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

2. THE ONLINE BOOK CHAPTER I GENERAL INTRODUCTION

3. Algorithms
Problem solving is coming with an algorithm that can serve as a general solution

4. High-level languages Very few programs are written in machine language
Prefer to use a more human-friendly high-level language
Cannot be directly executed by any computer
Must be first translated
Two approaches
Compiled languages
Interpreted languages

5. Compiled languages (I)
Programs written in old Fortran, C, C++ and so on must 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

6. Compiled languages (II)
C program
C compiler
Executable

7. 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

8. Interpreted languages (I)
Languages like Basic, Perl, Python and Ruby are not compiled
Translated again and again into bytecode Each time we execute them
Bytecode is interpreted by the language interpreter

9. Interpreted languages (II)
Python Program
Byte code
Python Interpreter:
translates program into bytecode
then executes it

10. 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

11. 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 a copies of a program without revealing its source code

12. 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

13. Neither fish nor fowl Java is compiled Like Fortran, C, …
into bytecode
Like Perl and Python
Bytecode is executed by Java Virtual Machine

14. 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

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

16. 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 file:

17. Debugging Finding what’s wrong in your program
Finding the roof of the problem
Fixing the error

18. Types of error Syntax errors
print(“Hello!” “How are you:”) # missing comma
Run-time errors
Division by zero
Semantic errors
Your solution is wrong
You expressed it poorly

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

20. 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.

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

22. 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

23. 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

24. 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

25. In-line comments
Anything following a pound sign (“#”) is ignored by the Python interpreter
print (“Hello World!”)
print (“Hello World!”) # not very original
r = d/2 # compute radius of circle
#Begin main loop

26. 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