Download Python3-Eng

Introduction to
Python 3.x
Mikio Kubo
Why Python?
• We can do anything by
importing some modules
• Optimization
• Data Analysis
• Statistics
• Everything is included!
• Also fly!
import antigravity ?
• Programming becomes
fun again!
http://xkcd.com/353
/
Why Python?
• Most popular programming language
• Including MIT (Introduction to
Computer Science and Programming
Using Python)
Why Python?
• To get more money!
Average Initial
Salary in Japan
Python!
Why Python?
• To get more and more money!
Top 10 Programming Languages to Learn in 2014
No 1. Python
Average Salary: $93,000
Job Count: 24,533
Top Employers: Amazon, Dell, Google, eBay &
Yahoo, Instagram, NASA, Yahoo
Example Sites: Google.com, Yahoo Maps,
Reddit.com, Dropbox.com, Disqus.com
No 2. Java, No 3. Ruby, No. 4 C++, No5. JavaScript
No 6. C#, No. 7 PHP, No8. Perl ….
http://tech.pro/blog/1885/top-10-programming-languages-to-learn-in-2014
Some Remarks
• Python 3.x is not compatible to Python 2.x
• We use Python 3
• Install Anaconda (Python 3 Package)
https://store.continuum.io/cshop/anaconda/
Free version is enough
It includes more than 100 useful modules
It supports Mac, Linux, and Windows
How to print 'Hello, world!'
C++ (You need not remember this! )
#include <iostream>
int main() {
std::cout << 'Hello, world!' << std::endl;
return 0;
}
Python – Just use function print()
print ('Hello, world!')
Indent = Beauty
Indent = The way for grouping the sentences in Python
A bad C++ programmer may write...
if (x > 1) { y=x+1;
z=x+y; } else { y=0; z=0; }
In Python, anyone must write like this:
if x > 1:
y=x+1
# This is a comment part
z=x+y
# Here is the first block
else:
y=z=0
# Now the second block
If the indentation was wrong, it causes an error！
Why Python Again !
• Number of keywords to be remembered is much less
than other language (about 30! Java has more than 120!)
• Indentation helps to write a easy-to-read program
• Interpreter (You need not to compile)
• Need not to declare variables
• Need not to manage memory (automatic garbage
collection)
• Works on Windows, Mac, Linux (and also iphone and
ipad!)
• Object oriented
• Free!
Data Type (1): Number
• Number
–
–
–
–
int : integer number in unlimited range
bool : True or False
float: floating point number, i.e., 5.4 or 1.0
complex: complex number, i.e.,
z= 0 + 1j (real part and imaginary part)
z.real is 0
z.imag is 1
z*z
is -1+0j
because j*j is minus 1
Data Type (2): Sequence
• Sequence Type
– string : (immutable) sequence of unicodes, i.e., 'Mickey'，
'Minny'，'Mickey's Birthday', and
''' We can write multiple lines
using triple quotes
like this'''
– list: (mutable) sequence of arbitrary objects generated
by brackets [ ], i.e., [1,2,3,'a'], [1,'b', [2,3,'c'], 6 ], [ ]
– tuple： (immutable) sequence of arbitrary objects
generated by parentheses ( ), i.e., (1,2,3,'a'), ( (1,2),
(2,'f','g'))
'Immutable' = cannot modify
String is immutable!
>>> a='Mickey'
>>> a[0]='D'
# a[0] is the first string
Traceback (most recent call last):
File '<pyshell#7>', line 1, in <module>
a[0]='D'
TypeError: 'str' object does not support item assignment
Tuple are immutable, while list is mutable!
• Set Type
Data Type (3)
– set ： (mutable) set generated by braces { }, i.e.,
{1,2,3}, {'a', 'b', 'Mickey'}
– frozenset: (immutable) set
• Map Type
– dictionary： hash function that maps keys to values
composed generated by {key: value, ... }, i.e.,
{ 'Mary': 126, 'Jane': 156},
{ 'Hello': 'Konnnichiwa', 1: 'One', 2: 'Two'}
Function type( ) and type conversion
• To check the type of objects, use function type():
 type(124)
 type(3.1415)
 type(3+5j)
• To convert the type
– int( ) : convert to integer number
– float( ) : convert to floating number
– complex() : convert to complex number
 int(3.14)
 float(3)
 complex(1)
Bool and String: type conversion
• Check the type of objects
 type(True)
 type('Hello')
• Convert the type
– bool( ) : 0 is False，otherwise True
– str( ） : return string
 bool(3.14)
 bool(0)
 bool('0')
 str(3+5)
 str(True)
String (1)
'abc' or 'abc' or
''' .....
