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ch05-Functions in python programming.ppt
- 1. Copyright © 2018Pearson Education, Ltd. C H A P T E R 5 Functions
- 2. Copyright © 2018Pearson Education, Ltd. Topics • Introduction to Functions • Defining and Calling a Void Function • Designing a Program to Use Functions • Local Variables • Passing Arguments to Functions • Global Variables and Global Constants • Turtle Graphics: Modularizing Code with Functions
- 3. Copyright © 2018Pearson Education, Ltd. Topics (cont’d.) • Introduction to Value-Returning Functions: Generating Random Numbers • Writing Your Own Value-Returning Functions • The math Module • Storing Functions in Modules
- 4. Copyright © 2018Pearson Education, Ltd. Introduction to Functions • Function: group of statements within a program that perform as specific task • Usually one task of a large program • Functions can be executed in order to perform overall program task • Known as divide and conquer approach • Modularized program: program wherein each task within the program is in its own function
- 5. Copyright © 2018Pearson Education, Ltd.
- 6. Copyright © 2018Pearson Education, Ltd. Benefits of Modularizing a Program with Functions • The benefits of using functions include: • Simpler code • Code reuse • write the code once and call it multiple times • Better testing and debugging • Can test and debug each function individually • Faster development • Easier facilitation of teamwork • Different team members can write different functions
- 7. Copyright © 2018Pearson Education, Ltd. Void Functions and Value- Returning Functions • A void function: • Simply executes the statements it contains and then terminates. • A value-returning function: • Executes the statements it contains, and then it returns a value back to the statement that called it. • The input, int, and float functions are examples of value-returning functions.
- 8. Copyright © 2018Pearson Education, Ltd. Defining and Calling a Function • Functions are given names • Function naming rules: • Cannot use key words as a function name • Cannot contain spaces • First character must be a letter or underscore • All other characters must be a letter, number or underscore • Uppercase and lowercase characters are distinct
- 9. Copyright © 2018Pearson Education, Ltd. Defining and Calling a Function (cont’d.) • Function name should be descriptive of the task carried out by the function • Often includes a verb • Function definition: specifies what function does def function_name(): statement statement
- 10. Copyright © 2018Pearson Education, Ltd. Defining and Calling a Function (cont’d.) • Function header: first line of function – Includes keyword def and function name, followed by parentheses and colon • Block: set of statements that belong together as a group – Example: the statements included in a function
- 11. Copyright © 2018Pearson Education, Ltd. Defining and Calling a Function (cont’d.) • Call a function to execute it • When a function is called: • Interpreter jumps to the function and executes statements in the block • Interpreter jumps back to part of program that called the function • Known as function return
- 12. Copyright © 2018Pearson Education, Ltd. Defining and Calling a Function (cont’d.) • main function: called when the program starts • Calls other functions when they are needed • Defines the mainline logic of the program
- 13. Copyright © 2018Pearson Education, Ltd. Indentation in Python • Each block must be indented • Lines in block must begin with the same number of spaces • Use tabs or spaces to indent lines in a block, but not both as this can confuse the Python interpreter • IDLE automatically indents the lines in a block • Blank lines that appear in a block are ignored
- 14. Copyright © 2018Pearson Education, Ltd. Designing a Program to Use Functions • In a flowchart, function call shown as rectangle with vertical bars at each side • Function name written in the symbol • Typically draw separate flow chart for each function in the program • End terminal symbol usually reads Return • Top-down design: technique for breaking algorithm into functions
- 15. Copyright © 2018Pearson Education, Ltd. Designing a Program to Use Functions (cont’d.) • Hierarchy chart: depicts relationship between functions • AKA structure chart • Box for each function in the program, Lines connecting boxes illustrate the functions called by each function • Does not show steps taken inside a function • Use input function to have program wait for user to press enter
- 16. Copyright © 2018Pearson Education, Ltd. Designing a Program to Use Functions (cont’d.)
