Announcements Project 2 Available Tomorrow (we will send mail) Will be due 11:59PM October 9 th (Sunday) Week 6 ( I will be traveling this week) Review.

AnnouncementsProject 2

Available Tomorrow (we will send mail)Will be due 11:59PM October 9th (Sunday)

Week 6 ( I will be traveling this week)Review Wednesday (by Dr. Martino)No Class Friday

Week 7Class Wed /Fri as normalMidterm Thursday Project due Sunday

Python Boot CampIntroduction to Ranges

Sequences

Definite LoopsFor Loops

Comparison of For and While

Our First For Loop# File: chaos.py# A simple program illustrating chaotic behavior

def main(): print("This program illustrates a chaotic

function") x = eval(input("Enter a number between 0 and 1:")) for i in range(10): x = 3.9 * x * (1 - x) print(x)

main()

Executing our First For Loop

>>> This program illustrates a chaotic functionEnter a number between 0 and 1: .50.9750.09506250.3354999222660.8694649252590.4426331091130.9621652553370.1419727793620.47508438620.9725789275370.104009713267>>>

Definite Loops

In chaos.py, what did range(10) do?>>> list(range(10))[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

range is a built-in Python function that generates a sequence of numbers, starting with 0.

list is a built-in Python function that turns the sequence into an explicit list (sequence)

The body of the loop executes 10 times.

RangesPython allows us to specify a range of values

range(n) 0, 1, …, n-1

Example:

list(range(10))

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Ranges ContinuedBut what if we don’t want to start from 0?

range(n) is short hand for range(0, n)

Example:

list(range(-4, 4))

[-4, -3, -2, -1, 0, 1, 2, 3]

Ranges Continued But what if I don’t want to count up by 1

Python allows us to “step” by a given integer range(start, end, step)

Example:

list(range(0, 10, 2))

[0, 2, 4, 6, 8]

Ranges … againBut what if I want to count down?

Python allows us to “step” by a given integer range(start, end, step)

Lets try:

list(range(0,10, -1))

[]

Example that works:

list(range(10, 0, -1))

[10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

Definite LoopsA definite loop executes a definite number of

times, i.e., at the time Python starts the loop it knows exactly how many iterations to do.

for <var> in <sequence>:<body>

The beginning and end of the body are indicated by indentation.

Sequences: ranges, lists, strings

Definite Loopsfor <var> in <sequence>:

<body>

The variable after the for is called the loop index. It takes on each successive value in sequence.

For (Definite) LoopsThese loops have a more controlled structure

and cannot run infinitely

This type of loop is commonly used with the range(x,y) function

for x in range(0,10):print (x)

for x in list(range(0,10)):print (x)

Definite Loops>>> for i in [0,1,2,3]:

print (i)

0

1

2

3

Definite Loops>>> for i in range(0,4):

print (i)

0

1

2

3

Definite Loops>>> for i in list(range(0,4)):

print (i)

0

1

2

3

Definite Loops>>> for odd in [1, 3, 5, 7]:

print(odd*odd)

1

9

25

49

CQ:Are these programs equivalent?

for a in range(0, 10, 1): print(a)

for a in range(10): print(a)

21

A: yes

B: no

Definite Loopsfor loops alter the flow of program execution, so

they are referred to as control structures.

more items in <sequence>

<var> = new item

<body>

yes

no

More Complex Examples

x = 0for a in range(10): x = x + aprint(x)

x = 1for a in range(1,10): x = x * aprint(x)

How to “visualize” a loopYou can use print statements to figure out what

is happening inside a loopThis works for For loops and While loops

What should we print?The loop index

This will give us information of how the loop index changes as the loop executes

Any calculations we may perform


x = 0for a in range(10): print(“a is: ”, a) x = x + a print(“x is: ”, x)print(x)

a is: 0x is: 0a is: 1x is: 1a is: 2x is: 3a is: 3x is: 6a is: 4x is: 10a is: 5x is: 15a is: 6x is: 21a is: 7x is: 28a is: 8x is: 36a is: 9x is: 45


x = 1for a in range(1,10): print(“a is: ”, a) x = x * a print(“x is: ”, x)print(x)

a is: 1x ix: 1a is: 2x ix: 2a is: 3x ix: 6a is: 4x ix: 24a is: 5x ix: 120a is: 6x ix: 720a is: 7x ix: 5040a is: 8x ix: 40320a is: 9x ix: 362880


A: Yes

B: No

x = 0y = 0

for k in range(5):x = x + ky = x + k

print (y)

x = 0y = 0

for k in range(5):x = x + k

y = x + kprint (y)

Loops can be nested

def complex(a, b): for x in range(0, a):

for y in range (0, b): print(x*y)

Visualizing a Nested Loop

def complex(a, b): for x in range(0, a): print(“x is: ”, x)

for y in range (0, b): print(“y is:”, y) print(x*y)

x is: 0y is: 00y is: 10y is: 20x is: 1y is: 00y is: 11y is: 22

CQ:Are these functions equivalent?

def complex(a,b): for x in range(0, a):

for y in range (0, b): print(x*y)

def complex2(a,b): for y in range(0,b):

for x in range (0, a): print(x*y)

A: yes

B: no

When might they be equivalent?