...... '''
Single (double) quote inside the double
(single) quotes is allows.
' My 'ideal' Place ' or 'My 'favorite' movie'
Triple quotes（ ''' ） allows multiple lines
String (2)
• String is a sequence type index from 0
For x= 'abcd', x[1] returns 'b'
a b c d
x[0] x[1] x[2] x[3]
Slicing (1)
• slicing of sequence type
[i:j] means i≦k<j
[ :j] means 0≦k<j
[i: ] means i≦k
For x= 'abcd', x[1:3] returns 'bc'
x[1:] returns 'bcd'
a b c d
x[0] x[1] x[2] x[3]
Slicing (2)
• len() returns the size of the object
len(x[i:j]) is equal to j-i
• The negative indices mean the index from the last.
The last character is x[-1]. The slicing is the strings
between the number i and j.
0
1
2
3
4
a b c d
-4
-3
-2
-1
x[0] x[1] x[2] x[3]
String Methods (1)
• Method (function associated with object using '.'
notation)
Example: s='abABBaac'
s.count(x)：returns how many times x occurs in string s
s.count('a') -> 3
s.index(x)：returns the first index x occurs in string s
s.index('A') -> 2
s.upper()：converts all the characters in string s to
uppercase
s.upper() -> 'ABABBAAC'
s.islower()： returns True iff all the characters in sting s
are lowercased
s.islower() -> False
String Methods (2)
• Example: s='abABBaac'
s.replace(old, new): returns a string by substituting
string <old> to <new> in string s
s.replace('AB','Mickey') -> 'abMickeyBaac'
s.split(d)： returns a list of strings using d as the
separator
s.split('a')| -> [' ', 'bABB', ' ', 'c']
s.join(L): returns a string that concatenates all the
string in list L using s as the separator
':'.join(['a','b','c'])| -> |'a:b:c'|
s.format(args): formats the string s using argument
args
'My friends are {0}, {1} and {0}'.format('Bob','Pedro','Bob)
-> 'My friends are Bob, Pedro and Bob'
List
• list： sequence of arbitrary objects
• Use bracket [] and separate the objects by
comma (,). i.e., [1,2,3,'a']，['a',['b',5,5],'c']
• mutable: can modify the items
For L=[1,1,5,4] , L[1]=100 => L=[1,100,5,4]
• Modify the elements by slicing
For L=[1,1,5,4] , L[1:3]=[100,200,300] =>
L=[1,100,200,300,4]
List Methods
• Method (function associated with object using '.'
notation)
Example: L=[1,1,5,4]
L.count(x)：returns how many times x occurs in list L
L.count(1) -> 2
L.index(x)：returns the first index x occurs in L
L.index(5) -> 2
L.reverse()： reverses the order of items in L
L.reverse() -> L=[4,5,1,1]
L.sort()： sorts the items in L
L.sort()-> L=[1,1,4,5]
Tuple
• tuple：sequence type of arbitrary objects
• Just separate the objects by comma (,) like
a,b
or use parentheses. i.e., (1,2,3,'a')
• Parentheses are necessarily for nested tuples
((1,5),('a','b',6))
• immutable： cannot modify
• Usage: a,b = b,a
Dictionary
• dictionary： map type composed of keys and values
• Use braces { } and input 'key : value' pairs separated
by comma, i.e.,
D={ 'Mary': 126, 'Jane': 156, 'Sara': 170}
• Same dictionary can be generate using dict() function
D=dict(Mary=126, Jane=156, Sara=170)
• D['Sara'] returns 170
• Keys must be immutable
• Dictionary is mutable：
D['Sara']=130 =>
{ 'Mary': 126, 'Jane': 156, 'Sara': 130}
Set
• set and frozenset: set type composed of
arbitrary objects
• Use braces S = {1,2,'Candy'}
• Use function set (or frozenset)
S=set( [1,2,'Candy'])
• set methods such as union, intersection,
difference，symmetric difference
• set is mutable
• fronzenset is immutable
Operation (1)
• Priority of operations
1. Parentheses ()
2. Exponential **
3. Multiply * or Division / or Integer Division // or
Mod (Division Remainder) %
4. Add + or Subtract -