- 17. Copyright © 2018Pearson Education, Ltd. Local Variables • Local variable: variable that is assigned a value inside a function • Belongs to the function in which it was created • Only statements inside that function can access it, error will occur if another function tries to access the variable • Scope: the part of a program in which a variable may be accessed • For local variable: function in which created
- 18. Copyright © 2018Pearson Education, Ltd. Local Variables (cont’d.) • Local variable cannot be accessed by statements inside its function which precede its creation • Different functions may have local variables with the same name • Each function does not see the other function’s local variables, so no confusion
- 19. Copyright © 2018Pearson Education, Ltd. Passing Arguments to Functions • Argument: piece of data that is sent into a function • Function can use argument in calculations • When calling the function, the argument is placed in parentheses following the function name
- 20. Copyright © 2018Pearson Education, Ltd. Passing Arguments to Functions (cont’d.)
- 21. Copyright © 2018Pearson Education, Ltd. Passing Arguments to Functions (cont’d.) • Parameter variable: variable that is assigned the value of an argument when the function is called • The parameter and the argument reference the same value • General format: • def function_name(parameter): • Scope of a parameter: the function in which the parameter is used
- 22. Copyright © 2018Pearson Education, Ltd. Passing Arguments to Functions (cont’d.)
- 23. Copyright © 2018Pearson Education, Ltd. Passing Multiple Arguments • Python allows writing a function that accepts multiple arguments • Parameter list replaces single parameter • Parameter list items separated by comma • Arguments are passed by position to corresponding parameters • First parameter receives value of first argument, second parameter receives value of second argument, etc.
- 24. Copyright © 2018Pearson Education, Ltd. Passing Multiple Arguments (cont’d.)
- 25. Copyright © 2018Pearson Education, Ltd. Making Changes to Parameters • Changes made to a parameter value within the function do not affect the argument • Known as pass by value • Provides a way for unidirectional communication between one function and another function • Calling function can communicate with called function
- 26. Copyright © 2018Pearson Education, Ltd. Making Changes to Parameters (cont’d.)
- 27. Copyright © 2018Pearson Education, Ltd. Making Changes to Parameters (cont’d.) • Figure 5-18 • The value variable passed to the change_me function cannot be changed by it
- 28. Copyright © 2018Pearson Education, Ltd. Keyword Arguments • Keyword argument: argument that specifies which parameter the value should be passed to • Position when calling function is irrelevant • General Format: • function_name(parameter=value) • Possible to mix keyword and positional arguments when calling a function • Positional arguments must appear first
- 29. Copyright © 2018Pearson Education, Ltd. Global Variables and Global Constants • Global variable: created by assignment statement written outside all the functions • Can be accessed by any statement in the program file, including from within a function • If a function needs to assign a value to the global variable, the global variable must be redeclared within the function • General format: global variable_name
- 30. Copyright © 2018Pearson Education, Ltd. Global Variables and Global Constants (cont’d.) • Reasons to avoid using global variables: • Global variables making debugging difficult • Many locations in the code could be causing a wrong variable value • Functions that use global variables are usually dependent on those variables • Makes function hard to transfer to another program • Global variables make a program hard to understand
- 31. Copyright © 2018Pearson Education, Ltd. Global Constants • Global constant: global name that references a value that cannot be changed • Permissible to use global constants in a program • To simulate global constant in Python, create global variable and do not re-declare it within functions
- 32. Copyright © 2018Pearson Education, Ltd. Introduction to Value-Returning Functions: Generating Random Numbers • void function: group of statements within a program for performing a specific task • Call function when you need to perform the task • Value-returning function: similar to void function, returns a value • Value returned to part of program that called the function when function finishes executing
- 33. Copyright © 2018Pearson Education, Ltd. Standard Library Functions and the import Statement • Standard library: library of pre-written functions that comes with Python • Library functions perform tasks that programmers commonly need • Example: print, input, range • Viewed by programmers as a “black box” • Some library functions built into Python interpreter • To use, just call the function
- 34. Copyright © 2018Pearson Education, Ltd. Standard Library Functions and the import Statement (cont’d.) • Modules: files that stores functions of the standard library • Help organize library functions not built into the interpreter • Copied to computer when you install Python • To call a function stored in a module, need to write an import statement • Written at the top of the program • Format: import module_name
- 35. Copyright © 2018Pearson Education, Ltd. Standard Library Functions and the import Statement (cont’d.)
