

  36. Sort then merge Subdivide

  37. Sort then merge Sort pieces By merging Subdivide

  38. Sort then merge Sort pieces Merge Subdivide

  39. SortMerge algorithm Function SortMerge(x,1,n) -- sort list of n elements x If n = 1 then Return -- nothing to sort Endif Mid = (1+ n)/2 -- midpoint of list SortMerge(x,1, Mid ) -- sort first half of list SortMerge(x, Mid +1, n) -- sort second half of list Merge(x,1, Mid , Mid +1, n) -- merge sorted halves End Function

  40. Does it work? • Have to be careful about stopping • There are always a lot of things going on

  41. Divide and conquer • Use recursion • reduce solving for problem of size n to solving two problems of size n/2 • then combine the solutions • S(n) = 2 S(n/2) + M(n/2,n/2) • Solving a sorting problem of size n requires solving 2 sorting problems of size n/2 and doing a merge of 2 sets of size n/2

  42. Comparing running times

  43. Comparing running times Reducing 20 hours to 3 seconds

  44. Searching • Once a list is in alphabetical order, how do you find things in it? • For example, is COS 111 on the list of courses that satisfy the (EC) Epistemology and Cognition requirement?

  45. EC courses AAS 391 ANT 201 COS 302 FRS 135 FRS 137 GER 306 HUM 365 LIN 213 LIN 302 LIN 306 LIN 315 PHI 200 PHI 201 PHI 204 PHI 301 PHI 304 PHI 312 PHI 321 PHI 333 PHI 338 PSY 255 PSY 306 PSY 307 PSY 316

  46. Searching for COS 111 AAS 391 ANT 201 COS 302 FRS 135 FRS 137 GER 306 HUM 365 LIN 213 LIN 302 LIN 306 LIN 315 PHI 200 PHI 201 PHI 204 PHI 301 PHI 304 PHI 312 PHI 321 PHI 333 PHI 338 PSY 255 PSY 306 PSY 307 PSY 316 COS 111 Compare to the middle

  47. Searching AAS 391 ANT 201 COS 302 FRS 135 FRS 137 GER 306 HUM 365 LIN 213 LIN 302 LIN 306 LIN 315 PHI 200 PHI 201 PHI 204 PHI 301 PHI 304 PHI 312 PHI 321 PHI 333 PHI 338 PSY 255 PSY 306 PSY 307 PSY 316 If larger search second half If smaller search first half COS 111 Compare to the middle

  48. Repeat If smaller search first half AAS 391 ANT 201 COS 302 FRS 135 FRS 137 GER 306 HUM 365 LIN 213 LIN 302 LIN 306 LIN 315 PHI 200 If larger search second half COS 111 Compare to the middle

  49. AAS ANT COS FRS GER HUM LIN PHI PSY Building indices AAS 391 ANT 201 COS 302 FRS 135 FRS 137 GER 306 HUM 365 LIN 213 LIN 302 LIN 306 LIN 315 PHI 200 PHI 201 PHI 204 PHI 301 PHI 304 PHI 312 PHI 321 PHI 333 PHI 338 PSY 255 PSY 306 PSY 307 PSY 316

  50. AAS ANT COS FRS GER HUM LIN PHI PSY Search indices then data AAS 391 ANT 201 COS 302 FRS 135 FRS 137 GER 306 HUM 365 LIN 213 LIN 302 LIN 306 LIN 315 PHI 200 PHI 201 PHI 204 PHI 301 PHI 304 PHI 312 PHI 321 PHI 333 PHI 338 PSY 255 PSY 306 PSY 307 PSY 316 COS 111

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