02. Python Basics, Branching, and Iteration

02. Python Basics, Branching, and Iteration#

Last time#

Course information
Setup the environment
- Python
- VS code
- Git and GitHub
- Windows terminal
Homework

Today#

Introduction to Python
String
Logistic operators
Branching and conditionals
Iterations and loops

Introduction to Python#

An interpreted programming language, not a compiled programming language
Can be typed directly in a Python Shell or in a file

print("Hello, world!")
print("Xi Winnie the Pooh")
print("蔡EE - 天棄之子")

Hello, world!
Xi Winnie the Pooh
蔡EE - 天棄之子

Objects#

It’s something that Python can manipulate. In other words, you can write a program to manipulate it.
There are different types of objects in Python, which defines what kinds of things the programs can do to them.

Common objects in Python:

Name	Type	Example
String	str	`9527`
Integer	int	`87`
Floating-point number	float	`3.14152`
Complex number	complex	`1.3+4j`
Boolean	bool	`True` or `False`
Nonetype	None	`None`
Tuple	tuple	`(1, 2, 3)`
List	list	`[4, 5, 6]`
Dictionary	dict	`{"key1": (1, 2, 3)}`

type(5487)

int

type("5487")

str

type("5487")
type(5487)

int

print(type(5487))
print(type("5487"))

<class 'int'>
<class 'str'>

Data type conversion#

You can convert the type of object.

print(type(5487))
print(type(str(5487)))
print(type(float(5487)))

<class 'int'>
<class 'str'>
<class 'float'>

Variables and assignment#

= is an assignment of an object to a variable.
Example:
Values are stored in the computer memory.
An assignment binds name to value.
Binding values can be invoked by typing variables.
Multiple assignments is also allowed in Python.

pi = 3
radius = 10
circum = 2*pi*radius
print("pi =", pi)
print("radius =", radius)
print("circum =", circum)

pi = 3
radius = 10
circum = 60

pi, radius = 3, 10
circum = 2*pi*radius
print("pi =", pi)
print("radius =", radius)
print("circum =", circum)

pi = 3
radius = 10
circum = 60

radius = 94
print("radius =", radius)
radius = radius + 1
print("radius =", radius)

radius = 94
radius = 95

String#

Letters, special characters, spaces, … can be a string object.
Using quotes to specify the object type
Strings can be concatenated or multiplied by an integer.

hi = "你好"
name = "Winnie the Pooh"

# Concatenated
greeting1 = hi + ", " + name
print(greeting1)

greeting2 = "Xi " + name
print(greeting2)

# Multiplied by an integer
greeting3 = "You f" + "o"*10 + "l"
print(greeting3)

你好, Winnie the Pooh
Xi Winnie the Pooh
You fooooooooool

More about `print`#

Different types of objects can be concatenated by print.
Use sep and end to change its functionality.

Try this#

# 1
print("hello," + 3)
# 2
print("hello,", 3)
# 3
print("hello,", 3, sep="", end="!")
# 4
print("塔綠班"*1.3)

Function `input`#

A function to interact with users
Be careful with the data type

xxx = input("Please tell me something: ")

print("type(xxx):", type(xxx))

Indexing#

The indexing in Python is zero-based

Slicing#

Using slicing to extract the objects

Try this

xxx = "0123456789"
# 1
print(xxx[0])
# 2
print(xxx[-1])
# 3
print(xxx[2:5])
# 4
print(xxx[:7])
# 5
print(xxx[7:])

String formatting#

# Example of %-formatting

text = 'world'
# %s: string
print('hello %s!' % text)
# %x: hexadecimal
print('%x' % 10)
# multiple %-formatting
print('Xi %s %s' % ('Winnie', 'the Pooh'))

hello world!
a
Xi Winnie the Pooh

str-format

# Example of str-formatting

text = 'world'
# {}: string
print('Hello {}!'.format(text))
# {:x}: hexadecimal
print('{:x}'.format(10))
# multiple str-formating
print('Xi {} {}'.format('Winnie', 'the Pooh'))

Hello world!
a
Xi Winnie the Pooh

f-string

# Example of f-string

text = 'world'
print(f'Hello {text}!')

first = 'Winnie'
title = 'the Pooh'
print(f'Xi {first} {title}')

