Lecture 1

• course Info
• what is computation
• python basics
• mathematical operations
• python variables and types

• Subject : Learn Python a programming language
• Classes :
  • All classes are face-to-face
  • All off-class communication will be on Hacettepe University Digital Learning Platform (HADİ)
    • hadi website
    • You have to follow hadi daily for new announcements

• Grading
  • 60%: Midterm (2x)
    • Midterms format will be announced during the semester
  • 40%: Final Exam (1x)

• You are strictly forbidden to cooperate on exams
• You are strictly forbidden to search for answers on the Internet
• You are strictly forbidden to copy/paste materials from the Internet

• Cooperation
  • you may cooperate with your friends to understand the course material
  • all exam work needs to be individual
    • cheaters will be harshly penalized -> -100, F3, disciplinary committee

• Position yourself to succeed!
  • do practice early
  • do not skip lectures
• New to programming? PRACTICE. PRACTICE? PRACTICE!
  • can’t passively absorb programming as a skill
  • download code before lecture and follow along
  • don’t be afraid to try out Python commands!

• represent knowledge with data structures
• iteration and recursion as computational metaphors
• abstraction of procedures and data types
• organize and modularize systems using object classes and methods
• [maybe] different classes of algorithms, searching and sorting
• [maybe] complexity of algorithms

• You can download your own copy, it is free
  • Download Python recommended
• More advanced tasks (data science, machine learning)
  • Download Anaconda
  • Not recommended at this level
• You can use online compilers/IDEs
  • Google Colab recommended will be used in course
  • Jupyter Notebook recommended
  • Python Fiddle
  • PyFiddle
  • Programiz
• Use a Linux distribution such as Ubuntu, Manjaro etc.
  • Python comes built in

• What is an IDE?
  • An Integrated Development Environment allows you to write programs in a programming language and provides extra tools to help the process
    • Syntax highlighting
    • Code completion
    • Bracket completion/matching
    • Debugging
    • Profiling
    • more…
• PyCharm is a very powerful and popular IDE for Python
• IDLE comes built-in with any Python distribution
• Eric, Spyder, Eclipse+PyDev
• Sublime Text, VS Code, Atom, etc. are generic editors that can be extended with Python capabilities

• Fundamentally:
  • performs calculations
    a billion calculations per second!
  • remembers results
    100s of gigabytes of storage!
• What kinds of calculations?
  • built-in to the language
  • ones that you define as the programmer
• computers only know what you tell them

• declarative knowledge is statements of fact.

  • someone will win a Trophy before class ends

• imperative knowledge is a recipe or “how-to”.

  1. Students sign up for lottery
  2. Burkay opens his IDE
  3. Burkay chooses a random number between 1^st and n^th responder
  4. Burkay finds the number in the responders sheet. Winner!

• square root of a number $x$ is $y$ such that $y*y = x$

• recipe for deducing square root of a number $x$

  1. Start with a guess, $g$
  2. If $g*g$ is close enough to $x$, stop and say $g$ is the answer
  3. Otherwise make a new guess by averaging $g$ and $x/g$
  4. Using the new guess, repeat process until close enough

• a programming language provides a set of primitive constructs
  • 2, 4.8, ‘a’, “burkay”, TRUE, +, -, *, …
• expressions are complex but legal combinations of primitives in a programming language
  • 2 + 4.7
  • TRUE && FALSE
  • 3.0 + 2 / 5.4
• expressions and computations have values and meanings in a programming language

• primitive constructs
  • English: words
  • programming language: numbers, strings, simple operators

• syntax

  • English:

    “cat dog boy”	not syntactically valid
    “cat hugs boy”	syntactically valid

  • programming language:

    “hi”5	not syntactically valid
    3.2 * 5	syntactically valid

• static semantics is which syntactically valid strings have meaning

  • English:

    “I are hungry”

    syntactically valid but static semantic error

  • programming language:

    3.2*5	syntactically valid
    3+“hi”	static semantic error

• semantics is the meaning associated with a syntactically correct string of symbols with no static semantic errors
  • English: can have many meanings "Flying planes can be dangerous"
  • programming languages: have only one meaning but may not be what programmer intended
  • Trying to print the square of 5:

x = 5
print(x * 2)

## 10

• Oops! The program runs, because there are no static semantic errors. But it doesn’t do the intended thing. Here is a corrected version:

x = 5
print(x ** 2)

## 25

• syntactic errors
  • common and easily caught
• static semantic errors
  • some languages check for these before running program
  • can cause unpredictable behavior
• no static semantic errors but different meaning than what programmer intended
  • program crashes, stops running
  • program runs forever
  • program gives an answer but different than expected

• syntactic errors

print(3.a)

SyntaxError: invalid syntax

a = 5
3a + 2

SyntaxError: invalid syntax

a - 3'a'

SyntaxError: invalid syntax

• static semantic errors

3 + 'a'

TypeError: unsupported operand type(s) for +: 'int' and 'str'

• a program is a sequence of definitions and commands
  • definitions evaluated
  • commands executed by Python interpreter in a shell
• commands (statements) instruct interpreter to do something
• can be typed directly in a shell or stored in a file that is read into the shell and evaluated

