Introduction to Python - condor.depaul.educondor.depaul.edu/slytinen/241f14/Ch2.pdf · We will use v 3.4.1 Downloads the Python language, plus an interactive shell called idle To

Introduction to Computing Using Python

Introduction to Python

Reading assignment: Perkovic text, Ch. 1 and 2.1-2.5


Python

Python is an interactive language.

Java or C++: compile, run

Also, a main function or method

Python: type expressions directly into interactive shell (or load them from a file)

Wherever you want your program to start, type this into the interactive shell (or in a file that is loaded into the shell)


Python Download and run

http://www.python.org/download. We will use v 3.4.1

Downloads the Python language, plus an interactive shell called idle

To run: type idle into Search programs and files

You can type Python expressions directly into the idle window

Or, you can open a file window by clicking New file under the File menu, or Ctrl-o

To load Python code from a file, hit F5 key

http://www.python.org/download


Algebraic expressions >>> 2 + 3

5

>>> 2 + 3

5

>>> 7 - 5

2

>>> 2*(3+1)

8

>>> 5/2

2.5

>>> 5//2

2

>>> 14//3

4

>>> 14%3

2

>>> 2 + 3

5

>>> 7 - 5

2

>>> 2*(3+1)

8

>>> 5/2

2.5

>>> 5//2

2

>>> 14//3

4

>>> 14%3

2

>>> 2**3

8

>>> abs(-3.2)

3.2

>>> 2 + 3

5

>>> 7 - 5

2

>>> 2*(3+1)

8

>>> 5/2

2.5

>>> 5//2

2

>>> 14//3

4

>>> 14%3

2

>>> 2**3

8

>>> abs(-3.2)

3.2

>>> min(23,41,15,24)

15

>>> 2 + 3

5

>>> 7 - 5

2

>>> 2*(3+1)

8

>>> 5/2

2.5

>>> 5//2

2

>>> 14//3

4

>>> 14%3

2

>>> 2**3

8

>>> abs(-3.2)

3.2

>>> min(23,41,15,24)

15

>>> max(23,41,15,24)

41

>>> 2 + 3

5

>>> 7 - 5

2

>>> 2 + 3

5

>>> 7 - 5

2

>>> 2*(3+1)

8

>>> 2 + 3

5

>>> 7 - 5

2

>>> 2*(3+1)

8

>>> 5/2

2.5

>>> 2 + 3

5

>>> 7 - 5

2

>>> 2*(3+1)

8

>>> 5/2

2.5

>>> 5//2

2

The Python interactive shell can be used to evaluate algebraic expressions

14//3 is the quotient when 14 is divided by 3 and 14%3 is the remainder

2**3 is 2 to the 3rd power

abs(), min(), and max() are functions

• abs() takes a number as input and returns its absolute value

• min() (resp., max()) take an arbitrary number of inputs and return the “smallest” (resp., “largest”) among them

>>> 2 + 3

5

>>> 7 - 5

2

>>> 2*(3+1)

8

>>> 5/2

2.5

>>> 5//2

2

>>> 14//3

4

>>> 14%3

2

>>> 2**3

8

>>> min(3, 2, 4)

2


Operator precedence In algrabraic expressions that use more than one operator, Python must have some way to determine the order in which operators are applied. For example: >>> 2 + 3 * 5 (is it 25 or 17?) It’s 17, because * has higher precedence than + In other words * is computed first You can always override precedence by using parentheses


Functions

A function is a sequence of simple statements that is given a name Many functions are built in to the Python language Examples: abs, min, max When you call a function, you must place parentheses after it; e.g., abs(-4) NOT abs -4


Functions Functions are often passed parameters which are pieces of information a function needs to run Example: >>> abs(-3) 3 Different functions take different numbers of parameters. For example:


Functions abs takes one parameter >>> abs(-1) 1 min takes one or more parameters >>> min(2,1) 1 >>> min(4, 2, 6) 2 Some take no parameters >>> import time >>> time.time() 1377628104.592236


Comments When you write non-trivial code, it is important to comment it. Comment symbol is # Anything on a line that is after # will be ignored by the computer


