Introduction to Computer Programming for Non-Majors

Transcription

1 Introduction to Computer Programming for Non-Majors CSC 2301, Fall 2016 Mid-term Review Instructor: Long Ma The Department of Computer Science

2 Basic Model of a Computer Input CPU Control Unit Arithmetic /Logic Unit (ALU) Output Memory

3 Programming Languages Natural language has ambiguity and imprecision problems when used to describe complex algorithms. Programs expressed in an unambiguous, precise way using programming languages. Every structure in programming language has a precise form, called its syntax. Every structure in programming language has a precise meaning, called its semantics.

4 Inside a Python Program # File: chaos.py # A simple program illustrating chaotic behaviour Lines that start with # are called comments Python skips text from # to end of line Frequently used in programming Organized Easy read Maintenance

5 The Software Development Process The process of creating a program is often broken down into stages according to the information that is produced in each phase. Analyze the Problem Determine Specifications Create a Design Implement the Design Test/Debug the Program Maintain the Program

6 Example Program: Temperature Converter Before we start coding, let s write a rough draft of the program in pseudocode Pseudocode is precise English that describes what a program does, step by step.

7 Elements of Programs Names Names are given to variables (celsius, fahrenheit), modules (main, convert), etc. These names are called identifiers Every identifier must begin with a letter or underscore ( _ ), followed by any sequence of letters, digits, or underscores. Identifiers are case sensitive.

8 Elements of Programs Complete list of Python Keywords

9 Elements of Programs Output Statements A print statement can print any number of expressions. print( 3+4 ) 3+4 print(3, 4, 3+4) Successive print statements will display on separate lines (\n), or separate by other symbol, like space print(3, 4, end= ), print(3 + 4) A bare print will print a blank line. print()

10 Assignment Statements Simple Assignment <variable> = <expr> variable is an identifier, expr is an expression x = 3.9 * x * (1-x) >>> myvar = 0 The expression on the RHS is evaluated to produce a value which is then associated with the variable named on the LHS.

11 Assigning Input The purpose of an input statement is to get input from the user and store it into a variable. Input string: <variable> = input(<prompt>) First the prompt is printed The input part waits for the user to enter a value and press <enter>

12 Assigning Input Input number: <variable> = eval(input(<prompt>)) The expression that was entered is evaluated to turn it from a string of characters into a Python value (a number). The value is assigned to the variable.

13 Data Type Numeric Data Types Int (while numbers) Float ( with fractions) How can we tell which is which? A numeric literal without a decimal point produces an int value A literal that has a decimal point is represented by a float (even if the fractional part is 0)

14 Math Library To access the sqrt library routine, we need to access it as math.sqrt(x). Using this dot notation tells Python to use the sqrt function found in the math library module. To calculate the root, you can do: x=16 y = math.sqrt(x)

15 Accumulating Results: Factorial What does range(n) return? 0, 1, 2, 3,, n-1 range has another optional parameter! range(start, n) returns start, start + 1,, n-1 But wait! There s more! range(start, n, step) start, start+step,, n-1 list(<sequence>) to make a list

16 Accumulating Results: Factorial Some examples! >>> list(range(10)) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] >>> list(range(5,10)) [5, 6, 7, 8, 9] >>> list(range(5,10,2)) [5, 7, 9]

17 The Object of Objects An object is a sort of active data type that combines data and operations. Objects know stuff (contain data) and they can do stuff (have operations). Basic idea view a complex system as the interaction of simpler objects. Objects interact by sending each other messages.

18 Using Graphical Objects Each object is an instance of some class, and the class describes the properties of the instance. Classes = Blueprints Example: Class of Car Object s of Car

19 GUI Most applications you re familiar with have Graphical User Interfaces (GUI) that provide windows, icons, buttons and menus. Python GUI framework: Tkinter There s a graphics library (graphics.py). It s based on Tkinter.

20 Simple Graphics Programming Since this is a library, we need to import the graphics commands import graphics A graphics window is a place on the screen where the graphics will appear. win = graphics.graphwin() Notice the use of dot notation to invoke the GraphWin function that lives in the graphics library. We can manipulate the window object through the variable win. Windows can be closed/destroyed by issuing the command win.close()

21 Simple Graphics Programming Doing the import this way eliminates the need to preface graphics commands with graphics. >>> from graphics import * >>> win = GraphWin()

22 Simple Graphics Programming >>> p = Point(50, 60) >>> p.getx() 50 >>> p.gety() 60 >>> win = GraphWin() >>> p.draw(win) >>> p2 = Point(140, 100) >>> p2.draw(win)

