IS 313 : Putting loops to work for you

IS 313: Putting loops to work for youW

hat'

s next?

[ 1, 11, 21, 1211, 111221, ? ]

[ -35, -24, -13, -2, 9, 20, 31, ? ]

[ 2, 22, 222, ? ]

[ 26250, 5250, 1050, 210, ? ]

[ 90123241791111 , 93551622, 121074, 3111, ? ]

and which one is not like the

others… ?

• Thinking loopily and cumulativelysounds natural enough to me!for a while +=

Schedule

9/27 Loops, some graphics, and webapps

9/28 Homework #2 due

10/4 Maximizing… and webforms


10/11 Language-processing on the web?


10/18 No class this day!

10/29 No HW due this day!

10/25 Objects & small projects


Self-altering statements?

Shortcuts for changing variables:

age = 41

age = age + 1

amoebas = 100000;amoebas = amoebas * 2

hwToGo = 8;hwToGo = hwToGo - 1

u235 = 10000000000000u235 = u235 / 2;

age += 1

or, even shortcuttier:

recursion is worse!

fore!

for x in range(8):

print 'x is', x

print 'Phew!'anatomy?

empty?

x unused?

fore!

for x in range(8):

print 'x is', x

print 'Phew!'anatomy?

empty?

x unused?

x is assigned each value from this

sequence

the BODY or BLOCK of the for loop runs with that x

Code AFTER the loop will not run until the loop is finished.

1

2

3

4

LOOP back to step 1 for EACH value in the list

four on for

for x in range(8):

print 'x is', x

how about 6*x?

sum the list?

construct the list?

Accumulating an answer…

def sum( L ):

""" returns the sum of L's elements """

sum = 0

for x in L:

sum = sum + x

return sum

Finding the sum of a list:

Accumulator! Liar! That's not the sum!

shortcuts?

vs. recursion?

sum every OTHER element?

for loops: selfless vs. selfish

Element-based Loops

sum = 0

for x in L: sum += x

L = [ 42, -10, 4 ]

x

"selfless"


Element-based Loops

L = [ 42, -10, 4 ]

x

L = [ 42, -10, 4 ]

i0 1 2

Index-based Loops

sum = 0


sum = 0

for i in : sum +=

these index-based loops seem so egocentric


Element-based Loops

L = [ 42, -10, 4 ]

x

L = [ 42, -10, 4 ]

i0 1 2

Index-based Loops

sum = 0


sum = 0

for i in range(len(L)): sum += L[i]

L[i]

Perspective on for loops

// 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… */

At the top of a project file …

and it is possible to avoid them entirely…

Extreme Looping

What does this code do?

print 'It keeps on'

while True: print 'going and'

print 'Phew! I\'m done!'

Extreme Looping

Anatomy of a while loop:

print 'It keeps on'

while True: print 'going and'

print 'Phew! I\'m done!'

“while” loop

the loop keeps on running as long as this test is True

alternative tests?

This won't print until the while loop finishes - in this case, never!

Making our escape!

import randomescape = 0

while escape != 42: print 'Help! Let me out!' escape = random.choice([41,42,43])

print 'At last!'

how could we make it easier/harder to escape?

how could we count the number of loops we run?

Procrastination Programming

Every while loop can be a

while True: loop!

- just be sure to break!

Robot Loops?!

Robots run programs that are variants of this very basic loop:

Robot of the Day...

while True: sense() plan() act()

This is the "spa" architecture: it's almost universal.

Which of those three pieces is the most

difficult!?How to escape ?

import randomescape = 0

while True: print 'Help! Let me out!' escape = random.choice([41,42,43]) if escape == 42: break

print 'At last!'

Give me a break !

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

compare return

Loops aren't just for lists…

for c in 'down with CS!':

print c

What does this return?

n = 0 for c in s: if c not in 'aeiou': n += 1return n

def mymin( L ):

Write a loop to find and return the min of a list, LL is a list of numbers.

while N > 1: if N%2==0: N /= 2 else: N = 3*N+1

how could you "accumulate" the minimum?

def cc( s ):

>>> cc( 'forty-two' )

def odd( N ):

steps = 0

steps += 1

return steps

>>> odd( 3 )What does this return?

Extra!

n = 0 for c in s: if c not in 'aeiou': n += 1return n

def cc( s ):

>>> cc( 'forty-two' )

What does this return?

Extra!

while N > 1: if N%2==0: N /= 2 else: N = 3*N+1

def odd( N ):

steps = 0

steps += 1

return steps

>>> odd( 3 )

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

Homework 3

Sequences… #2

What is this stuff?

Hooray… Sanity!

or you could be saying both…

Sound… #3

Graphics… #4

one note on these...

111211211111221312211?