Comments Comments are used to make code more understandable for people (including you) Example: x = 3.141 * radius * radius # compute the area of a circle Don’t overcomment: y = 3 # set the value of y to 3 Even the first example of a comment might not be necessary


Boolean expressions

In addition to algebraic expressions, Python can evaluate Boolean expressions

• Boolean expressions evaluate to True or False

• Boolean expressions often involve arithmetic comparison operators

<, >, ==, !=, <=, and >=

>>> 2 < 3

True

>>> 2 > 3

False

>>> 2 == 3

False

>>> 2 != 3

True

>>> 2 <= 3

True

>>> 2 >= 3

False

>>> 2+4 == 2*(9/3)

True In an expression containing algebraic and comparison operators:

• Algebraic operators are evaluated first • Comparison operators are evaluated next


Logical operators

Logical operators can also be used in Boolean expressions - and, or, not

>>> 2<3 and 3<4

True

>>> 4==5 and 3<4

False

>>> False and True

False

>>> True and True

True

>>> 4==5 or 3<4

True

>>> False or True

True

>>> False or False

False

>>> not(3<4)

False

>>> not(True)

False

>>> not(False)

True

>>> 4+1==5 or 4-1<4

True

In a an expression containing algebraic, comparison, and Boolean operators:

• Algebraic operators are evaluated first • Comparison operators are evaluated next • Boolean operators are evaluated last


Exercise >>> 14.99 + 27.95 + 19.83

62.769999999999996

>>> 20*15

300

>>> 2**10

1024

>>> min(3, 1, 8, -2, 5, -3, 0)

-3

>>> 3 == 4-2

False

>>> 17//5 == 3

True

>>> 17%5 == 3

False

>>> 284%2 == 0

True

>>> 284%2 == 0 and 284%3 == 0

False

>>> 284%2 == 0 or 284%3 == 0

True

Translate the following into Python algebraic or Boolean expressions and then evaluate them:

a) The total of 14.99, 27.95, and 19.83 b) The area of a rectangle of length 20 and width 15 c) 2 to the 10th power d) The minimum of 3, 1, 8, -2, 5, -3, and 0 e) 3 equals 4-2 f) The value of 17//5 is 3 g) The value of 17%5 is 3 h) 284 is even i) 284 is even and 284 is divisible by 3 j) 284 is even or 284 is divisible by 3

Introduction to Computing Using Python Variables and assignments

<variable> = <expression>

Just as in algebra, a value can be assigned to a variable, such as x

When variable x appears inside an expression, it evaluates to its assigned value

A variable (name) does not exist until it is assigned

The assignment statement has the format <expression> is evaluated first, and the resulting value is assigned to variable <variable>

>>> x = 3

>>> x

3

>>> 4*x

12

>>> y

Traceback (most recent call last):

File "<pyshell#18>", line 1, in

<module>

y

NameError: name 'y' is not defined

>>> y = 5*x

15

>>> y

15

Introduction to Computing Using Python Naming rules

Variable names can contain these characters: • a through z • A through Z • the underscore character _ • digits 0 through 9

Names cannot start with a digit

For a multiple-word name, use • either the underscore as the delimiter • or camelCase capitalization

Short and meaningful names are ideal

>>> My_x2 = 21

>>> My_x2

21

>>> My_x2 = 21

>>> My_x2

21

>>> 2x = 22

SyntaxError: invalid syntax

>>>

>>> My_x2 = 21

>>> My_x2

21

>>> 2x = 22


>>> new_temp = 23

>>> newTemp = 23

>>>

>>> My_x2 = 21

>>> My_x2

21

>>> 2x = 22


>>> new_temp = 23

>>> newTemp = 23

>>> counter = 0

>>> temp = 1

>>> price = 2

>>> age = 3

"Hello, World!"

Introduction to Computing Using Python Strings

In addition to number and Boolean values, Python supports string values

A string value is represented as a sequence of characters enclosed within quotes

>>> 'Hello, World!'

'Hello, World!'

>>>

'Hello, World!'

A string value can be assigned to a variable

String values can be manipulated using string operators and functions

>>> 'Hello, World!'

'Hello, World!'

>>> s = 'rock'

>>> t = 'climbing'

>>>

Introduction to Computing Using Python String operators

>>> 'Hello, World!'