23 Simple Graphics Programming >>> ### Open a graphics window >>> win = GraphWin('Shapes') >>> ### Draw a red circle centered at point (100, 100) with radius 30 >>> center = Point(100, 100) >>> circ = Circle(center, 30) >>> circ.setfill('red') >>> circ.draw(win) >>> ### Put a textual label in the center of the circle >>> label = Text(center, "Red Circle") >>> label.draw(win) >>> ### Draw a square using a Rectangle object >>> rect = Rectangle(Point(30, 30), Point(70, 70)) >>> rect.draw(win) >>> ### Draw a line segment using a Line object >>> line = Line(Point(20, 30), Point(180, 165)) >>> line.draw(win) >>> ### Draw an oval using the Oval object >>> oval = Oval(Point(20, 150), Point(180, 199)) >>> oval.draw(win)

24 Using Graphical Objects The assignment righteye = lefteye makes righteye and lefteye refer to the same circle! The situation where two variables refer to the same object is called aliasing.

25 Interactive Graphics The graphics module hides the underlying, low-level window management Two simple way to get users input win.getmouse() Entry()

26 Interactive Graphics -win.getmouse() An event is generated whenever a user moves the mouse, clicks the mouse, or types a key on the keyboard. An event is an object that encapsulates information about what just happened! The event object is sent to the appropriate part of the program to be processed, for example, a button event.

27 Getting Mouse Clicks The following code reports the coordinates of a mouse click: from graphics import * win = GraphWin("Click Me!") p = win.getmouse() print("you clicked", p.getx(), p.gety()) We can use the accessors like getx and gety or other methods on the point returned.

28 Getting Mouse Clicks # triangle.pyw # Interactive graphics program to draw a triangle from graphics import * def main(): win = GraphWin("Draw a Triangle") win.setcoords(0.0, 0.0, 10.0, 10.0) message = Text(Point(5, 0.5), "Click on three points") message.draw(win) # Get and draw three vertices of triangle p1 = win.getmouse() p1.draw(win) p2 = win.getmouse() p2.draw(win) p3 = win.getmouse() p3.draw(win)

29 Getting Mouse Clicks # Use Polygon object to draw the triangle triangle = Polygon(p1,p2,p3) triangle.setfill("peachpuff") triangle.setoutline("cyan") triangle.draw(win) # Wait for another click to exit message.settext("click anywhere to quit.") win.getmouse() main()

30 Handling Textual Input -Entry() The triangle program s input was done completely through mouse clicks. There s also an Entry object that can get keyboard input. The Entry object draws a box on the screen that can contain text. It understands settext and gettext, with one difference that the input can be edited.

31 The String Data Type Text is represented in programs by the string data type. A string is a sequence of characters enclosed within quotation marks( or ) >>> str1="hello" >>> str2='spam' >>> print(str1, str2) Hello spam >>> type(str1) Getting a string as input >>> firstname = input("please enter your name: ") Please enter your name: John >>> print("hello", firstname) Hello John 10/3/16-31-

32 The String operations We can access the individual characters in a string through indexing. The positions in a string are numbered from the left, starting with 0. The general form is <string>[<expr>], where the value of expr determines which character is selected from the string. 10/3/16-32-

33 The String Data Type H e l l o B o b >>> greet = "Hello Bob" >>> greet[0] 'H' >>> print(greet[0], greet[2], greet[4]) H l o >>> x = 8 >>> print(greet[x - 2]) B 10/3/16-33-

34 The String Data Type H e l l o B o b In a string of n characters, the last character is at position n-1 since we start counting with 0. We can index from the right side using negative indexes. >>> greet[-1] 'b' >>> greet[::-1] 'bob olleh >>> greet[:-1] Hello Bo >>> str[1:-2] 'ello b' 10/3/16-34-

35 The String Data Type Can we put two strings together into a longer string? Two ways Concatenation glues two strings together (+) >>> "spam" + "eggs" 'spameggs' >>> "Spam" + "And" + "Eggs" 'SpamAndEggs Repetition: 5 * spam 10/3/16-35-

36 The String Data Type String variable is kinds of object! Operator Meaning + Concatenation * Repetition <string>[] <string>[:] len(<string>) for <var> in <string> Indexing Slicing Length Iteration through characters 10/3/16-36-

37 Split to List Read the input as a single string, then split it apart into substrings, each of which represents one words? Strings are objects and have useful methods associated with them One of these methods is split. This will split a string into substrings based on spaces. >>> "Hello string methods!".split() ['Hello', 'string', 'methods!'] 10/3/16-37-

