Today s Topics... Intro to Computer Science (cont.) Python exercise. Reading Assignment. Ch 1: 1.5 (through 1.5.2) Lecture Notes CPSC 121 (Fall 2011)

Transcription

1 Today s Topics... Intro to Computer Science (cont.) Python exercise Reading Assignment Ch 1: 1.5 (through 1.5.2) S. Bowers 1 of 8

2 Computation (cont.) What parts do recipes usually have? A description (the requirements) An ingredient list (the input) A set of ordered steps to perform (a procedure) A recipe is an example of procedural (imperative) knowledge Knowledge of how to perform some task (or process) A recipe gives a procedure to create ( compute ) an output from input Declarative knowledge only tells us about the output, not how to produce it Computation is a process for performing certain tasks Like the process of making your favorite dessert We typically think of computation as a process that manipulates data That is, computing (deriving) new data from existing data Computation is directed by a series of instructions Often called procedures, methods, or algorithms We write these down using a programming language A procedure written in a programming language is called a program Computers execute programs for us S. Bowers 2 of 8

3 At the 10,000 foot level, as programmers We figure out what the instructions should be (the algorithm) We write them down as a program (in a programming language) We give the program to a computer with input data The computer runs the program on input data (the computation process) And supplies the output A computer is a general-purpose machine (aka universal machine ) Instead of being able to execute only one or a few types of processes e.g., like a toaster or a car wash A computer can perform any computation According to the Church-Turing Thesis Everything computable is computable by a Turing Machine But you have to give it the instructions :-) S. Bowers 3 of 8

4 Another example... Consider the following two statements 1. x is y such that y 2 = x for y 0 2. To find an approximation of x, make a guess g, improve the guess by averaging g and x/g, and continue until g is good enough What type of knowledge do each of these represent? Another way to think of computation... (from Abelson & Sussman) Computation is like a sorcerer s idea of a magical spirit Cannot be seen or touched, is not composed of matter The programs we use to conjure computations are like a sorcerer s spells From symbolic expressions in arcane, esoteric programming langauges Used to prescribe the tasks we want our computations to perform We ll conjure our (computational) spirits in a magical language called Python Learning to program is considerably less dangerous than learning sorcery Computational spirits are conveniently contained within a computer In case you aren t convinced... def square_root(x, d): g = 1 # not a great guess for i in range(1, d): g = (g + (x/g))/2 return g S. Bowers 4 of 8

5 Getting started in the CS Lab The CS lab guide The computers run the Linux (CentOS) operating system Similar to MS Windows and MacOS, but different Login using GU userid and password If you can t login, send to Jason Schnagl (schnagl@gonzaga.edu, PC 216, ) Double-click on the IDLE 3.2 desktop icon Starts a basic Python Shell Shows a prompt >>> called a chevron You can start typing statements at the prompt (e.g., 3+4) IDLE includes a simple code editor From the Python Shell window click File > New Window In the new window, note there is no prompt You can enter statements into the window You are creating/editing a Python module (more later) S. Bowers 5 of 8

6 After you enter statements, you can run the module You need to save the module file first You might create a cpsc121 folder and save the file, e.g., as lab1.py Run the module by clicking Run > Run Module in the editor (not the shell) You can also hit the F5 shortcut key After you run the module, you ll see RESTART in the Python Shell This is where you wold see the output of your program Print statements The Python Shell prints the result of every statement entered E.g., >>> prints 7 In a module, you have to explicitly tell Python to print the result of a statement E.g., print(3 + 4) prints 7 in the Python Shell Whereas the statement doesn t print anything S. Bowers 6 of 8

7 Basic file management Double-click on the Home folder icon on the desktop For me, this is bowers s Home Printing Select File > Print Window from within Python Shell or the editor and select the Default Printer Don t forget to logout! Select System > Log Out from the menu bar on the desktop Don t need to shut down the computer, but do need to log out S. Bowers 7 of 8

8 Exercise: The Hello World! Program By convention, the first program written when learning a new langauge >>> print("hello World!") Hello World! >>> In Python, this is a super-easy program to write Exercise: 1. Start IDLE 2. Using the Python Shell, use print to display Hello World! 3. Try print with other strings 4. Send your session to the printer 5. Repeat the above, but use the editor instead of the Python Shell 6. Put your name on both hardcopies and turn them in Notes: print is a function We call the print function We pass arguments to function calls The number of arguments depends on the function S. Bowers 8 of 8