'Hello, World!'

>>> s = 'rock'

>>> t = 'climbing'

>>> s == 'rock'

True

>>> s != t

True

>>> s < t

False

>>> s > t

True

>>> s + t

'rockclimbing'

>>> s + ' ' + t

'rock climbing'

>>> 5 * s

'rockrockrockrockrock'

>>> 30 * '_'

'______________________________'

>>> 'o' in s

True

>>> 'o' in t

False

>>> 'bi' in t

True

>>> len(t)

8

Usage Explanation

x in s x is a substring of s

x not in s x is not a substring of s

s + t Concatenation of s and t

s * n, n * s Concatenation of n copies of s

s[i] Character at index i of s

len(s) (function) Length of string s

>> help(str)

Help on class str in module builtins:

class str(object) | str(object=‘’)-> str

...

To view all operators, use the help() tool


Exercise >>> s1

'good'

>>> s2

'bad'

>>> s3

'silly'

>>>

Write Python expressions involving strings s1, s2, and s3 that correspond to:

a) 'll' appears in s3 b) the blank space does not

appear in s1 c) the concatenation of s1, s2,

and s3 d) the blank space appears in the

concatenation of s1, s2, and s3

e) the concatenation of 10 copies of s3

f) the total number of characters in s1, s2, and s3 combined

>>> s1

'good'

>>> s2

'bad'

>>> s3

'silly'

>>> 'll' in s3

True

>>> ' ' not in s1

True

>>> s1 + s2 + s3

'goodbadsilly’

>>> ' ' in s1 + s2 + s3

False

>>> 10*s3

'sillysillysillysillysillysillysillysillysillysilly'

>>> len(s1+s2+s3)

12

>>>

Introduction to Computing Using Python Index and indexing operator

'A'

'p'

'p'

'l'

'e'

s[0] =

s[1] =

s[2] =

s[3] =

s[4] =

s =

0 1 3 4 2

The index of an item in a sequence is its position with respect to the first item

The index of an item in a sequence is its position with respect to the first item • The first item has index 0,

The index of an item in a sequence is its position with respect to the first item • The first item has index 0, • The second has index 1,

The index of an item in a sequence is its position with respect to the first item • The first item has index 0, • The second has index 1, • The third has index 2, …

The indexing operator [] takes a nonnegative index i and returns a string consisting of the single character at index i

>>> s = 'Apple'

>>> s[0]

'A'

>>> s[1]

'p'

>>> s[4]

'e'

' A p p l e '

Introduction to Computing Using Python Negative index

'A'

'l'

'e' s[-1] =

s[-2] =

s[-5] =

s =

0 1 3 4 2

' A p p l e '

A negative index is used to specify a position with respect to the “end” • The last item has index -1, • The second to last item has index -2, • The third to last item has index -3, …

-5 -4 -2 -1 -3

>>> s = 'Apple'

>>> s[-1]

'e'

>>> s[-2]

'l'

>>> s[-5]

'A'


Exercise

>>> s = 'abcdefgh'

>>>

String s is defined to be

'abcdefgh' Write expressions using s and the indexing operator [] that return the following strings:

a) 'a'

b) 'c'

c) 'h'

d) 'f'

>>> s = 'abcdefgh'

>>> s[0]

'a'

>>> s[2]

'c'

>>> s[7]

'h'

>>> s[-1]

'h'

>>> s[-3]

'f'

>>>

[ ' a n t ' , ' b a t ' , ' c o d ' , ' d o g ' , ' e l k ' ]

Introduction to Computing Using Python Lists

In addition to number, Boolean, and string values, Python supports lists

>>> pets = ['ant', 'bat', 'cod', 'dog', 'elk']

>>> lst = [0, 1, 'two', 'three', [4, 'five']]

>>>

A comma-separated sequence of items enclosed within square brackets

The items can be numbers, strings, and even other lists

>>> pets = ['ant', 'bat', 'cod', 'dog', 'elk’]

>>>

[ 0 , 1 , ' t w o ' , ' t h r e e ' , [ 4 , ' f i v e ' ] ] [ 0 , 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 1 0 ]



>>> nums = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

>>>

Introduction to Computing Using Python List operators and functions

Like strings, lists can be manipulated with operators and functions

>>> lst = [1, 2, 3]

>>> lstB = [0, 4]

>>> 4 in lst

False

>>> 4 not in lst

True

>>> lst + lstB

[1, 2, 3, 0, 4]

>>> 2*lst

[1, 2, 3, 1, 2, 3]

>>> lst[0]

1

>>> lst[1]

2

>>> lst[-1]

3

>>> len(lst)

3

>>> min(lst)

1

>>> max(lst)

3

>>> sum(lst)

6

>>> help(list

...

