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CSCI/CMPE 4341 Topic: Programming in Python Chapter 5 : Functions

CSCI/CMPE 4341 Topic: Programming in Python Chapter 5 : Functions. Xiang Lian The University of Texas – Pan American Edinburg, TX 78539 lianx@utpa.edu. Objectives. In this chapter, you will: Understand how to construct programs modularly from small pieces called functions

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CSCI/CMPE 4341 Topic: Programming in Python Chapter 5 : Functions

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  1. CSCI/CMPE 4341 Topic: Programming in PythonChapter 5: Functions Xiang Lian The University of Texas – Pan American Edinburg, TX 78539 lianx@utpa.edu

  2. Objectives • In this chapter, you will: • Understand how to construct programs modularly from small pieces called functions • Create new functions • Learn how to exchange information between functions • Know how to generate random numbers • Explore the visibility of identifiers in programs • Use recursive functions • Become familiar with default and keyword arguments

  3. Introduction • The best way to maintain a large program is to break it down into smaller more manageable pieces • These pieces are called components • Problem solving • Divide and conquer

  4. Program Components in Python • Components • Consist of functions, classes, modules and packages • In most cases programs are a combination of programmer defined functions and classes with predefined ones • Programmer defined functions • Programs the perform specific tasks and execute at various points in the program • Modules • Used to perform common tasks • Help to eliminate code rewriting • The standard library • A collection of modules provided with the python language

  5. Program Components in Python (cont'd) • Functions • Invoked by a function call • Specifies the function name and its arguments • Arguments • Additional information the function needs to compete its task • The return task • Information that the function returns to the source that invoked it for use elsewhere in the program

  6. boss worker1 worker2 worker3 worker4 worker5 Example of Program Components in Python Fig. 4.1 Hierarchical boss-function/worker-function relationship.

  7. Functions • Functions • Allow a programmer to modularize a program • Variables created in a function are local to that function • Parameters • Also local variables • Provide a means for one function to communicate with another • Purpose • Functions support the divide and conquer strategy • Help enhance software reusability • Acts as building blocks for the program • Avoids code repetition

  8. Module math Functions • Module • Contains function definitions and other elements • All of which are related in some way • Calling a function • functionName ( argument1, argument2 ) • The import keyword is used to include a module • Invoking functions from a module • Use the module name followed by the dot operator (.) • moduleName.functionName( argument )

  9. Examples of Module math Functions import math print (math.sqrt(900)) c=13 d=3 f=4 print(math.sqrt(c+d*f))

  10. More math Functions

  11. More math Functions (cont'd)

  12. Examples of Floor and Ceiling • floor function: • math.floor(2.10) = 2 • math.floor(2.00) = 2 • math.floor(1.90) = 1 • math.floor(1.80) = 1 • ceil function: • math.ceil(0.00) = 0 • math.ceil(0.10) = 1 • math.ceil(0.20) = 1 • math.ceil(0.30) = 1

  13. User-Defined Functions • Definitions • Functions must be defined before they are used • def functionName ( paramList ): • functionName is a valid identifier • paramList is a comma separated list of parameters received • The actions of the functions then follows • They should all be indented appropriately • The actions are also called the block or the function body

  14. This is a function definition, the function is called square and is passed the value y The function returns the passed value multiplied by itself This calls the square function and passes it the value x # Fig. 4.4: fig04_04.py # Creating and using a programmer-defined function. # function definition def square( y ): return y * y for x in range( 1, 11 ): print (square( x )) print() Fig04_04.pyProgram Output 1 4 9 16 25 36 49 64 81 100

  15. This is a function that receives tree values The function determines the greater of three numbers and returns it Gets three integers, passes them to the maximumValue function, and displays the results to the user The same process is performed on float and string variables to show the diversity of the function # Fig. 4.5: fig04_05.py # Finding the maximum of three integers. def maximumValue( x, y, z ): maximum = x if y > maximum: maximum = y if z > maximum: maximum = z return maximum a = int( input( "Enter first integer: " ) ) b = int( input( "Enter second integer: " ) ) c = int( input( "Enter third integer: " ) ) # function call print ("Maximum integer is:", maximumValue( a, b, c ) ) print () # print new line d = float( input( "Enter first float: " ) ) e = float( input( "Enter second float: " ) ) f = float( input( "Enter third float: " ) ) print ("Maximum float is: ", maximumValue( d, e, f )) print () g = input( "Enter first string: " ) h = input( "Enter second string: " ) i = input( "Enter third string: " ) print ("Maximum string is: ", maximumValue( g, h, i )) Fig04_05.py