When does the first 4 appear?

str versus float or int

Look-And-Say Sequences(aka “Read-It-And-Weep”)

Homework 3

Sequences… #2


Sound… #3

Graphics… #4

DEMO!

Representing Sound

continuous plot of air pressure vs. time

samples taken every ~ 1/22050th of a

second

Each sample is measured on a scale from -32,768 to 32,767.

(This fits into 2 bytes.)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.

sampling

quantization

storage

physics

pulse code modulation = PCM data

Homework 3

Sequences… #2


Sound… #3

Graphics… #4

DEMO!

Etch-a-Sketch ?www.gvetchedintime.com

No way this is real… but it is !

Python's Etch-a-Sketch

from turtle import *

Be SURE to start IDLE with "no subprocess"

(Step 2) Enter idle –n &

Then, you can type

(Step 1) menus: Go – Utilities – Terminal

Start this!

Mac instructions... Windows instructions...

(Step 1) Go to the Start menu's search field

(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

Hit return; you'll see a console window and IDLE open.

Python's Etch-a-Sketch

Turtle Canvasreset()

left(90)

forward(50)

right(90)

backward(50)

down() or up()

color('green')

tracer(1) or tracer(0)

width(5)

and lots more!

www.cs.hmc.edu/twiki/bin/view/CS5/TurtleDirections

for turtle installation and help

degrees!

states if the pen draws or not

states if the pen animates

or not

from turtle import *

(0,0)

win

dow

_hei

ght(

)

window_width()

(42,42)

Recursive Graphics

def tri(): """ draws a polygon """ forward(100) left(120) forward(100) left(120) forward(100) left(120)

there is no tri … Could we tri this with recursion?(1)

Could we create any regular n-gon?(2)

I don't know about tri, but there sure's NO return … !

def tri( ): """ draws a polygon """

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)

What does chai draw?(1)

Turtle Graphics

Finish rwalk to draw a "stock-market-type" random path of nsteps steps. Use recursion!

(2)

Name(s):

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'

Ex Cr: How could you make it a bull (or a bear) market?

one possible result of rw(20)

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)

What does chai draw?(1)

How could you add more to each end?

Why are there two identical commands in a row?

Finish rwalk to draw a "stock-market-type" random path of nsteps steps. (2)

one possible result of rw(20)

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'

Ex Cr: How could you make it a bull (or a bear) market?

What if we didn't turn back to face forward each

time?

hw3pr4 spiral

100

90

81

72.9

spiral( initLength, angle, multiplier )

close-up of innermost part of the spiral…

spiral( 100, 90, 0.9 )

hw3pr4 svTree

svTree( trunkLength, levels )

svTree( 100, 4 )

I wonder what happened to the leaves on that tree… ?

Level 1

Level 2

Level 3

Level 4

hw2pr3 spiral

100

90

81

72.9

spiral( initLength, angle, multiplier )

close-up of innermost part of the spiral…

spiral( 100, 90, 0.9 )

hw2pr3 svTree

svTree( trunkLength, levels )

svTree( 100, 4 )

I wonder what happened to the leaves on that

tree… ?

Level 1

Level 2

Level 3

Level 4

The Koch curve

snowflake( 100, 0 ) snowflake( 100, 1 ) snowflake( 100, 2 )

snowflake( 100, 3 ) snowflake( 100, 4 ) snowflake( 100, 5 )

CGI == Python + Web

a very common technology for webapplications is the Common Gateway Interface or CGI

webserver

Apache

browser

FirefoxPython Last weekThis week

Next weekNext week

any programs!

CGI == Python + Web

Basic idea

Python

Your Python file prints the HTML page you want!

This week

CGI

#!C:/Python27/python.exe

import cgiimport 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>"""

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!

Monty Hall

Let’s make a deal ’63-’86 Sept. 1990

inspiring the “Monty Hall paradox”

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" ?

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 """

while True: menu() uc = input('Which option? ') print 'The inner eye does not see upon command!'

def seer(): """ handles user input for our menu """

while True: menu() uc = input('Which option? ') print 'The inner eye does not see upon command!'

if uc ==

Clearer Vision

the gift…

LightPath, September 1999

watching the charts…

LightPath, six months later…

"brokerage" seems apt…

LightPath, now…

T. T. Securities (TTS)

Input stock prices for a number of days in a row, and then analyze that data… .

Input stock prices for a number of days in a row, and then analyze that data… .

Software

Menu

(0) Input a new list(1) Print the current list(2) Find the average price(3) Find the standard deviation(4) Find the min and its day(5) Find the max and its day(6) Your TTS investment plan(9) QuitEnter your choice:


Functions

There are a number of formulas, but we will use this one:

functions

def average( L )

Menu


def stdev( L )

def mini( L )

def maxi( L )

Standard Deviation

There are a number of formulas, but we will use this one:

functions

(L[i] - Lav)2

len(L)i

def stdev( L ):Menu


Investment analysis for the 21st century…

Software

Menu


Hardware


Investment analysis for the 21st century…

Software

Menu

Hardware



The TTS advantage!