What about when a=b? Ie we call the functions and provide the same

values for a and bcomplex(2,2) = complex2(2,2)

Differences between While and For

For loops provide a a finite and enumerated “range” of valuesThis determines the “length” of the loop

For loops explicitly rebind the loop indexfor X in …

While loops express an execution condition It executes until that condition no longer holds

Chaos with a While# File: chaos.py# A simple program illustrating chaotic behavior

def main(): print("This program illustrates a chaotic

function") x = eval(input("Enter a number between 0 and 1:")) a = 0 while a < 10: x = 3.9 * x * (1 - x) print(x) a = a+1

main()


a = 0while(a < 10): print(a) a = a+1

for a in range(10): print(a)

21

A: yes

B: no

HomeworkRead “Loops” in the python wiki book

http://en.wikibooks.org/wiki/Python_Programming/Loops






Announcements

Python Boot CampHow to specify strings

Special Characters

ASCII

Unicode

StringsIn addition to basic data types such as Integers

most languages provide Strings

Strings are sequences of characters. We have seen a few examples already.“This is a string”

Strings are designated using “”

Strings and how we specify them

Strings are defined with quotations“hello”

There are four ways we can define a string ‘hello’“hello”“””hello””” ‘’’hello’’’

Why do we have three versions?

We can use ‘ ‘ to print strings that contain “ Example: print ’ “ ‘Will print “Example: print “ ‘ “Will print ‘

We can use “”” “”” to print strings that span multiple lines

What about special characters?

There are more characters than we have on our keyboardsOther languages (Greek, Chinese, etc)Characters with accent marks

Recall from recitation that we can encode charactersWe presented ASCII

UnicodeUnicode is another way to encode characters

Just like there is a limit to the largest integer we can encode in 32 bits there is a limit to the amount of characters we can encode in ASCII

Full Unicode encoding is available at:http://www.unicode.org/charts/

Unicode Examples

How do we use Unicode in Strings?

“\uXXXX” is a Unicode character, where XXXX is a code and each X can be 0-9 or A-FWe can use such characters in strings“This is a unicode character: \uFFFF”

One Catch (only in Python 2.x):We need to specify a ‘u’ before the string to tell

python it contains unicode charactersprint u”This is a unicode character: \uFFFF”

Try this:

print (”This is a unicode character: \uFFFF”)

print (”This is a unicode character: \uFFFF”)

There are other special Characters

“\b” is backspaceExample: print (“hello\bworld”)prints hellworld

“\n” is a newline (like pressing the Enter key”Example: print (“hello\nworld”)prints hello

world

There are other special Characters

“\t” is a tabExample: print (“hello\tworld”)prints hello world

String OperationsWe already know how to concatenate strings

Example: print “Hello”+”World”Will print HelloWorld

We also know how to get the length of a stringExample: print len(“HelloWorld”)Will print 10

String OperationsWe also know how to print the ASCII encoding of a

characterExample: print ord(‘b’)Will print 98

We can leverage this to print the ASCII encoding of a string:

str = “hello”

for char in str:

print ord(char)

Advanced String Operations

But first we need to introduce some new syntax

Object.Method(arguments)

We will study Object Oriented programming more formally later in the course

Objects and MethodsMethods are similar to functions

We can think of a Method as a function or an action performed on an Object

An Object can be anythingMore concretely, any python value

We will revisit this definitionFor now we will consider strings as Objects

Different types support different methods


print (“Hello World”.capitalize())Hello world

print (“hello world”.capitalize())Hello world

print (“Hello World”.endswith(“World”))True

print (“Hello World”.endswith(“Hello”))False


1.capitalize() “1”.capitalize()

21

A: yes

B: no


print (“HeLlO WoRlD”.lower())hello world

print (“hello world”.lower())hello world

print (“Hello World”.upper())HELLO WORLD

print (“hello world”.upper())HELLO WORLD


print (“Hello World”.find(“World”))6

print (“Hello World”.find(“or”))7

print (“Hello World”.find(“Hello”))0

print (“Hello World”.find(“o”))4

Example

b = “HELLO”a = b.lower()print(b)print(a)

>>> b = "HELLO">>> a = b.lower()>>> print(b)HELLO>>> print(a)hello

Variable StorageIn week 2 we had this

example. Note that b is a string. 10a

Sbally

a = 10b = “Sally”

String StorageWe can see that strings are

really individual characters stored into different locations in memory, in order

There is another way to store values in memory in this way.

10aSbally

The Range Function Revisited

Consider this example

10a0b1234

a = 10b = range(0,5)

String StorageLike a string, we have a

single variable assigned to the start of multiple values in memory

10a0b1234Sca

a = 10b = range(0,5)c = “Sally”

….

HomeworkRead 5.1, 5.2, 5.3, 5.4

Read 8.1 – 8.4 but skip 8.3.3

Announcements Project 2 Available Tomorrow (we will send mail) Will be due 11:59PM October 9 th (Sunday) Week 6 ( I will be traveling this week) Review.

Documents

printx slide

strings slide

x printx main slide

printx x

chaotic function x

loops comparison

ranges python

definite number of times