Chapter 4 Repetition Structures. Dr. Zhang COSC 1436 Spring 2017 June 15, 2017

Transcription

1 Chapter 4 Repetition Structures Dr. Zhang COSC 1436 Spring 2017 June 15, 2017

2 Review Python: about Indentation The indentation level of your statements is significant (i.e. the whitespace at the very left of your statements). Everywhere else, whitespace is not significant and can be used as you like, just like in any other language. You can also insert empty lines that contain nothing (or only arbitrary whitespace) anywhere. What is indentation in Python? if pwd == 'apple': print('logging on...') else: print('incorrect password.') print('all done!') The two blocks of code in our example if-statement are both indented four spaces, which is a typical amount of indentation for Python. In most other programming languages, indentation is used only to help make the code look pretty.

3 Review # In this program, user inputs a number. # If the number is positive, #we print an appropriate message num = float(input("enter a number: ")) if num > 0: print("positive number") print("this is always printed") What is the output when num>0? What is the output when num < 0

4 Review Python Nested if Example # In this program, we input a number # check if the number is positive or # negative or zero and display # an appropriate message # This time we use nested if num = float(input("enter a number: ")) if num >= 0: if num == 0: print("zero") else: print("positive number") else: print("negative number")

5 Indenting Code Review Python uses a different principle. Python programs get structured through indentation, i.e. code blocks are defined by their indentation. Okay that's what we expect from any program code, isn't it? Yes, but in the case of Python it's a language requirement not a matter of style. This principle makes it easier to read and understand other people's Python code.

6 Indenting Code Review So, how does it work? All statements with the same distance to the right belong to the same block of code, i.e. the statements within a block line up vertically. The block ends at a line less indented or the end of the file. If a block has to be more deeply nested, it is simply indented further to the right

7 Chapter 4: Repetition Structures A repetition structure causes a statement or set of statements to execute repeatedly Instead of writing the same sequence of statements over and over, a better way to repeatedly perform an operation is to write the code for the operation once, and then place that code in a structure that makes the computer repeat it as many times as necessary. This can be done with a repetition structure, which is more commonly known as a loop.

8 Condition-Controlled and Count- while Loop o o o Controlled Loops A Condition-Controlled Loop Causes a statement or set of statements to repeat as long as a condition is true. The syntax of a while loop in Python programming language is while expression: statement(s) It tests its condition before performing an iteration, which is also known as a protest loop.

9 While loop Flowchart while loop is Condition-Control

10 While Loop code example # This program calculates sales commissions. # Create a variable to control the loop. keep_going = 'y # Calculate a series of commissions. while keep_going == 'y or keep_going == Y': # Get a salesperson's sales and commission rate. sales = float(input('enter the amount of sales: ')) comm_rate = float(input('enter the commission rate: ')) # Calculate the commission. commission = sales * comm_rate # Display the commission. print('the commission is $', \ format(commission, ',.2f'), sep='') # See if the user wants to do another one. keep_going = input('do you want to calculate another ' + \ 'commission (Enter y for yes): )

11 Avoid Infinite Loops code example # This program demonstrates an infinite loop. # Create a variable to control the loop. keep_going = 'y' # Warning! Infinite loop! while keep_going == 'y': # Get a salesperson's sales and commission rate. sales = float(input('enter the amount of sales: ')) comm_rate = float(input('enter the commission rate: ')) # Calculate the commission. commission = sales * comm_rate # Display the commission. print('the commission is $', \ format(commission, ',.2f'), sep='')

12 The for Loop: A Count-Controlled Loop For Loop: A count-controlled loop iterates a specific number of times. Syntax: for variable in [value1, value2, etc ]: statement statement etc Code example: 1. # This program demonstrates a simple for loop # that uses a list of numbers. print('i will display the numbers 1 through 5.') for num in [1, 2, 3, 4, 5]: print(num) 2. # This program also demonstrates a simple for # loop that uses a list of strings. for name in ['Winken', 'Blinken', 'Nod']: print(name)

13 The for Loop Flowchart: A Count- Controlled Loop

14 Using the range Function with the for Loop Python's range() Function The range() function has two sets of parameters, as follows: range(stop) stop: Number of integers (whole numbers) to generate, starting from zero. eg. range(5) == [0, 1, 2,3,4] range([start, ] stop[, step]) start: Starting number of the sequence. stop: Generate numbers up to, but not including this number. step: Difference between each number in the sequence. Note that: All parameters must be integers. All parameters can be positive or negative. range() (and Python in general) is 0-index based, meaning list indexes start at 0, not 1. eg. The syntax to access the first element of a list is mylist[0]. Therefore the last integer generated by range() is up to, but not including, stop. For example range(0, 5) generates integers from 0 up to, but not including, 5

15 Using the range Function with the for Loop Python's range() Function Using the range Function with the for Loop for num in range(5) print(num) Code example: # This program demonstrates how the range # function can be used with a for loop. # Print a message five times. 1. for x in range(1, 5): print(x) output: for x in range(1, 10, 2): print(x) output:

16 Calculating a Running Total Concept: A running total is a sum of numbers that accumulates with each iteration of a loop. The variable used to keep the running total is called an accumulator. Two elements: A loop that reads each number in the series. A variable that accumulates the total of the numbers as they are read.