Usage Explanation

x in lst x is an item of lst

x not in lst x is not an item of lst

lst + lstB Concatenation of lst and lstB

lst*n, n*lst Concatenation of n copies of lst

lst[i] Item at index i of lst

len(lst) Number of items in lst

min(lst) Minimum item in lst

max(lst) Maximum item in lst

sum(lst) Sum of items in lst

Introduction to Computing Using Python Lists are mutable, strings are not



>>>

The elements can be numbers, strings, and even other lists

>>> pets = ['ant', 'bat', 'cod', 'dog', 'elk’]

>>>


>>>

pets = ['ant', 'bat', 'cod', 'dog', 'elk'] pets = ['ant', 'bat', 'cow', 'dog', 'elk']


>>> pets[2] = 'cow'

>>> pets

['ant', 'bat', 'cow', 'dog', 'elk']

>>>


>>> pets[2] = 'cow'

>>> pets


>>> pet = 'cod'

>>>


>>> pets[2] = 'cow'

>>> pets


>>> pet = 'cod'

>>> pet[2] = 'w'

Traceback (most recent call last):

File "<pyshell#155>", line 1, in <module>

pet[2] = 'w'

TypeError: 'str' object does not support item assignment

>>>

pet = 'cod'

Lists can be modified; they are said to be mutable

Strings can’t be modified; they are said to be immutable

Introduction to Computing Using Python List methods

len()and sum() are examples of functions that can be called with a list input argument; they can also be called on other type of input argument(s)

>>> lst = [1, 2, 3]

>>> len(lst)

3

>>> sum(lst)

6

>>>

`

There are also functions that are called on a list; such functions are called list methods (note the different syntax). Two of these methods are append and remove

>>> lst = [1, 2, 3]

>>> len(lst)

3

>>> sum(lst)

6

>>> lst.append(7)

>>> lst

[1, 2, 3, 7]

>>> lst.remove(3)

>>> lst

[1, 2, 7]

Later we will discuss in more detail what the difference is between a method and a function

Introduction to Computing Using Python List methods >>> lst = [1, 2, 3]

>>> lst.append(7)

>>> lst.append(3)

>>> lst

[1, 2, 3, 7, 3]

>>> lst.count(3)

2

>>> lst.remove(2)

>>> lst

[1, 3, 7, 3]

>>> lst.reverse()

>>> lst

[3, 7, 3, 1]

>>> lst.index(3)

0

>>> lst.sort()

>>> lst

[1, 3, 3, 7]

>>> lst.remove(3)

>>> lst

[1, 3, 7]

>>> lst.pop()

7

>>> lst

[1, 3]

Usage Explanation

lst.append(item) adds item to the end of lst

lst.count(item) returns the number of times item occurs in lst

lst.index(item) Returns index of (first occurrence of) item in lst

lst.pop() Removes and returns the last item in lst

lst.remove(item) Removes (the first occurrence of) item from lst

lst.reverse( ) Reverses the order of items in lst

lst.sort( ) Sorts the items of lst in increasing order

Methods append(), remove(), reverse(), and sort() do not return any value; they, along with method pop(), modify list lst


Exercise List lst is a list of prices for an item at different online retailers lst = [159.99, 160.00, 205.95, 128.83, 175.49]

>>> lst = [159.99, 160.00, 205.95, 128.83, 175.49]

>>> lst.append(160.00)

>>> lst.count(160.00)

2

>>> min(lst)

128.83

>>> lst.index(128.83)

3

>>> lst.remove(128.83)

>>> lst

[159.99, 160.0, 205.95, 175.49, 160.0]