• Comparison (returns Boolean)
<= less than or equal to
>= greater than or eqial to
== equal
!= not equal
Operator (2)
• 'in' and 'not in'
– True if an element is in the object (string, list,
tuple, set, dictionary)
– True if an element is not in the object
• Boolean operators
and
or
(1<4) or (5<4)
('a' in 'banana') and ('c' in [1,5,'c'])
If /else statesment
if <True or False Expression 1>:
Expression 1 is True
elif if <True or False Expression 2>:
Expression 1 is False and Expression 2 is True
else:
Expression 1 is False and Expression 2 is False
Example:
if x<0:
print ( 'Negative Number' )
else:
print ( 'Non-negative' )
for-loop
for <element> in <iterative object such as list or dict>:
Loop Block
range(i,j): generator function that returns integers k such that
i ≦k < j
Example： print 0,1,2,3,4
for x in range(5):
print(x)
Comprehension (1)
List comprehension:
generate a list using 'for' within [ ]
Example:
A list that contains tuples
2
x, x ,2
x
[ (x, x**2, 2**x) for x in range(5)]
>>>
[(0, 0, 1), (1, 1, 2), (2, 4, 4), (3, 9, 8), (4, 16, 16)]
Comprehension (2)
Generator comprehension:
'for' loop within ( )
Example using 'sum' function
sum(x for x in range(11))
>>>
55
while-loop
while <True or False Expression>:
Loop Block
Example： print 2*x while x is positive
x=10
while x>0:
print (2*x)
x =x-1
break and continue
• Keywords to escape from a loop
– break： finish the loop
– continue： jump to the next iteration
Example： print 2*x while x is positive and x is odd
x=10
while True:
x =x-1
if x%2==0:
continue # do not print when x is even
print (2*x)
if x<=0:
break
# finish the loop when x reaches 0
for-else
When for (or while) loop is not finished by 'break',
execute 'else' block after the loop
Example: check whether y is a prime number
y=11
for x in range(2,y):
if y % x==0:
# if y is divisible by x, y is a composite number
break
else:
print ('Prime Number' )
Function
• To define a fuction ...
def <name of function>( arguments ）:
function block
return <return value> # optional
Example： Concatenate given two strings and repeat three
times.
def concatenate3(a,b):
c=a+b
return 3*c
concatenate3('a','b') returns 'ababab'
Recursion (1)
• Recursion = function that calls itself
Example： factorial n! =n×(n-1) ×・・・×2×1
that can be defined recursively :
n!=n × (n-1)! and the initial condition 0!=1
def factorial(n):
if n==0:
return 1
else:
return n*factorial(n-1)
Recursion (2)
• Fibonacci number defined by：
F(0)=1, F(1)=1, F(n)=F(n-1)+F(n-2)
def fibonacci (n):
if n == 0 or n == 1:
return 1
else:
return fibonacci(n-1) + fibonacci(n-2)
Module
• Module is a library to be imported;
that is just a program named <filename.py> written in Python
Method 1： import module
import <filename>
Example： import 'math' module and use square root function 'sqrt'
import math
print math.sqrt(2)
Method 2： import function (and/or class) in module
from <filename> import <function/class name> or wildcard (*)
from math import *
print sqrt(2)
(Pseudo) Random Number
Generator Module
• import random or from random import *
random()： returns a uniform random number in [0.0, 1.0)
Example： random() -> 0.48777947886
randint(i,j)： returns a uniform random integer in i ≦k <=j
Example： randint(-5,1) -> -3
shuffle(L)： mix list L randomly．
Example： L=[1,2,3,4], shuffle(L) -> L=[4, 1, 3, 2]
choice(L)： Choose one item from list L randomly
Example： L=[1,2,3,4], choice(L) -> 3
Exercises
• CodingBat
http://codingbat.com/python/
• PySchool
http://www.pyschools.com/
(requires a google account)