38 Strings, Lists, and Sequences It turns out that strings are really a special kind of sequence, so these operations also apply to sequences! >>> [1,2] + [3,4] [1, 2, 3, 4] >>> [1,2]*3 [1, 2, 1, 2, 1, 2] >>> grades = ['A', 'B', 'C', 'D', 'F'] >>> grades[0] 'A' >>> grades[2:4] ['C', 'D'] >>> len(grades) 5 10/3/16-38-

39 Strings, Lists, and Sequences Lists are mutable, meaning they can be changed. Strings can not be changed. >>> mylist = [34, 26, 15, 10] >>> mylist[2] 15 >>> mylist[2] = 0 >>> mylist [34, 26, 0, 10] >>> mystring = "Hello World" >>> mystring[2] 'l' >>> mystring[2] = "p" Traceback (most recent call last): File "<pyshell#16>", line 1, in -toplevelmystring[2] = "p" TypeError: object doesn't support item assignment 10/3/16-39-

40 Append vs Extend Append: appends object at end Extend: extends list by appending elements from the iterable. >>> x = [1, 2, 3] >>> x.append([4, 5]) >>> x.extend([4, 5]) 10/3/16-40-

41 String Representation and Message Encoding The ord function returns the numeric (ordinal) code of a single character. The chr function converts a numeric code to the corresponding character. >>> ord("a") 65 >>> ord("a") 97 >>> chr(97) 'a' >>> chr(65) 'A' 10/3/16-41-

42 Input/Output as String Manipulation We now have a complete set of type conversion operations: Function float(<expr>) int(<expr>) str(<expr>) eval(<string>) Meaning Convert expr to a floating point value Convert expr to an integer value Return a string representation of expr Evaluate string as an expression 10/3/16-42-

43 String Formatting We can format our output by modifying the print statement as follows: print("the total value of your change is ${0:0.2f}".format(total)) Now we get something like: The total value of your change is $1.50 Key is the string format method. 10/3/16-43-

44 File Processing Working with text files in Python Associate a disk file with a file object using the open function <filevar> = open(<name>, <mode>) Name is a string with the actual file name on the disk. Sometimes you need to specify the path of this file. The mode is either r or w depending on whether we are reading or writing the file. Infile = open( input.txt, r ) Outfile = open( output, w ) 10/3/16-44-

45 File Methods <file>.read() reads the entire file at once, (possibly large, multi-line) string Eg. Infile.read() <file>.readline() reads in just only single line of the file at a time. This is all text up to and including the next newline character <file>.readlines() automatically parse the read contents into a list of the lines in the file. Each list item is a single line including the newline characters. 10/3/16-45-

46 File Processing Opening a file for writing prepares the file to receive data If you open an existing file for writing, you wipe out the file s contents. If the named file does not exist, a new one is created. Outfile = open("mydata.out", "w") print(<expressions>, file=outfile) 10/3/16-46-

47 The Function of Functions 47 So far, we ve seen four different types of functions: Our programs comprise a single function called main(). Built-in Python functions (abs,print,eval ) Functions from the standard libraries (math.sqrt ) Functions from the graphics module (p.getx() ) 10/3/16-47-

48 The reasons of defining functions 48 Having similar or identical code in more than one place has some drawbacks. Issue one: writing the same code twice or more. Issue two: This same code must be maintained in two separate places. Functions Can be used to reduce code duplication Make programs more easily understood Make programs more easily to be maintained. 10/3/16-48-

49 Defining Functions 49 The part of the program that creates a function is called a function definition. A function definition looks like this: def <name>(<formal-parameters>): <body> When the function is used in a program, we say the definition is called or invoked. A function is called by using its name followed by a list of actual parameters or arguments <name>(<actual-parameters>) 10/3/16-49-

50 Parameters Formal parameter -- The parameter defined as part of the function definition, e.g., x in: def cube(x): return x*x*x Actual Parameter -- The actual data sent to a function. It's found in the function call, e.g., 7 in result = cube(7) or y in: y=5 res = cube(y) 10/3/16-50-

51 Function Call 51 When Python comes to a function call, it initiates a four-step process. The calling program suspends execution at the point of the call. The formal parameters of the function get assigned the values supplied by the actual parameters in the call. The body of the function is executed. Control returns to the point just after where the function was called. 10/3/16-51-

52 Functions and Parameters The scope of a variable refers to the places in a program a given variable can be referenced. The variables used inside of a function are local to that function, even if they happen to have the same name as variables that appear inside of another function. Formal parameters, like all variables used in the function, are only accessible in the body of the function. Multiple parameters: In this case the formal and actual parameters are matched up based on position 10/3/16-52-

53 Function: Return Values Sometimes a function needs to return more than one value. To do this, simply list more than one expression in the return statement. When calling this function, use simultaneous assignment. As before, the values are assigned based on position 10/3/16-53-