Hello world!
Xi Winnie the Pooh

# String formatting
a = 50
b = 4

# %-formatting
print("%d/%d = %f" % (a, b, a/b))
# str-formatting
print("{:d}/{:d} = {:f}".format(a, b, a/b))
# f-string
print(f'{a}/{b} = {a/b}')

50/4 = 12.500000
50/4 = 12.500000
50/4 = 12.5

Exercise 2.1#

Please write a program that aims to print an arbitrary number into the following format:
1. Binary (b)
2. Octal (o)
3. Decimal (d)
4. Hexadecimal (x)
For example, 9487:
1. Binary: 10010100001111
2. Octal: 22417
3. Decimal: 9487
4. Hexadecimal: 250F

Logistic operators#

Logical operators

and, or, not

A = 0 #False
B = 1 #True
print(A and B)
print(A or B)
print(not A)

Logical operators with none boolean objects

Object

Result

0

False

(),[],{}

False

None

False

False

False

others

True

print(not 0)
print(not ())
print(not None)
print(not 8)
print(not "string")

True
True
True
False
False

Multiple objects#

From left to right
Priority:
1. not
2. and
3. or
and:
- return the far right object if all objects are True, return the first False object otherwise.
or:
- return the far right object if all objects are False, return the first True object otherwise.

print(3 and 4 and 5)        # return 5
print(3 and 0 and 5)        # return 0
print(3 or 4 or 5)          # return 3
print(() or None or {})     # return {}
print(2 and 3 or 4)         # return 3
print(2 and 0 or 4)         # return 4

Bitwise operators
- and: &
- or: |
- xor: ^

Input	Result
`A`	1	0	1
`B`	0	1	1
`A & B`	0	0	1
`A \| B`	1	1	1
`A ^ B`	1	1	0

A = 5
B = 3

# format(Number, '{number to filled}{width}{format}')
print('  A binary:', format(A, '03b'), A)

# f-string
print('  B binary:', f'{B:03b}', B)
print('A&B binary:', f'{A & B:03b}', A & B)
print('A|B binary:', f'{A | B:03b}', A | B)
print('A^B binary:', f'{A ^ B:03b}', A ^ B)

  A binary: 101 5
  B binary: 011 3
A&B binary: 001 1
A|B binary: 111 7
A^B binary: 110 6

A = 5
B = 3

# str-format
print('  A binary: {:03b} {}'.format(A, A))
print('  B binary: {:03b} {}'.format(B, B))
print('A&B binary: {:03b} {}'.format(A & B, A & B))
print('A|B binary: {:03b} {}'.format(A | B, A | B))
print('A^B binary: {:03b} {}'.format(A ^ B, A ^ B))

  A binary: 101 5
  B binary: 011 3
A&B binary: 001 1
A|B binary: 111 7
A^B binary: 110 6

Exercise 2.2#

Please write a program that can print a variable a as the following string:
1. 94.8700
2. 0.949%
3. 00095
Here is your sample code:

a = 94.87

print("自己想")
print("自己想")
print("自己想")

Identity operators
- is
Operators

Meaning

Example

is

Is A same object as B?

True is True

is not

Is A not same objest as B?

True is not False

Do not confuse with comparison operator ==.

a = [1, 2, 3]
b = [1, 2, 3]
print(a == b)
print(a is b)
print(id(a), id(b))

True
False
2461526278464 2461526224192

Branching & conditional#

Using branching to control the flow of algorithm

your_id = "113514087"

if your_id[:3] == "112":
    # branch 1
    print("Welcome, freshman!")
elif your_id[0:3] == "108":
    # branch 2
    print("Why are you still here?")
else:
    # branch 3
    print("You have to pass this if you want to graduate.")

You have to pass this if you want to graduate.

