Designing a Program: Input, Processing, and Output

2. Topics • Designing a Program • Input, Processing, and Output • Displaying Output with print Function • Comments • Variables • Reading Input from the Keyboard • Performing Calculations • More About Data Output

3. Designing a Program • Programs must be designed before they are written • Program development cycle: • Design the program • Write the code • Correct syntax errors • Test the program • Correct logic errors

4. Designing a Program (cont’d.) • Design is the most important part of the program development cycle • Understand the task that the program is to perform • Work with customer (user) to get a sense what the program is supposed to do • Ask questions about program details • Create one or more software requirements • In college course make sure you understand your professor’s requirements

5. Designing a Program (cont’d.) • Determine the steps that must be taken to perform the task • Break down required task into a series of steps • Create an algorithm, listing logical steps that must be taken • Algorithm: set of well-defined logical steps that must be taken to perform a task

6. Algorithms • Set of well-defined logical steps • Must be performed in order to perform a task • Example: Making a cup of instant coffee • Remove lid from coffee jar • Put lid down on counter • Remove 1 tsp of coffee from jar • Place that coffee in a cup • Add 8 oz of boiling water to cup • Use teaspoon to stir water and coffee mixture • Stir 10 revolutions • Remove teaspoon from cup and place on counter Is this adequate? Is it detailed enough? Is anything ambiguous?

7. Class Exercise • Write an algorithm that explains how to make a Peanut Butter and Jelly sandwich • Assume the person does not know anything about how to make sandwiches

8. Pseudocode • Pretend code • Informal language that has no syntax rule • Not meant to be compiled or executed • Used to create model program • No need to worry about syntax errors, can focus on program’s design • Can be translated directly into actual code in any programming language

9. Flowcharts • Diagram that graphically depict the steps in a program • Ovals are terminal symbols • Parallelograms are input and output symbols • Rectangles are processing symbols • Symbols are connected by arrows that represent the flow of the program

11. Input, Processing, and Output • Typically, computer performs three-step process • Receive input • Any data that the program receives while it is running • Perform some process on the input • Example: mathematical calculation • Produce output

12. print Function • Function: piece of prewritten code that performs an operation • Calling a function: when programmers ask that a function execute • print function: displays output on the screen • Notice it is all lower case • Argument: data given to a function • Ex: print ("Hello from Python")

13. String Literals • String: sequence of characters • Can be anything you can type on keyboard • Can be a word, sentence, name, nonsense • String literal: string that appears in program code • Must be enclosed in single (') or double (") quote marks • Putting a quote in string requires special instructions • Must be enclosed in triple quotes (''' or """) • Allows enclosed string to contain both single and double quotes and can have multiple lines Examples: print("Kate Austen") print('''Cara O'Brian''' "Cara O’Brian") print('Asheville, NC 28899')

14. Comments • Notes of explanation within a program • Ignored by Python interpreter • Intended for a person reading the program’s code • Begin with a # character • End-line comment: appears at the end of a line of code • Typically explains the purpose of that line # This program displays a person's name and address. print('Kate Austen') # Name print('123 Full Circle Drive') # Street address print('Asheville, NC 28899') # City, state, and ZIP

15. Variables • Name that represents a value stored in computer memory • Used to remember, access and manipulate data stored in memory • Variable references the value • Assignment statement - used to create a variable and make it reference the desired data • General format is • variable = expression • Expression is the value, mathematical equation or function that results in a value • Assignment operator: the equal sign (=) • Examples: • age = 29 • age = current_year– birth_year

16. Variables • In assignment statement, variable receiving value must be on left side • You cannot use a variable UNTIL a value is assigned to it • Otherwise will "get name 'variable name' is not defined" • A variable can be passed as an argument to a function, like print • Do not enclose variable name in quotes

17. Variable Naming Rules • Rules for naming variables in Python • Should be short, but descriptive • First character must be a letter or an underscore • After first character may use letters, digits, or underscores • Variable name cannot be a Python keyword • Variable name cannot contain spaces • Use underscore instead • Variable names are case sensitive • Variable name should reflect its use • Ex: grosspay, payrate • Separate words: gross_pay, pay_rate

18. Example Program message = "Hello Python world!" print(message) Output: Hello Python world!

19. Change Variable Value message = "Hello Python world!" print(message) message = "Hello world!” # message has new value print(message) # something different is printed Output: Hello Python world! Hello world!

20. Displaying Multiple Items with the print Function • Python can display multiple items on one line • Items to print are separated by commas • The items are passed as arguments • Displayed in order they are passed to print function • Items are automatically separated by a space when displayed on screen # This program demonstrates printing # multiple items on one line room = 503 print('I am staying in room number', room) Output: I am staying in room number 503

21. Print String and Variable width = 10 length = 5 print('width is', width) print('length is', length) print('width') Output: width is 10 length is 5 width

22. Variable Naming Errors • 3dGraph # What is wrong with this variable? • Error: variable can’t start with number • Mixture#3 # What is wrong with this variable? • Error: variable may only use letters, digits or underscores • temperature = 74.5 # Create a variable • print(tempereture) # What is wrong with this variable? • Error: misspelled variable name • Gives error: NameError: name ‘temperature’ is not defined • temperature = 74.5 # Create a variable • print(Temperature) # What is wrong with this variable? • # Error: inconsistent use of case

23. Variable Reassignment • Variables can reference different values while program is running # This program demonstrates variable reassignment. # Assign a value to the dollars variable. dollars = 2.75 print('I had $', dollars, 'in my account.') # Reassign dollars so it references a different value. dollars = 99.95 print('But now I have $', dollars, 'in my account!') Output: I had $ 2.75 in my account. But now I have $ 99.95 in my account!

