IS 313 : Putting loops to work for you

# IS 313 : Putting loops to work for you

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## IS 313 : Putting loops to work for you

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1. IS 313: Putting loops to work for you [ -35, -24, -13, -2, 9, 20, 31, ? ] [ 26250, 5250, 1050, 210, ? ] and which one is not like the others… ? What's next? [ 1, 11, 21, 1211, 111221, ? ] [ 2, 22, 222, ? ] [ 90123241791111 , 93551622, 121074, 3111, ? ] • Thinking loopily and cumulatively += for a while sounds natural enough to me!

2. Schedule 9/27 Loops, some graphics, and webapps 9/28 Homework #2 due 10/4 Maximizing… and webforms 10/5 Homework #3 due 10/11 Language-processing onthe web? 10/12 Homework #4 due 10/18 No class this day! 10/29 No HW due this day! 10/25 Objects & small projects 10/26 Homework #5 due

3. Self-altering statements? recursion is worse! Shortcuts for changing variables: or, even shortcuttier: age = 41 age = age + 1 age += 1 amoebas = 100000; amoebas = amoebas * 2 hwToGo = 8; hwToGo = hwToGo - 1 u235 = 10000000000000 u235 = u235 / 2;

4. fore! for x in range(8): print'x is', x print'Phew!' anatomy? empty? x unused?

5. fore! x is assigned each value from this sequence 1 for x in range(8): print'x is', x print'Phew!' 3 LOOP back to step 1 for EACH value in the list the BODY or BLOCK of the for loop runs with that x 2 anatomy? empty? x unused? Code AFTER the loop will not run until the loop is finished. 4

6. four on for for x in range(8): print'x is', x how about 6*x? sum the list? construct the list?

7. Accumulating an answer… Finding the sum of a list: def sum( L ): """ returns the sum of L's elements """ sum = 0 for x in L: sum = sum + x return sum Accumulator! Liar! That's not the sum! shortcuts? vs. recursion? sum every OTHER element?

8. for loops: selfless vs. selfish Element-based Loops sum = 0 for x in L: sum += x L = [ 42, -10, 4 ] x "selfless"

9. for loops: selfless vs. selfish Element-based Loops Index-based Loops sum = 0 for x in L: sum += x sum = 0 for i in : sum += i 1 2 0 L = [ 42, -10, 4 ] L = [ 42, -10, 4 ] x these index-based loops seem so egocentric

10. for loops: selfless vs. selfish Element-based Loops Index-based Loops sum = 0 for x in L: sum += x sum = 0 for i inrange(len(L)): sum += L[i] i 1 2 0 L = [ 42, -10, 4 ] L = [ 42, -10, 4 ] x L[i]

11. Perspective on for loops At the top of a project file … // Author: Matt Beaumont // Purpose: To get me out of CS... // ...no, really... // Purpose: To create and maintain a list // of films and directors /* Notes: * I haven't liked for-loops since the day I met them. * They bother me for some reason. Hence, no for-loops… */ and it is possible to avoid them entirely…

12. Extreme Looping What does this code do? print'It keeps on' whileTrue: print'going and' print'Phew! I\'m done!'

13. Extreme Looping Anatomy of a while loop: the loop keeps on running as long as this test is True print'It keeps on' whileTrue: print'going and' print'Phew! I\'m done!' “while” loop This won't print until the while loop finishes - in this case, never! alternative tests?

14. Making our escape! import random escape = 0 whileescape != 42: print'Help! Let me out!' escape = random.choice([41,42,43]) print'At last!' how could we count the number of loops we run? how could we make it easier/harder to escape?

15. Procrastination Programming Every while loop can be a whileTrue: loop! - just be sure to break!

16. Robot Loops?! Robots run programs that are variants of this very basic loop: while True: sense() plan() act() This is the "spa" architecture: it's almost universal. Which of those three pieces is the most difficult!? Robot of the Day... How to escape ?

17. Give me a break ! import random escape = 0 whileTrue: print'Help! Let me out!' escape = random.choice([41,42,43]) if escape == 42: break print'At last!' I'll figure out later how to get out of this loop! OK – I'll stop the loop now and continue with the code after the loop comparereturn

18. Loops aren't just for lists… for c in'down with CS!': print c

19. def cc( s ): Write a loop to find and return the min of a list, L n = 0 for c in s: if c not in'aeiou': n += 1 return n L is a list of numbers. def mymin( L ): how could you "accumulate" the minimum? What does this return? >>> cc( 'forty-two' ) def odd( N ): Extra! steps = 0 while N > 1: if N%2==0: N /= 2 else: N = 3*N+1 steps += 1 return steps What does this return? >>> odd( 3 )

20. What does this return? def cc( s ): n = 0 for c in s: if c not in'aeiou': n += 1 return n >>> cc( 'forty-two' )

21. Extra! def odd( N ): steps = 0 while N > 1: if N%2==0: N /= 2 else: N = 3*N+1 steps += 1 return steps >>> odd( 3 )

22. L is a list of numbers. def min( L ):

23. Homework 3 Hooray… Sanity! Sequences… #2 one note on these... What is this stuff? or you could be saying both… Sound… #3 Graphics… #4

24. Look-And-Say Sequences(aka “Read-It-And-Weep”) str versus float or int 1 11 21 1211 111221 312211 ? When does the first 4 appear?