Your stock's prices:

The TTS investment strategy:

L = [ 40, 80, 10, 30, 27, 52, 5, 15 ]

Day Price 0 40.0 1 80.0 2 10.0 3 30.0 4 27.0 5 52.0 6 5.0 7 15.0

but, you must sell after you buy.

def minday( L ):

min = L[0]

for x in L:

if x < min:

min = x

return x

Alter this code to return the index of L's minimum.

Example:

>>> minday( [9,8,1,7] )

2

min-value loop

Alter this code to return the index of L's minimum.

using an index-based loop

def minday( L ):

min = L[0]

for :

if < min:

min =

return

INDEX-BASED LOOP

Example:

>>> minday( [9,8,1,7] )

2

What does this code print?

for x in range(3):

for y in range(3):

print x, y

>>> diff( [7,3],[0,6] )

1

for x in range(3):

for y in range(3):

print x, y

def diff( X, Y ):

Return the minimum difference between one value from X and one value from Y.

X and Y will be lists of numbers

Hint: adapt this code

Only consider unsigned differences.

Example:

Lab & Questions

Extra (!) funky series

• Harmonic Sequence:

1/1 + 1/2 + 1/3 + 1/4 + 1/5 + 1/6 + …

• Without composites (primes only):

1/1 + 1/2 + 1/3 + 1/4 + 1/5 + 1/6 + 1/7 +…

• Without 9’s…

1/1 + 1/2 + … + 1/8 + 1/9 + … +1/18 + 1/19 + … + 1/88 + 1/89 + 1/90 + 1/91 + …

Diverges!

???

???

Monte Carlo A engineering challenge: to estimate using everyday items...

Hw8 Pr3

Visualize:

Easy as

(-1,-1)

(1,1)

iPhone, geohot, and Von Neumann

George Hotz's iPhone

In red: one memory-access bit

After

Before

soldered to be +1.8v (1)



When starting up, the phone checks 4 locations in memory to see if its software is already there. Only if it sees the software is NOT there, does the phone use software from general-purpose memory to start. Otherwise, it loads its software from read-only memory.

There's not much you can do to change what is in those areas of read-only memory. Instead, the idea was to change one of the address lines to be high while it checked these four locations.

All of these locations are in a read/write (accessible) portion of the phone's memory -- so, they can be written to the "reset" signal. This reloads the phone's start-up software from read/write memory -- allowing arbitrary network access, not just AT&T.

The 4 32-bit memory locations it checks are

10100000000000000000000000110000101000000000000010100101101000001010000000000001010111000101100010100000000000010111001101110000

0xA0000030

0xA000A5A0

0xA0015C58

0xA0017370

hex binary

1

10100000000001000000000000110000101000000000010010100101101000001010000000000101010111000101100010100000000001010111001101110000

The memory locations it actually checks are thus



the trade

public static double sum(double[] A){ double s = 0.0;

for ( {

} return s;}

public static double average(double[] A)

“Extra Credit”: How concise can this method be?

Finish these two methods… “Quiz”• This method returns the sum of the elements in the input array.

• This method returns the average of the elements in the input array.

public static double max(double[] A)

Extra: What is something unusual and unrelated to CS 5 that you & the person next to you have in common ?!

• This method returns the maximum element from the input array.

Loopy thinking

s = 'gattacaaggtaaaatgca'0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

How could we find the longest sequence of 'a's ?

How could we find the number of 'a's ? How about 'a's and 't's?How could we find the number of 'ta's ?

Loopy thinking




s = 'gattacaaggtaaaatgca' N = 0for i in range(0,len(s)): if s[i] == 'a': N = N + 1

print 'N is', N

Loopy thinking




s = 'gattacaaggtaaaatgca' N = 0for i in range(0,len(s)): if s[i] == 'a' or s[i] == 't': N = N + 1

print 'N is', N

Loopy thinking




s = 'gattacaaggtaaaatgca' N = 0for i in range(1,len(s)): if s[i] == 'a' and s[i-1] == 't': N = N + 1

print 'N is', N

Loopy thinking



Planning in "pseudocode"


Loop through the string:

if we do see an 'a'

if the PREVIOUS letter is NOT an 'a'

if the PREVIOUS letter IS an 'a'

Keep track of CurRun, MaxRun




if we do see an 'a'




Start a Run! CurRun = 1

Continue our run! CurRun = CurRun + 1Check for a new maximum…



MAX = 0cur = 0for i in range(0,len(s)): if s[i] == 'a': if s[i-1] != 'a': cur = 1 else: cur = cur + 1 if cur > MAX: MAX = cur print 'Max is', MAX


if we do see an 'a'




Start a Run!

