COSC 1306 COMPUTER SCIENCE AND PROGRAMMING JehanFranois Pris

COSC 1306 COMPUTER SCIENCE AND PROGRAMMING Jehan-François Pâris jfparis@uh. edu Fall 2017

THE ONLINE BOOK CHAPTER I GENERAL INTRODUCTION

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

Why? n 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 n Algorithms existed before computers existed ¨ Euclid’s algorithm, … ¨ Computers made them more important

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

These are not algorithms n On a shampoo bottle: ¨ Lather ¨ Rinse ¨ Repeat

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

These are not algorithms n On fuel tank cap: ¨ Turn until three o'clock

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

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

High-level languages n n 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 n Two approaches ¨Compiled languages ¨Interpreted languages

Compiled languages n n 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

C program C compiler Once Executable Computer Results Every time

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

Interpreted languages n 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

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

Advantages n n n 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

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

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

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

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

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

Example n Informal ¨ sales_tax = total_taxable × tax_rate with tax_rate = 0. 0825 n Python ¨ tax. Rate = 0. 0825 ¨ sales. Tax = total. Taxable*tax. Rate

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

Types of instructions n n 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:

Debugging n n n Finding what’s wrong in your program Finding the problem ¨ Hardest Fixing the error

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

Types of errors n Run-time errors/Exceptions ¨ Division by zero ¨ A forgotten special case

Types of errors n Semantic errors ¨ Your solution is wrong ¨ You expressed it poorly ¨ tax. Rate = 8. 25 # in percent! ¨ sales. Tax = total. Taxable*tax. Rate

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

Experimental debugging (II) n n n 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.

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

Natural and formal languages n 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? n Formal languages are simpler, have much more strict syntax rules, and want to be unambiguous ¨ What they say will be taken literally

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

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

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

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

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

Multi-line comments n 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 2017 """ ¨ At the beginning of each program