- 36. Copyright © 2018Pearson Education, Ltd. Generating Random Numbers • Random number are useful in a lot of programming tasks • random module: includes library functions for working with random numbers • Dot notation: notation for calling a function belonging to a module • Format: module_name.function_name()
- 37. Copyright © 2018Pearson Education, Ltd. Generating Random Numbers (cont’d.) • randint function: generates a random number in the range provided by the arguments • Returns the random number to part of program that called the function • Returned integer can be used anywhere that an integer would be used • You can experiment with the function in interactive mode
- 38. Copyright © 2018Pearson Education, Ltd. Generating Random Numbers (cont’d.)
- 39. Copyright © 2018Pearson Education, Ltd. Generating Random Numbers (cont’d.)
- 40. Copyright © 2018Pearson Education, Ltd. Generating Random Numbers (cont’d.) • randrange function: similar to range function, but returns randomly selected integer from the resulting sequence • Same arguments as for the range function • random function: returns a random float in the range of 0.0 and 1.0 • Does not receive arguments • uniform function: returns a random float but allows user to specify range
- 41. Copyright © 2018Pearson Education, Ltd. Random Number Seeds • Random number created by functions in random module are actually pseudo- random numbers • Seed value: initializes the formula that generates random numbers • Need to use different seeds in order to get different series of random numbers • By default uses system time for seed • Can use random.seed() function to specify desired seed value
- 42. Copyright © 2018Pearson Education, Ltd. Writing Your Own Value- Returning Functions • To write a value-returning function, you write a simple function and add one or more return statements • Format: return expression • The value for expression will be returned to the part of the program that called the function • The expression in the return statement can be a complex expression, such as a sum of two variables or the result of another value- returning function
- 43. Copyright © 2018Pearson Education, Ltd. Writing Your Own Value- Returning Functions (cont’d.)
- 44. Copyright © 2018Pearson Education, Ltd. How to Use Value-Returning Functions • Value-returning function can be useful in specific situations • Example: have function prompt user for input and return the user’s input • Simplify mathematical expressions • Complex calculations that need to be repeated throughout the program • Use the returned value • Assign it to a variable or use as an argument in another function
- 45. Copyright © 2018Pearson Education, Ltd. Using IPO Charts • IPO chart: describes the input, processing, and output of a function • Tool for designing and documenting functions • Typically laid out in columns • Usually provide brief descriptions of input, processing, and output, without going into details • Often includes enough information to be used instead of a flowchart
- 46. Copyright © 2018Pearson Education, Ltd. Using IPO Charts (cont’d.)
- 47. Copyright © 2018Pearson Education, Ltd. Returning Strings • You can write functions that return strings • For example:
- 48. Copyright © 2018Pearson Education, Ltd. Returning Boolean Values • Boolean function: returns either True or False • Use to test a condition such as for decision and repetition structures • Common calculations, such as whether a number is even, can be easily repeated by calling a function • Use to simplify complex input validation code
- 49. Copyright © 2018Pearson Education, Ltd. Returning Multiple Values • In Python, a function can return multiple values • Specified after the return statement separated by commas • Format: return expression1, expression2, etc. • When you call such a function in an assignment statement, you need a separate variable on the left side of the = operator to receive each returned value
- 50. Copyright © 2018Pearson Education, Ltd. The math Module • math module: part of standard library that contains functions that are useful for performing mathematical calculations • Typically accept one or more values as arguments, perform mathematical operation, and return the result • Use of module requires an import math statement
- 51. Copyright © 2018Pearson Education, Ltd. The math Module (cont’d.)