25. Homework 3 Sequences… #2 or you could be saying both… Sound… #3 DEMO! Graphics… #4

26. Representing Sound physics continuous plot of air pressure vs. time sampling samples taken every ~ 1/22050th of a second quantization Each sample is measured on a scale from -32,768 to 32,767. (This fits into 2 bytes.) storage These two bytes are called a frame. Raw audio data - such as what is written to the surface of a CD - is simply a list of these frames. pulse code modulation = PCM data

27. Homework 3 Sequences… #2 or you could be saying both… Sound… #3 DEMO! Graphics… #4

28. Etch-a-Sketch ? www.gvetchedintime.com No way this is real… but it is !

29. Python's Etch-a-Sketch Be SURE to start IDLE with "no subprocess" Mac instructions... Windows instructions... (Step 1) Go to the Start menu's search field (Step 1) menus: Go – Utilities – Terminal Start this! (Step 2) Type or paste the full path to IDLE If you used the standard install, it will be C:\Python27\Lib\idlelib\idle.pyw -n (Step 2) Enter idle –n & Hit return; you'll see a console window and IDLE open. Then, you can type from turtle import *

30. Python's Etch-a-Sketch from turtle import * Turtle Canvas reset() left(90) forward(50) right(90) backward(50) down() or up() color('green') tracer(1)or tracer(0) width(5) and lots more! degrees! (42,42) states if the pen draws or not window_height() (0,0) states if the pen animates or not window_width() www.cs.hmc.edu/twiki/bin/view/CS5/TurtleDirections for turtle installation and help

31. Recursive Graphics there is no tri … Could we tri this with recursion? (1) def tri(): """ draws a polygon """ forward(100) left(120) forward(100) left(120) forward(100) left(120) def tri( ): """ draws a polygon """ Could we create any regular n-gon? (2) I don't know about tri, but there sure's NO return … !

32. Turtle Graphics Name(s): Finish rwalk to draw a "stock-market-type" random path of nsteps steps. Use recursion! (1) What does chai draw? (2) from random import * def rw(nsteps): """ moving for nsteps steps, randomly 45 deg. left/up or right/down """ if nsteps == 0: return if choice(['left','right']) == 'left': else: # 'right' def chai(size): """ mystery! """ forward(size) left(90) forward(size/2.0) right(90) right(90) forward(size) left(90) left(90) forward(size/2.0) right(90) backward(size) Ex Cr: How could you make it a bull (or a bear) market? one possible result of rw(20)

33. (1) What does chai draw? def chai(size): """ mystery! """ forward(size) left(90) forward(size/2.0) right(90) right(90) forward(size) left(90) left(90) forward(size/2.0) right(90) backward(size) Why are there two identical commands in a row? How could you add more to each end?

34. (2) Finish rwalk to draw a "stock-market-type" random path of nsteps steps. from random import * def rw(nsteps): """ moving for nsteps steps, randomly 45 deg. left/up or right/down """ if nsteps == 0: return if choice(['left','right']) == 'left': else: # 'right' one possible result of rw(20) What if we didn't turn back to face forward each time? Ex Cr: How could you make it a bull (or a bear) market?

35. hw3pr4 spiral 81 72.9 90 close-up of innermost part of the spiral… spiral( 100, 90, 0.9 ) 100 spiral( initLength, angle, multiplier )

36. hw3pr4 svTree Level 1 Level 2 Level 3 Level 4 svTree( 100, 4 ) I wonder what happened to the leaves on that tree… ? svTree( trunkLength, levels )

37. hw2pr3 spiral 81 72.9 90 close-up of innermost part of the spiral… spiral( 100, 90, 0.9 ) 100 spiral( initLength, angle, multiplier )

38. hw2pr3 svTree Level 1 Level 2 Level 3 Level 4 svTree( 100, 4 ) I wonder what happened to the leaves on that tree… ? svTree( trunkLength, levels )

39. The Koch curve snowflake( 100, 0 ) snowflake( 100, 1 ) snowflake( 100, 2 ) snowflake( 100, 3 ) snowflake( 100, 4 ) snowflake( 100, 5 )

40. CGI == Python + Web a very common technology for webapplications is the Common Gateway Interface or CGI Python Apache Firefox This week Last week Next week Next week any programs! webserver browser

41. CGI == Python + Web Basic idea Your Python file prints the HTML page you want! #!C:/Python27/python.exe import cgi import cgitb; cgitb.enable() print "Content-Type: text/html; \ charset=ISO-8859-1\n\n" print print""" <html> <body> <h1>Hello from Python!</h1> </body> </html> """ This week Python CGI

42. Lab/HW time I will be happy to help get your webserver running Python scripts. – That will help a lot with problem #1! Have fun!

43. Monty Hall Let’s make a deal ’63-’86 Sept. 1990 inspiring the “Monty Hall paradox”

44. Monty Hall Getting the user's input: answer = raw_input( 'What is your name?' ) door = input( 'Which door do you choose?' ) response = raw_input( 'Switch or stay?' ) Making decisions if response == 'switch': print 'So you switch to door', other_door But how to get the "other door" ?

45. Seeing into the future… def menu(): """ prints our menu of options """ print"(1) Input a list of numbers" print"(2) Guess the next element" print"(9) Quit" def seer(): """ handles user input for our menu """ whileTrue: menu() uc = input('Which option? ') print'The inner eye does not see upon command!'

46. Clearer Vision def seer(): """ handles user input for our menu """ whileTrue: menu() uc = input('Which option? ') print'The inner eye does not see upon command!' if uc ==

47. the gift… LightPath, September 1999

48. watching the charts… LightPath, six months later…

49. "brokerage" seems apt… LightPath, now…