• programs manipulate data objects
• objects have a type that defines the kinds of things programs can do to them
  • Burkay is a human so he can walk, speak English, etc.
  • Chewbacca is a wookie so he can walk, “mwaaarhrhh”, etc.
• objects are
  • scalar (cannot be subdivided)
  • non-scalar (have internal structure that can be accessed)

• int – represent integers, ex. 5
• float – represent real numbers, ex. 3.27
• bool – represent Boolean values True and False
• NoneType – special and has one value, None
• can use type() to see the type of an object

type(5)

## <class 'int'>

type(3.0)

## <class 'float'>

• Strings are non-scalar objects. They have internal structures.

name = "burkay"
print(name[2:4])

## rk

• We can construct new non-scalar objects.
• Object oriented programming is the art of programming using non-scalar objects.

• We can convert object of one type to another
  • Not all types are convertible!
• float(3) converts integer 3 to float 3.0
• int(3.9) truncates float 3.9 to integer 3

float(3)

## 3.0

int(3.9)

## 3

int("burkay")

## ValueError: invalid literal for int() with base 10: 'burkay'

• to show output from code to a user, use print command

print("Hello World!")

## Hello World!

print(3 + 2)

## 5

print("My age is", 41)

## My age is 41

• combine objects and operators to form expressions
• an expression has a value, which has a type
• syntax for a simple expression
  <object> <operator> <object>

• i+j $\rightarrow$ the sum

• i-j $\rightarrow$ the difference

• i*j $\rightarrow$ the product

• i/j $\rightarrow$ the division

• For the sum, the difference and the product, if both objects are integers then the result is an integer. If one or both are floats, then the result is a float.

• For the division the result is always a float.

• i%j $\rightarrow$ the remainder when i is divided by j

• i**j $\rightarrow$ i to the power of j

3 + 5

## 8

3 - 5

## -2

3 * 5

## 15

3 / 5

## 0.6

32 % 5

## 2

3 ** 4

## 81

• parentheses are used to tell Python to prioritize operations

3 * (2 + 5)

## 21

• operator precedence without parentheses
  • **
  • *
  • /
  • + and – executed left to right, as appear in expression

3 * 2 + 5

## 11

• equal sign is an assignment of a value to a variable name

# variable  =  value
pi        = 3.14159

pi_approx = 22/7

• value stored in computer memory
• an assignment binds variable name to value
• retrieve value associated with variable name by invoking the name, by typing pi

pi

## 3.14159

pi_approx

## 3.142857142857143

• you can assign multiple values to variables at once

a, b = 3, 5
a

## 3

b

## 5

• it is important to use clear and understandable names for variables
• also, using parenthesis in expressions helps with code readability

a, b, c = 5, 8, 3.14
d = c * a ** 2 * b

vs.

r, h, pi = 5, 8, 3.14
V_cyl = pi * (r ** 2) * h

• you can enrich your code by adding comments
  • to add a comment, start a line with #
  • lines starting with # are ignored by Python

# Radius, height and pi are defined
r, h, pi = 5, 8, 3.14

# The volume of the cylinder is computed
# Volume is equal to height times the base area
V_cyl = pi * (r ** 2) * h

• why give names to values of expressions?
• to reuse names instead of values
• easier to change code later

pi = 3.14159
radius = 2.2
area = pi * (radius ** 2)
print(area)

## 15.205295600000001

• in programming, variables do not get automatically updated

pi = 3.14159
radius = 2.2
# area of circle
area = pi * (radius**2)
print(area)

## 15.205295600000001

radius = radius + 1
print(radius)

## 3.2

print(area)

## 15.205295600000001

area = pi * (radius**2)
print(area)

## 32.169881600000004

• can re-bind variable names using new assignment statements
• previous value may still be stored in memory but lost the handle for it
• value for area does not change until you tell the computer to do the calculation again

pi = 3.14
radius = 2.2
area = pi * (radius**2)
radius = radius + 1

1. What is printed when the code snippet below is executed?

type(5)
print(3.0 - 1)

1. <class ‘int’>
2. 2.0
3. <class ‘int’> then 2.0
4. nothing

Try the code: https://www.tutorialspoint.com/python/online-python-compiler.php

1. Which expression is allowed in Python?

1. x + y = 2
2. x*x = 2
3. 2 = x
4. xy = 2

1. What is printed when the code snippet below is executed?

a = 6
b = 7
c = 1
total = a + b + c
print(total)
c += 1 # Same as: c = c + 1
print(total)

1. 14 then 14
2. 14 then 15
3. 14
4. 15

These slides are a direct adaptation of the slides used for MIT 6.0001 course present (as of February 2020) on MIT OCW web site.

Original work by:

Ana Bell, Eric Grimson, and John Guttag. 6.0001 Introduction to Computer Science and Programming in Python. Fall 2016. Massachusetts Institute of Technology: MIT OpenCourseWare. License: Creative Commons BY-NC-SA.

Adapted by and for:

Assoc. Prof. Dr. Burkay Genç. MUH101 Introduction to Programming, Fall 2022 Hacettepe University, Computer Engineering Department.