- 52. Copyright © 2018Pearson Education, Ltd. The math Module (cont’d.) • The math module defines variables pi and e, which are assigned the mathematical values for pi and e • Can be used in equations that require these values, to get more accurate results • Variables must also be called using the dot notation • Example: circle_area = math.pi * radius**2
- 53. Copyright © 2018Pearson Education, Ltd. Storing Functions in Modules • In large, complex programs, it is important to keep code organized • Modularization: grouping related functions in modules • Makes program easier to understand, test, and maintain • Make it easier to reuse code for multiple different programs • Import the module containing the required function to each program that needs it
- 54. Copyright © 2018Pearson Education, Ltd. Storing Functions in Modules (cont’d.) • Module is a file that contains Python code • Contains function definition but does not contain calls to the functions • Importing programs will call the functions • Rules for module names: • File name should end in .py • Cannot be the same as a Python keyword • Import module using import statement
- 55. Copyright © 2018Pearson Education, Ltd. Menu Driven Programs • Menu-driven program: displays a list of operations on the screen, allowing user to select the desired operation • List of operations displayed on the screen is called a menu • Program uses a decision structure to determine the selected menu option and required operation • Typically repeats until the user quits
- 56. Copyright © 2018Pearson Education, Ltd. Turtle Graphics: Modularizing Code with Functions • Commonly needed turtle graphics operations can be stored in functions and then called whenever needed. • For example, the following function draws a square. The parameters specify the location, width, and color. def square(x, y, width, color): turtle.penup() # Raise the pen turtle.goto(x, y) # Move to (X,Y) turtle.fillcolor(color) # Set the fill color turtle.pendown() # Lower the pen turtle.begin_fill() # Start filling for count in range(4): # Draw a square turtle.forward(width) turtle.left(90) turtle.end_fill() # End filling
- 57. Copyright © 2018Pearson Education, Ltd. Turtle Graphics: Modularizing Code with Functions • The following code calls the previously shown square function to draw three squares: square(100, 0, 50, 'red') square(-150, -100, 200, 'blue') square(-200, 150, 75, 'green')
- 58. Copyright © 2018Pearson Education, Ltd. Turtle Graphics: Modularizing Code with Functions • The following function draws a circle. The parameters specify the location, radius, and color. def circle(x, y, radius, color): turtle.penup() # Raise the pen turtle.goto(x, y - radius) # Position the turtle turtle.fillcolor(color) # Set the fill color turtle.pendown() # Lower the pen turtle.begin_fill() # Start filling turtle.circle(radius) # Draw a circle turtle.end_fill() # End filling
- 59. Copyright © 2018Pearson Education, Ltd. Turtle Graphics: Modularizing Code with Functions • The following code calls the previously shown circle function to draw three circles: circle(0, 0, 100, 'red') circle(-150, -75, 50, 'blue') circle(-200, 150, 75, 'green')
- 60. Copyright © 2018Pearson Education, Ltd. Turtle Graphics: Modularizing Code with Functions • The following function draws a line. The parameters specify the starting and ending locations, and color. def line(startX, startY, endX, endY, color): turtle.penup() # Raise the pen turtle.goto(startX, startY) # Move to the starting point turtle.pendown() # Lower the pen turtle.pencolor(color) # Set the pen color turtle.goto(endX, endY) # Draw a square
- 61. Copyright © 2018Pearson Education, Ltd. Turtle Graphics: Modularizing Code with Functions • The following code calls the previously shown line function to draw a triangle: TOP_X = 0 TOP_Y = 100 BASE_LEFT_X = -100 BASE_LEFT_Y = -100 BASE_RIGHT_X = 100 BASE_RIGHT_Y = -100 line(TOP_X, TOP_Y, BASE_LEFT_X, BASE_LEFT_Y, 'red') line(TOP_X, TOP_Y, BASE_RIGHT_X, BASE_RIGHT_Y, 'blue') line(BASE_LEFT_X, BASE_LEFT_Y, BASE_RIGHT_X, BASE_RIGHT_Y, 'green')
- 62. Copyright © 2018Pearson Education, Ltd. Summary • This chapter covered: • The advantages of using functions • The syntax for defining and calling a function • Methods for designing a program to use functions • Use of local variables and their scope • Syntax and limitations of passing arguments to functions • Global variables, global constants, and their advantages and disadvantages
- 63. Copyright © 2018Pearson Education, Ltd. Summary (cont’d.) • Value-returning functions, including: • Writing value-returning functions • Using value-returning functions • Functions returning multiple values • Using library functions and the import statement • Modules, including: • The random and math modules • Grouping your own functions in modules • Modularizing Turtle Graphics Code