24. Data Types • Categorize value in memory • Written in memory differently • int for integer • float for real number • str used for storing strings in memory

25. Numeric Literal • Number written in a program • int - whole number. Doesn’t have decimal point • Examples: 7, 124, -9 • If number with decimal point is placed in int variable, number is truncated to smallest number • float - has decimal point • Examples: 1.5. 3.14159, 5.0

26. Changing Case in String • You can change lower case letters in a string to upper case and vice versa • First assign a string to a variable • Ex: name = “adalovelace” • To upper case first letter of each word in a string • Add suffix .title() to variable name • Example: print (name.title()) • Produces: Ada Lovelace • print(name.upper()) • Produces: ADA LOVELACE • print(name.lower()) • Produces: adalovelace

27. Combining/Concatenating Strings • You can merge two strings together • Example first_name = “Ada” last_name = “Lovelace” full_name = first_name + “ “ + last_name print (full_name) • Produces Ada Lovelace

28. Syntax Error • When the Python interpreter doesn’t recognize something in your program as recognizable Python code • Ex: print('Cara O’Brian') has a single quote between single quotes. Python sees the third quote and doesn’t know how to handle that • Computers are very picky on syntax • Thonny IDE gives help with errors

29. Reading Input from the Keyboard • Most programs need to read input from the user • Built-in input function reads input from keyboard • Returns the data as a string • Format: • variable= input(prompt) • prompt is typically a string telling user to enter a value • Does not automatically display a space after the prompt • You will need to add one at the end of the string so what user types doesn't merge with the string prompt

30. Input Example Program # Get the user's first name. first_name = input('Enter your first name: ') # Get the user's last name. last_name = input('Enter your last name: ') # Print a greeting to the user. print('Hello', first_name, last_name) Output: Enter your first name: Anthony Enter your last name: Manno Hello Anthony Manno

31. Reading Numbers with input • inputfunction always returns a string • Need to convert strings to numbers (type conversion) • int(item) converts item to an int • float(item) converts item to a float • This requires a Nested Function Call where you get input from the user AND convert it to a number in same line • General format: • function1(function2(argument)) • value returned by function2 is passed to function1 • Example: int( input("what is the temperature? ")) • This gets what they user typed and sends it immediately to function that converts it to an integer • Type conversion only works if item is valid numeric value, otherwise, gives error

32. Numeric Input Example # Get the user's name, age, and income. name = input('What is your name? ') age = int(input('What is your age? ')) income = float(input('What is your income? ')) # Display the data. print('Here is the data you entered:') print('Name:', name) print('Age:', age) print('Income:', income) Output: What is your name? Anthony What is your age? 20 What is your income? 10000 Here is the data you entered: Name: Anthony Age: 20 Income: 10000.0

33. Performing Calculations • Math expression: performs calculation and gives a value • Math operator: tool for performing calculation • Resulting value typically assigned to variable • Example: 12 + 2 • Value returned is 14 • Operands: values surrounding operator • Variables can be used as operands • Example with variables “PayRate” and “HoursWorked” • Salary = PayRate * HoursWorked • Two types of division: • / operator performs floating point division • // operator performs integer division • Positive results truncated, negative rounded away from zero

34. Math Program # Assign a value to the salary variable. salary = 2500.0 # Assign a value to the bonus variable. bonus = 1200.0 # Calculate the total pay by adding salary # and bonus. Assign the result to pay. pay = salary + bonus # Display the pay. print('Your pay is $', pay) Output: Your pay is $ 3700.0

35. Exponentiation and Remainder • Exponent operator (**): Raises a number to a power • x ** y = xy • Remainder operator (%): Performs division and returns the remainder • a.k.a. modulus operator • Ex: 4%2=0, 5%2=1 • Example uses • Convert times and distances • Detect odd or even numbers

36. Operator Precedence • Python operator precedence • Operations enclosed in parentheses • Forces operations to be performed before others • Exponentiation (**) • Multiplication (*), division (/ and //), and remainder (%) • Addition (+) and subtraction (-) • Higher precedence performed first • Same precedence operators execute from left to right

37. Zen of Python • Guiding principles for writing code with Python to write well-designed, elegant and efficient code • Beautiful is better than ugly • Explicit is better than implicit • Simple is better than complex • Complex is better than complicated • Readability counts • In the face of ambiguity, refuse the temptation to guess • Now is better than never. • Although never is often better than right now

38. Exercises • Do exercises for lab 2a • It is due next class