17 Accumulator Code Example # This program calculates the sum of a series # of numbers entered by the user. max = 5 # The maximum number # Initialize an accumulator variable. total = 0.0 # Explain what we are doing. print('this program calculates the sum of') print(max, 'numbers you will enter.') # Get the numbers and accumulate them. for counter in range(max): number = float(input('enter a number: ')) total = total + number # Display the total of the numbers. print('the total is', format(total, 0.2f ) )

18 The Augmented Assignment Operators Augmented assignment operators

19 Sentinels Concept: A sentinel is a special value that marks the end of a sequence of values In computer programming, a sentinel value (also referred to as a flag value, trip value, rogue value, signal value, or dummy data) is a special value in the context of an algorithm which uses its presence as a condition of termination, typically in a loop or recursive algorithm.

20 Sentinels Concept: A sentinel is a special value that marks the end of a sequence of values Code example: # This program displays property taxes. TAX_FACTOR = # Represents the tax factor. # Get the first lot number. print('enter the property lot number') print('or enter 0 to end.') lot = int(input('lot number: ')) # Continue processing as long as the user # does not enter lot number 0. while lot!= 0: # Get the property value. value = float(input('enter the property value: ')) # Calculate the property's tax. tax = value * TAX_FACTOR # Display the tax. print('property tax: $', format(tax, ',.2f'), sep='') # Get the next lot number. print('enter the next lot number or') print('enter 0 to end.') lot = int(input('lot number: '))

21 Input Validation Loops Concept: Input validation is the process of inspecting data that has been input to a program, to make sure it is valid before it is used in a computation. Input validation is commonly done with a loop that iterates as long as an input variable references bad data. Logic containing an input validation loop

22 Validation Code Example Retail no validation: # 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 the retail price. print('retail price: $', format(retail, ',.2f')) # Do this again? another = input('do you have another item? ' + \ '(Enter y for yes): ')

23 Nested Loops Concept: A loop that is inside another loop is called a nested loop

24 Nested Loops

25 Validation Code Example Retail with validation: # 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 wholesale cost: ")) # Validate the wholesale cost. while wholesale < 0: print('error: the cost cannot be negative.') wholesale = float(input('enter the correct ' + \ 'wholesale cost:')) # Calculate the retail price. retail = wholesale * mark_up # Display the retail price. print('retail price: $', format(retail, ',.2f')) # Do this again? another = input('do you have another item? ' + \ '(Enter y for yes): ')

26 Nested loop Code example # This program averages test scores. It asks the user for the number of students and the number of test scores per student. #Get the number of students. num_students = int(input('how many students do you have? ')) # Get the number of test scores per student. num_test_scores = int(input('how many test scores per student? ')) # Determine each students average test score. for student in range(num_students): # Initialize an accumulator for test scores. total = 0.0 # Get a student's test scores. print('student number', num_students + 1) print(' ') for test_num in range(num_test_scores): print('test number', test_num + 1, end='') score = float(input(': ')) # Add the score to the accumulator. total += score # Calculate the average test score for this student. average = total / num_test_scores # Display the average. print('the average for student number', student + 1, \ 'is:', format(average, '.1f')) print()

27 Nested loop Lab Example We want to print * on the screen in the following pattern: * ** *** **** ***** ****** ******* ********

28 Lab Example Code total_rows=8 for row in range(total_rows): # generate row for asterrisks in range(row+1): #print * in each row print("*",end="") print() # print new line

29 Lab Example Write a Loop that calculates the total of the following series of numbers: 1/30 + 2/29 + 3/ /1

30 Lab Example ##Write a Loop that calculates the total of the ##following series of numbers: ##1/30 + 2/29 + 3/ /1 total = 0.0 numerator = 30 for num in range(1,31,1): total = total + (num/numerator) numerator = numerator -1 print("1/30 + 2/29 + 3/ /1 =",format(total,'.2f'))

31 Lab Example (Programming Exercices: P161 #3) Write a program that asks the user to enter the amount that he or she has budgeted for a month. A loop should then prompt the user to enter each his or her expenses for the month and keep a running total. When the loop finishes, the program should display the amount that the user is over or under budget.

32 Lab Example print("programming Exercises: P161 #3") budget=float(input("please enter your budget for a month :")) totalexpense = 0 answer = 'Y' while(answer=='y' or answer=='y'): expense=float(input("please enter your expense: ")) totalexpense += expense answer=input("please enter your 'Y' for more expense: ") underorover = budget-totalexpense if underorover > 0: print("under budget!! Good!!. You have saved ", underorover) else: print("over budget!! Bad. You over budget ", underorover)

33 Chapter 4 lab and assignments Lab: Programming Exercises : #1, # 3, #13,#14 Assignments: Programming Exercises 9, 10,11, 12, (p163) Due on 06/20/2017