1. COSC 1306 COMPUTER SCIENCE AND PROGRAMMING
Jehan-François Pâris
Fall 2016 1

2. THE ONLINE BOOK CHAPTER I GENERAL INTRODUCTION

3. Problem solving Given a problem, find a solution Should be Correct
Accurate
Complete
We want an algorithm

4. Why?
Once we have an algorithmic solution for a problem, we can convert it into a program
Let a computer solves many instances of the problem
Algorithms existed before computers existed
Euclid’s algorithm, …
Computers made them more important

5. What defines an algorithm
Must always produce the right result
Under all circumstances
Instructions should be well-defined
Anybody using the algorithm should obtain the same answer
Should have a finite number of steps
Cannot run forever

6. These are not algorithms
On a shampoo bottle:
Lather
Rinse
Repeat

7. These are not algorithms
On a shampoo bottle:
Lather
Rinse
Repeat
How many times?

8. These are not algorithms
On fuel tank cap:
Turn until three o'clock

9. These are not algorithms
On fuel tank cap:
Turn until three o'clock
Ambiguous!

10. Python High-level interpreted language Created by Guido Van Rossum
Named after Monty Python's Flying Circus
BBC comedy series of early 70’s

11. High-level languages Have replaced assembler
Increase programmer’s productivity
Can run on multiple architectures
Intel/AMD and ARM
Must be translated into machine language before being executed
Two approaches
Compiled languages
Interpreted languages

12. Compiled languages
Programs written in old Fortran, C, C++ and so go through a program that translates them into directly executable code
The compiler
Doing g++ myprogram.cpp –o myprogram produces an executable file called myprogram that can run anytime
No need to recompile

13. C program
C compiler
Executable
Results
Computer
Once
Every time

14. Advantages The executable can run on any computer with
Same CPU instruction set
Same—or compatible—operating system
We can sell copies of the executable without revealing the secrets of our program
Reverse engineering is possible but very time-consuming

15. Interpreted languages
Languages like Python and Ruby are not compiled
Translated again and again into bytecode each time we execute them
Bytecode is interpreted by the language interpreter

16. translates and executes
Source code
Python Interpreter:
translates and executes
the program
Every time
Results

17. Advantages Platform independence:
Bytecode can run on any platform for which there is an interpreter
Dynamic typing:
Same variable can refer to a string then an integer then …
Smaller executable sizes

18. Disadvantages Portability limitations :
Bytecode will not run on any machine on which the interpreter was not installed.
Speed:
Bytecode is not optimized for a specific architecture
Just-in-time compilation introduces delays
Cannot sell copies of a program without revealing its source code

19. A partial solution
In many cases, speed is not an issue outside of loops than get executed thousand and thousands of times
Loops inside other loops
Can rewrite code for these inner loops in C and include this code into a Python program
Use Python C API

20. Neither fish nor fowl Java is compiled Like C into bytecode
Like Python
Bytecode is executed by Java Virtual Machine

21. A comparison Compiled languages
Translate once the whole program into an executable
Fast
Can run on any machine having same architecture and same OS
Interpreted languages
Translate programs line by line each time they are executed
Much slower
Can run on any machine having the interpreter installed

22. Programs
Specify a sequence of instructions to be executed by a computer
The order of the instructions specify their order of execution

23. Example
Informal
sales_tax = total_taxable × tax_rate with tax_rate =
Python
taxRate =
salesTax = totalTaxable*taxRate

24. My first Python program
print("I add two numbers, 2 and 3")
print(2 + 3)
It’s “print” and not “Print”
Cannot use smart quotes
Semicolons are optional and nobody uses them

25. Types of instructions Input & output name = input("Enter your name: ")
print("Hello! ")
Math and logic
celsius = (fahrenheit - 32)*5/9
Conditional execution
if celsius > 100 :
Repetition
for all lines in myfile:

26. Debugging Finding what’s wrong in your program Finding the problem
Hardest
Fixing the error

27. Types of errors Syntax errors print("Hello!) # missing closing quotes
print("Hello!") # correct
A big annoyance
Especially for beginners
Everyone ends better at it

28. Types of errors Run-time errors/Exceptions Division by zero
A forgotten special case

29. Types of errors Semantic errors Your solution is wrong
You expressed it poorly
taxRate = 8.25 # in percent!
salesTax = totalTaxable*taxRate

30. Experimental debugging (I)
Modify your program until it works?

31. Experimental debugging (II)
Look at your program
Try to explain it to yourself, to friends
Do not let them copy your code!
Add print statements after each step
Remove them or better comment them out when they are not needed anymore.

32. Experimental debugging (III)
Act as if you are investigating a mystery

33. Natural and formal languages
Natural languages are complex, tolerate ambiguities, often have to be taken figuratively
I am literally dying of thirst
Could you bring me a glass of water?
Formal languages are simpler, have much more strict syntax rules, and want to be unambiguous
What they say will be taken literally

34. Natural language ambiguities
KIDS MAKE NUTRITIOUS SNACKS
STOLEN PAINTING FOUND BY TREE
QUEEN MARY HAVING BOTTOM SCRAPED
MILK DRINKERS ARE TURNING TO POWDER
SQUAD HELPS DOG BITE VICTIM

35. My first program print("I add two numbers, 2 and 3") print(2 + 3)
It’s “print” and not “Print”
Cannot use smart quotes
Semicolons are optional and nobody uses them

36. Interactive Python
Requires Javascript to be authorized for at least interactivepython.org

37. Commenting Main purpose To help human readers understand your programs
Becomes more important as program sizes grow
To record who wrote the program, when it happened and so on

38. In-line comments
Anything following a pound sign (“#”) is ignored by the Python interpreter
print ("Hello World!")
print ("Hello World!") # what to say?
r = d/2 # compute radius of circle
#Begin main loop

39. Multi-line comments
Anything between three double quotes—"""—is ignored by the Python interpreter
""" Tell what the program does Jehan-Francois Paris Assignment # 1 COSC 1306 MW 2:30-4:00 Fall """
At the beginning of each program