>>> lst.sort()

>>> lst

[159.99, 160.0, 160.0, 175.49, 205.95]

>>>

a) You found another retailer selling the item for $160.00; add this price to list lst

b) Compute the number of retailers selling the item for $160.00

c) Find the minimum price in lst d) Using c), find the index of the

minimum price in list lst e) Using c) remove the minimum price

from list lst f) Sort list lst in increasing order

Introduction to Computing Using Python Objects and classes

Terminology: object X is of type int = object X belongs to class int

An object’s type determines what values it can have and how it can be manipulated

A class is a kind of data (int, list, str, …) An object is a particular value associated with a class (3, [1, 2, 3], ‘abc’) All values in Python are objects Unlike many other programming languages

Introduction to Computing Using Python Values of number types

An object of type int can be any integer number value (with no decimal point) Another type for numbers is called float. Any number with a decimal point is a float.

>>> 3

3

>>> type(3)

<class 'int'>

>>> 3.

3.0

>>> type(3.)

<class 'float'>

>>> type(3.0)

<class 'float'>

An object’s type determines what values it can have and how it can be manipulated

Introduction to Computing Using Python Input from a user Function for user input: input(prompt) where prompt is a string Example (assume the user enters ‘Hello’) >>> inp = input(‘Enter a word: ’) (user types Hello) >>> inp + ‘ was your input’ Hello was your input The user’s input is always interpreted as a string Example (the user enters ‘1’) >>> x = input(‘Enter a number: ’) (user types 1) >>> x * 2 11

Introduction to Computing Using Python Output to a user

Function: print(<output>)

where <output> is a string For example: >>> print(‘Hello world’) Hello world

Introduction to Computing Using Python .py Files

You can write Python statements in a file To start a new file, type Ctrl-n (hole down the Ctrl key and type n) Or, to open an existing file, Ctrl-o File name should end in .py To execute the code, hit F5 Example: Compose a file that, when loaded, will ask the user to enter 3 numbers, and prints the sum.

Introduction to Computing Using Python Example program

Compose a file that, when loaded, will ask the user to enter 3 numbers, and prints the sum. (user input is in red)

>>> Enter a number: 1 Enter a number: 2 Enter a number: 3 The sum is 6

Does anyone’s program print The sum is 123

Introduction to Computing Using Python Conversion of types

To change a string into an integer: int(<string>) For example: >>> int(‘3’) 3 Example (the user enters ‘1’) >>> x = int(input(‘Enter a number ’)) >>> x * 2 2

Introduction to Computing Using Python Example program

x = int(input(‘Enter a number: ’)) y = int(input(‘Enter a number: ’)) z = int(input(‘Enter a number: ’)) print(‘The sum is ’ + (x + y + z))

Introduction to Computing Using Python Conversion of types, continued

What if you want to print something that isn’t a string? The str function does this >>> str(1) ‘1’ For example: >>> int(‘3’) 3 >>> int(‘3’) + 2 5

Introduction to Computing Using Python Example using str Assume user types 3 >>> x = int(input(‘Enter an integer ’)) >>> y = x + 2 >>> print(x + ' plus 2 is ' + y) Traceback (most recent call last): File "<pyshell#58>", line 1, in <module> print(x + ' plus 2 is ' + y) TypeError: unsupported operand type(s) for +: 'int' and 'str' >>> print(str(x) + ‘ plus 2 is ’ + str(y)) 3 + 2 is 5

Introduction to Computing Using Python Special characters

If a string contains a ‘\’, then the character after that is interpreted differently Example: ‘\n’ means a newline >>> print(‘abc\ndef’) abc def (‘\’ is an escape character within print function)

Introduction to Computing Using Python Special characters, continued

‘\t’: tab character >>> print(‘abc\tdef’) abc def However: >>> ‘abc\ndef’ abc\ndef (‘\’ is an escape character within print function)

Introduction to Computing Using Python Homework assignment 1

See D2L Dropbox

Documents

Introduction to Python - condor.depaul.educondor.depaul.edu/slytinen/241f14/Ch2.pdf · We will use v 3.4.1 Downloads the Python language, plus an interactive shell called idle To