Topics

2. Topics • Augmented Assignment Operators • Sentinels • Input Validation Loops • Nested Loops • Turtle Graphics: Using Loops to Draw Designs

3. Augmented Assignment Operators • Shorthand of mathematical expression where variable on the left side of '=' operator also appears on the right side of the '=' operator • Ex: • x = x +1 becomes x += 1 • balance = balance – withdrawal • Becomes: balance -= withdrawal

4. Sentinels • Special value that marks the end of a sequence of items • Identifies when to terminate loop • Must be distinctive enough so as not to be mistaken for a regular value in the sequence • Ex: when reading a string a null (\0) can be used as a sentinel for the end of the string

5. Sentinel Example: Property Tax Program # This program displays property taxes. TAX_FACTOR = 0.0065 # Represents the tax factor. # Get first lot number print('Enter the property lot number or enter 0 to end.') lot = int(input('Lot number: ')) # Continue processing as long as user doesn't enter lot no 0 while lot != 0: value = float(input('Enter the property value: ')) tax = value * TAX_FACTOR # Calculate the # property's tax. # Display the tax print('Property tax: $', format(tax, ',.2f'), sep='') # Get the next lot number. print('Enter the next lot number or enter 0 to end.') lot = int(input('Lot number: '))

6. Input Validation Loops • Computer cannot tell the difference between good data and bad data • If user provides bad input, program will produce bad output • GIGO: garbage in, garbage out • It is important to design a program such that bad input is never accepted

7. Input Validation Loops • Input validation: inspecting input before it is processed by the program • If input is invalid, prompt user to enter correct data • Commonly accomplished using a while loop which repeats as long as the input is bad • If input is bad, display error message and receive another set of data • If input is good, continue to process the input • Sometimes called error trap or error handler

8. Flow Chart

9. Retail Price Program With Problem # This program calculates retail prices. mark_up = 2.5 # The markup percentage another = 'y' # Variable to control the loop. # Process one or more items. while another == 'y' or another == 'Y': # Get the item's wholesale cost. wholesale = float(input("Enter the item's wholesale cost: ")) # Calculate the retail price. retail = wholesale * mark_up # Display retail price. print('Retail price: $', format(retail, ',.2f'), sep='') # Do this again? another = input('Do you have another item? (Enter y for yes): ')

10. With Validation mark_up = 2.5 # The markup percentage another = 'y' # Variable to control the loop. while another == 'y' or another == 'Y': # Process one or more items. # Get the item's wholesale cost. wholesale = float(input("Enter the item's wholesale cost: ")) # Validate the wholesale cost. while wholesale < 0: print('ERROR: the cost cannot be negative.') wholesale = float(input('Enter the correct wholesale cost:')) retail = wholesale * mark_up # Calculate the retail price. # Display retail price. print('Retail price: $', format(retail, ',.2f'), sep='') # Do this again? another = input('Do you have another item? (Enter y for yes): ')

11. Nested Loops • Loop that is contained inside another loop • Example: analog clock works like a nested loop • Hours hand moves once for every sixty times the minute hand moves • Minute hand moves once every sixty times the second hand moves

12. Clock Program for hours in range (24): for minutes in range(60): for seconds in range(60): print(hours, ":", minutes, ":", seconds)

13. Nested Loops • Key points about nested loops: • Inner loop goes through all of its iterations for each iteration of outer loop • Inner loops complete their iterations faster than outer loops • Total number of iterations in nested loop: number_iterations_inner x number_iterations_outer

14. Test Score Program With Validation Problems # This program averages test scores. num_students = int(input('How many students do you have? ')) num_test_scores = int(input('How many test scores per student? ')) # Determine each students average test score. for student in range(num_students): total = 0.0 # Initialize total for test scores. print('Student number', student + 1) print('-----------------') for test_num in range(num_test_scores): # Total test scores # Allows using variables when asking for input print('Test number', test_num + 1, end='') # Prompt is only ':' when asking for scor score = float(input(': ')) total += score # compound addition expression average = total / num_test_scores print('The average for student number', student + 1, \ 'is:', format(average, '.1f')) print()

15. Standard Library Functions • 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” • Don’t know what is inside the box or function • Some library functions are built into Python interpreter • Just call the function to use it

16. Modules and ImportStatement • Files that store functions of standard library • Help organize library functions not built into the interpreter • Turtle module has functions that use the python turtle • Math module has functions related to math • Copied to computer when you install Python • Use an import statement to call a function stored in a module • Written at the top of the program • Format: import module_name

17. Generating Random Numbers • Random numbers 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: random.function_name() • Use import random to use random functions

18. Generating Random Numbers • randint function: generates a random number in the range provided by the arguments • Returns the random number to where the function was called • Returns an integer • You can experiment with the function in interactive mode • Format random.randint(start number, end number)

19. Random Numbers Program # This program displays five random numbers # in the range of 1 through 100. import random for count in range(5): # Get a random number from 1 to 100 number = random.randint(1, 100) print(number) # Display the number.

20. Rolling Dice Example # This program simulates the rolling of dice. import random MIN = 1 # Constant for minimum numbers MAX = 6 # Maximum random number again = 'y’ # Create a variable to control the loop # Simulate rolling the dice while again == 'y' or again == 'Y': print('Rolling the dice...') print('Their values are:') print(random.randint(MIN, MAX)) print(random.randint(MIN, MAX)) # Do another roll of dice again = input('Roll them again? (y = yes): ')

21. Coin Toss Example # This program simulates 10 tosses of a coin. import random # Constants HEADS = 1 TAILS = 2 TOSSES = 10 for toss in range(TOSSES): # Simulate the coin toss. if random.randint(HEADS, TAILS) == HEADS: print('Heads') else: print('Tails')

22. Functions:randrange,random,uniform • randrange function • Similar to range function • Returns randomly selected integer from the resulting sequence • Same three types of 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

23. Random Number Seeds • Random number created by functions in random module are not really random • They are pseudo-random numbers • Calculated by a formula • Seed value: initializes the formula • 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

24. Turtle Graphics: Using Loops to Draw Designs • You can use loops with the turtle to draw both simple shapes and elaborate designs. • Example, the following for loop iterates four times to draw a square that is 100 pixels wide for x in range(4): turtle.forward(100) turtle.right(90)

25. Turtle Graphics: Using Loops to Draw Designs • This for loop iterates eight times to draw the octagon: for x in range(8): turtle.forward(100) turtle.right(45)

26. Turtle Graphics: Using Loops to Draw Designs • You can create interesting designs by repeatedly drawing a simple shape, with the turtle tilted at a slightly different angle each time it draws the shape. NUM_CIRCLES = 36 # Number of circles to draw RADIUS = 100 # Radius of each circle ANGLE = 10 # Angle to turn for x in range(NUM_CIRCLES): turtle.circle(RADIUS) turtle.left(ANGLE)

27. Turtle Graphics: Using Loops to Draw Designs • This code draws a sequence of 36 straight lines to make a "starburst" design. START_X = -200 # Starting X coordinate START_Y = 0 # Starting Y coordinate NUM_LINES = 36 # Number of lines to draw LINE_LENGTH = 400 # Length of each line ANGLE = 170 # Angle to turn turtle.hideturtle() turtle.penup() turtle.goto(START_X, START_Y) turtle.pendown() for x in range(NUM_LINES): turtle.forward(LINE_LENGTH) turtle.left(ANGLE)