Iterations and loops#

A method for executing repeatable instructions

`while` loop#

Keep iterating the loop while the condition is True

# Syntax
while <condition>:
    <expression>
    <expression>
    <expression>
    ...

# Example of while loop

# Example 1
n = 0
ans = 0

while n < 11:
    print("(n, ans) = ({}, {})".format(n, ans))
    ans += n
    n += 1

(n, ans) = (0, 0)
(n, ans) = (1, 0)
(n, ans) = (2, 1)
(n, ans) = (3, 3)
(n, ans) = (4, 6)
(n, ans) = (5, 10)
(n, ans) = (6, 15)
(n, ans) = (7, 21)
(n, ans) = (8, 28)
(n, ans) = (9, 36)
(n, ans) = (10, 45)

# Example 2
n = 0
ans = 0

while n < 11:
    ans += n
    n += 1
    print("(n, ans) = ({}, {})".format(n, ans))

(n, ans) = (1, 0)
(n, ans) = (2, 1)
(n, ans) = (3, 3)
(n, ans) = (4, 6)
(n, ans) = (5, 10)
(n, ans) = (6, 15)
(n, ans) = (7, 21)
(n, ans) = (8, 28)
(n, ans) = (9, 36)
(n, ans) = (10, 45)
(n, ans) = (11, 55)

Exercise 2.3: sum of harmonic series#

Please write a program with while that aims to return you the sum of a harmonic series

\[ f(n) = \sum_{k=1}^{n}\frac{1}{k} \]

Note: \(n\) is a positive integer
Test your program with different \(n\), for example, \(n = 1, 2, 3, ...\)

`for` loop#

Keep iterating the loop for a specific number of times

# Syntax
for idx in <iterable object>:
    <expression>
    <expression>
    <expression>
    ...
# Simple version
for idx in <number of times>:
    <expression>
    <expression>
    <expression>
    ...

We will explain what is iterable object in the future

# Example of for loop

# Example 1
ans = 0

for n in range(11):
    print("(n, ans) = ({}, {})".format(n, ans))
    ans += n

(n, ans) = (0, 0)
(n, ans) = (1, 0)
(n, ans) = (2, 1)
(n, ans) = (3, 3)
(n, ans) = (4, 6)
(n, ans) = (5, 10)
(n, ans) = (6, 15)
(n, ans) = (7, 21)
(n, ans) = (8, 28)
(n, ans) = (9, 36)
(n, ans) = (10, 45)

# Example 2
ans = 0

for n in range(11):
    ans += n
    print("(n, ans) = ({}, {})".format(n, ans))

(n, ans) = (0, 0)
(n, ans) = (1, 1)
(n, ans) = (2, 3)
(n, ans) = (3, 6)
(n, ans) = (4, 10)
(n, ans) = (5, 15)
(n, ans) = (6, 21)
(n, ans) = (7, 28)
(n, ans) = (8, 36)
(n, ans) = (9, 45)
(n, ans) = (10, 55)

Exercise 2.4: sum of harmonic series#

Please write a program with for that aims to return you the sum of a harmonic series

\[ f(n) = \sum_{k=1}^{n}\frac{1}{k} \]

Note: \(n\) is a positive integer
Test your program with different \(n\), for example, \(n = 1, 2, 3, ...\)

Function `enumerate`#

# Example of for loop
text = "3.14159265359"

# 1
for i in range(len(text)):
    print("{}: {}".format(i, text[i]))

# 2
for idx, string in enumerate(text):
    print(str(idx) + ": " + string)

Exercise 2.5: the factorial#

Please write a program that aims to return you a factorial of a non-negative integer \(n\).

\[\begin{split} \begin{aligned} n! &= n \times (n-1) \times (n-2) \times (n-3) \times \cdots \times 3 \times 2 \times 1 \\ &= n \times (n-1)! \end{aligned} \end{split}\]

Note: \(n\) is a non-negative integer
Test your program with different \(n\), for example, \(n = 1, 2, 3, ...\)

Object	Result
`0`	`False`
`()`,`[]`,`{}`	`False`
`None`	`False`
`False`	`False`
others	`True`

Operators	Meaning	Example
`is`	Is A same object as B?	`True is True`
`is not`	Is A not same objest as B?	`True is not False`

02. Python Basics, Branching, and Iteration

Contents

02. Python Basics, Branching, and Iteration#

Last time#

Today#

Introduction to Python#

Objects#

Data type conversion#

Variables and assignment#

String#

More about print#

Try this#

Function input#

Indexing#

Slicing#

String formatting#

Exercise 2.1#

Logistic operators#

Multiple objects#

Exercise 2.2#

Branching & conditional#

Iterations and loops#

while loop#

Exercise 2.3: sum of harmonic series#

for loop#

Exercise 2.4: sum of harmonic series#

Function enumerate#

Exercise 2.5: the factorial#

More about `print`#

Function `input`#

`while` loop#

`for` loop#

Function `enumerate`#