Continue our run!

Check for a new maximum…

Challenge Print the first N numbers in this series…

Example: >>> fib(11)

1 1 2 3 5 8 13 21 34 55 89

Quiz, part 2

0 1 2 3 6 7 14 15 30

Write any loop to print the first N terms of

N terms total

7 9 11 13 15 17 …

N terms total

Write an index-based loop to print the first N terms of

def seven(N):

for i in range(N):

feel free to add an accumulator, if you'd like!

input vs. raw_input

reply = raw_input('Enter a string and I\'ll tell you what I see.')

for c in reply:

print 'I see a(n) ', c

interprets what the user types as a

string of characters

reply = input('Enter any list and I\'ll tell you what I see.')

for c in reply:

print 'I see a(n) ', c

processes what the user types just as

python would

Why Assembly Language ?

It’s only the foolish that never climb Mt. Fuji -- or that climb it again.

Who writes most of the assembly language used?

the compiler

a program that translates from human-usable language into assembly language and machine langauge

x = 6y = 7z = x*yprint z

the code assembly or byte-code

executable machine code

machine code

loadn r1 6loadn r2 7mul r3 r1 r2write r3

0000 0001 0000 00011000 0010 0001 00010110 0010 0010 00010000 0010 0000 00100000 0000 0000 0000

0000 0001 …0010 0001 …0110 0010 …1000 0001 …

interpreting byte code

Interpreter

Compiler

Register r0 is always 0.

Pure jumps not allowed!

Assembly language

GOTO #jump #

Fortran, C, C++, Basic, …

Who writes

asembly?

people who need to talk to the processors directly…

I think the question is, "Who can spell assembly?"

Von Neumann Architecture

CPU RAMcentral processing unit random access memory

programs stored hereinstructions executed here

Von Neumann bottleneck

read r10

1

2

3

4

5

…6

255

jgtz r1 2

factorial program

r0

…

16 registers,

each 16 bits

r15

they can hold values

from -32768 upto

32767

r1

r2

Program

CounterInstruction

RegisterHolds the current instruction

Holds address of the next instruction

loadn r2 1

mul r2 r2 r1

addn r1 -10

the bus!

write r2

halt






Before






10100000000000000000000000110000101000000000000010100101101000001010000000000001010111000101100010100000000000010111001101110000

0xA0000030

0xA000A5A0

0xA0015C58

0xA0017370

hex binary




After

Before

soldered to be +1.8v (1)





All of these locations are in a read/write (accessible) portion of the phone's memory -- so, they can be written to the "reset" signal. This reloads the phone's start-up software from read/write memory -- allowing arbitrary network access, not just AT&T.


10100000000000000000000000110000101000000000000010100101101000001010000000000001010111000101100010100000000000010111001101110000

0xA0000030

0xA000A5A0

0xA0015C58

0xA0017370

hex binary

1

10100000000001000000000000110000101000000000010010100101101000001010000000000101010111000101100010100000000001010111001101110000

The memory locations it actually checks are thus



the trade

Thinking about Hmmm

• Repeated actions occur through jumps

• Functions (and recursion) pretend to have a separate processor on which to work

r1

r2

• Results can be set, reset, and accumulated

add r2 r2 r1

19

2342

Thinking about Hmmm


"Functional" Programming

results from composing individual functions

recursion is common

abstraction allows humans to think about 1 thing at a time

Examples

Fast matrix multiplicationFast median findingFast Fourier Transform…

I may be seeing the theme here…

Thinking about Hmmm


"Functional" Programming

"Imperative" Programming

results from composing individual functions

recursion is common

abstraction allows humans to think about 1 thing at a time

Python ~ processor.

variables ~ registers

accumulation is common

jumps & loops require people to do the work of the stack

You mean this is NON-functional programming?!?

Input and typing

What is python thinking ?!?

meters = raw_input('How many m? ')

cm = meters * 100

print 'That is', cm, 'cm.'

I'm thinking I like these units better then light years per

year!

Fix #1: use a type converter

meters = float(raw_input('How many m? '))

cm = meters * 100

print 'That is', cm, 'cm.'check out my newly installed

float converter!

The type of variable (box) matters!name: meters

type: floatname: cmtype: float

1.0 100.0

Homework 3

Sequences… #1

What is this stuff?

Hooray… Sanity!


Graphics… #2

Fix #2: use input()

# gets processed input from user

meters = input('How many m? ')

cm = meters * 100


I always use input -- but don't quote me on that.

Fix #2: use input()

# gets processed input from user

meters = input('How many m? ')

cm = meters * 100


raw_input always returns input as a string!

input processes the input as if typed into Python

both allow you to specify a prompt string

I always use input -- but don't quote me on that.

Documents

IS 313 : Putting loops to work for you