  16. Random-Number Generation • The random module • Used to generate a random number for the programmer • Function randrange • Generates a number from the first argument up to, but not including, the second argument • Each number in the range has the same likelihood of being selected by the function

  17. The random module is imported The randrange function is called passing the values 1 and 7 As shown by the output the number range is really from 1 to 6 not 7 # Fig. 4.6: fig04_06.py # Random integers produced by randrange. import random for i in range( 1, 21 ): # simulates 20 die rolls print ("%10d" % (random.randrange( 1, 7 )), end ="") if i % 5 == 0: # print newline every 5 rolls print () Fig04_06.pyProgram Output 5 3 3 3 2 3 2 3 3 4 2 3 6 5 4 6 2 4 1 2

  18. Creates a loop that executes 6000 times Again the randrange function is called with values 1 and 7 passed to it This nested if is used to keep track of the occurrence of each number generated This else statement should never be used by the program but is there for good programming purposes # Fig. 4.7: fig04_07.py # Roll a six-sided die 6000 times. import random frequency1 = 0 frequency2 = 0 frequency3 = 0 frequency4 = 0 frequency5 = 0 frequency6 = 0 for roll in range( 1, 6001 ): # 6000 die rolls face = random.randrange( 1, 7 ) if face == 1: # frequency counted frequency1 += 1 elif face == 2: frequency2 += 1 elif face == 3: frequency3 += 1 elif face == 4: frequency4 += 1 elif face == 5: frequency5 += 1 elif face == 6: frequency6 += 1 else: # simple error handling print ("should never get here!") print(frequency1) print(frequency2) print(frequency3) print(frequency4) print(frequency5) print(frequency6) Fig04_07.py

  19. The start of the rollDice function Prints out the value of each random number along with their sum, which is also returned An if statement that determines the next step in the game based on what the player rolled The while statement will loop until the player has wither lost or won the game # Fig. 4.8: fig04_08.py # Craps. import random def rollDice(): die1 = random.randrange( 1, 7 ) die2 = random.randrange( 1, 7 ) workSum = die1 + die2 print ("Player rolled %d + %d = %d" % ( die1, die2, workSum )) return workSum sum = rollDice() # first dice roll if sum == 7or sum == 11: # win on first roll gameStatus = "WON" elif sum == 2or sum == 3or sum == 12: # lose on first roll gameStatus = "LOST" else: # remember point gameStatus = "CONTINUE" myPoint = sum print ("Point is", myPoint) while gameStatus == "CONTINUE": # keep rolling sum = rollDice() if sum == myPoint: # win by making point gameStatus = "WON" elif sum == 7: # lose by rolling 7: gameStatus = "LOST" if gameStatus == "WON": print ("Player wins") else: print ("Player loses") Fig04_08.py

  20. Scope Rules • Namespaces store information about an identifier and a value to which it is bound • Local namespace • Contains values that were created in a block • Each function has a unique local namespace • Global namespace • Stores the names of date, functions and classes defined within the file or module • Each module contain a __name__ • It holds the name of the module • Function dir() to show variables under the current namespace • The global keyword • Used to automatically search the global namespace

  21. Scope Rules (cont'd) • Built-in namespace • Not usually modified by programmers • Contains functions such as input, int, and range • The built-in namespace is included when the interpreter starts

  22. This is a global variable and can be used by any function in the program Has its own value of x therefore the global value is hidden Function b uses and modifies the value of the global x Changes the value of x to 7 # Fig. 4.10: fig04_10.py # Scoping example. x = 1 # global variable # alters the local variable x, shadows the global variable def a(): x = 25 print ("\nlocal x in a is", x, "after entering a") x += 1 print ("local x in a is", x, "before exiting a") # alters the global variable x def b(): global x print ("\nglobal x is", x, "on entering b") x *= 10 print ("global x is", x, "on exiting b") print ("global x is", x) x = 7 print ("global x is", x) a() b() a() b() print ("\nglobal x is", x) Fig04_10.py

