Introduction to Computer Science. Course Goals. Python. Slides02 - Intro to Python.key - September 15, 2015

Transcription

1 Introduction to Computer Science Barbara Lerner 1 Office Clapp 227, x3250 Mon 1-2 Tues 4-5 Wed Thurs 3:30-4:30 or by appointment Course Goals 2 Learn how to describe a solution as an algorithm Become familiar with the concept of abstraction in computer science Be able to develop simple programs in Python Become more aware of how computing affects our daily lives Python 3 The programming language we will be using is called Python ( It s used by companies like Google, Industrial Light & Magic, Pixar, Nextel, and others The kind of Python we re using is called Jython It s Java-based Python (

2 What is Programming? 4 Writing instructions that manipulate data Examples: Data has a Type 5 34 and 56 are integers Hello and world are strings 3.14 is a floating point number The same operator (like +) can mean different things depending on the type of the data it is operating on Variables 6 A variable is a name and has a value An assignment statement gives a value to a variable birthyear = 1995 When a variable is used in a calculation, its current value is used age = birthyear A later assignment statement changes the variable s value

3 Syntax and Semantics Syntax - the grammar rules of the language 7 Semantics - the meaning of something stated in the language Assignment statement: Syntax: <variable> = <expression> Semantics: the expression is evaluated and the variable remembers the value birthyear = 1995 age = birthyear = does not mean equals! 8 = means is assigned the value age = age + 1 Take the current value of the variable age Add 1 to that value Put the new value in the variable age Functions 9-1

4 Functions 9-2 function name Functions 9-3 argument Functions 9-4 return value

5 Functions 9-5 Calling the same function with a different argument gives a different return value Calling Functions: Syntax and Semantics 10 Syntax: <function-name> ( <arguments> ) The number of arguments depends on the function. Some functions have 0 arguments, some have many. Multiple arguments are separated by commas. Semantics: Execute the statements defined in the function, using the arguments passed in. If called from the command area, print the value that the function returns. sqrt ( 999 ) max ( age, birthyear ) JES 11 Program Area - Enter Python code that we save in files Command Area - Enter Python commands to be executed

6 Defining Functions 12-1 return Defining Functions Defining the function return 12-2 Defining Functions 12-3 return function name

7 Defining Functions 12-4 return instructions to execute when the function is called Defining Functions 12-5 return Calling the function Defining Functions: Syntax and Semantics Syntax: def <function-name> ( <parameters> ) : <instructions> 13 Semantics: Give a name to a set of instructions so they can be used later. A function defines an abstraction and implements an algorithm. return

8 Matching Arguments and Parameters 14 Function definition determines the number of parameters and their order return hello has no parameters Matching Arguments and Parameters 15 Function definition determines the number of parameters and their order def plus (num1, num2) : return num1 + num2 >>> plus(3, 6) 9 plus has two numeric parameters Matching Arguments and Parameters 16 Function definition determines the number of parameters and their order def reportage (name, birthyear) : age = birthyear return name + is + str(age) >>> reportage ( Greg, 1995) Greg is 20 reportage has a String and a numeric parameter

9 Summary of Today s Functions 17 pickafile () - Loads a file browser. Returns the full path to the file that the user selected. makepicture ( <filename> ) - Pass in a filename as a parameter. Returns a Picture object. show ( <Picture> ) - Pass in a Picture object. Displays the Picture in a window.