  23. Keyword import and Namespaces • Importing • Affects a programs namespace • Use the keyword import followed by the desired module • import math • dir() • dir(math) • import random • dir(random) • dir(__builtins__)

  24. Recursion • Method that calls itself • A recursive method solves only a simple problem (base case) • For any thing other than the base case, it calls itself with a slightly simpler problem • Eventually it becomes the base case for which it knows the answer

  25. Example of Factorial # n! def Factorial (number): # base case if number <= 1: return 1 else: return number * Factorial(number -1) print (Factorial (4))

  26. Example Using Recursion: The Fibonacci Series • The Fibonacci series • Each number is composed of the sum of the two previous numbers • Fibonacci( n ) = Fibonacci( n – 1 ) + Fibonacci( n – 2 ) • Fibonacci( 1 ) = 1 and Fibonacci( 0 ) = 0 • 0, 1, 1, 2, 3, 5, 8, 13, …

  27. If n is either 0 or 1 then return that value If the value is neither zero or one then two recursive calls are made # Fig. 4.18: fig04_18.py # Recursive fibonacci function. def fibonacci( n ): if n < 0: print ("Cannot find the fibonacci of a negative number.") if n == 0or n == 1: # base case return n else: # two recursive calls return fibonacci( n - 1 ) + fibonacci( n - 2 ) number = int(input( "Enter an integer: " ) ) result = fibonacci( number ) print ("Fibonacci(%d) = %d" % ( number, result )) Fig04_18.pyProgram Output Enter an integer: 0 Fibonacci(0) = 0 Enter an integer: 1 Fibonacci(1) = 1 Enter an integer: 2 Fibonacci(2) = 1

  28. Default Arguments • Function arguments • Functions may commonly receive a particular value type • When this is true a default argument can be set • Must appear to the right of any other arguments • A default value can also be set • If passes a value then the default value is overridden

  29. Sets the three values, defaulted to 1 When this is called the default values will be used The 10 will replace the left most 1 and the other default values will be used Here two values are sent meaning only height will use its default value In this case no default values were used # Fig. 4.20: fig04_20.py # Using default arguments. # function definition with default arguments def boxVolume( length = 1, width = 1, height = 1 ): return length * width * height print ("The default box volume is:", boxVolume() ) print ("\nThe volume of a box with length 10," ) print ("width 1 and height 1 is:", boxVolume( 10 ) ) print ("\nThe volume of a box with length 10," ) print ("width 5 and height 1 is:", boxVolume( 10, 5 ) ) print ("\nThe volume of a box with length 10," ) print ("width 5 and height 2 is:", boxVolume( 10, 5, 2 ) ) Fig04_20.pyProgram Output The default box volume is: 1 The volume of a box with length 10, width 1 and height 1 is: 10 The volume of a box with length 10, width 5 and height 1 is: 50 The volume of a box with length 10, width 5 and height 2 is: 100

  30. Keyword Arguments • Keyword arguments • Just as a programmer specifies default arguments keyword arguments can be specified as well • Allows parameter to be passed in any order so long as they are explicitly stated • Will set the values that were not passed to the default

  31. The definition of this function specifies keywords, some of which have default values as well Sets Deitel as the first value and uses the defaults for all the others In order the variables are given new values and CGI uses the default The values need not be entered in order # Fig. 4.21: fig04_21.py # Keyword arguments example. def generateWebsite( name, url = "www.deitel.com", Flash = "no", CGI = "yes" ): print ("Generating site requested by", name, "using url", url) if Flash == "yes": print ("Flash is enabled" ) if CGI == "yes": print ("CGI scripts are enabled" ) print () # prints a new line generateWebsite( "Deitel" ) generateWebsite( "Deitel", Flash = "yes", url = "www.deitel.com/new" ) generateWebsite( CGI = "no", name = "Prentice Hall" ) Fig04_21.pyProgram Output Generating site requested by Deitel using url www.deitel.comCGI scripts are enabled Generating site requested by Deitel using url www.deitel.com/newFlash is enabledCGI scripts are enabled Generating site requested by Prentice Hall using url www.deitel.com

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