Input, Processing, and Output Joan Boone jpboone@email.unc.edu - - PowerPoint PPT Presentation

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Input, Processing, and Output

Joan Boone

jpboone@email.unc.edu

Summer 2020

INLS 560

Programming for Information Professionals

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Topics

Part 1

• Program design and development
• Variables and print statement

Part 2

• Reading input from the keyboard
• Calculations, data conversion

Part 3

• Formatting, expressions

Slide 3

Topics

Part 1

• Program design and development
• Variables and print statement

Slide 4

Program Development Cycle

Iterative process: Design, code, test, fix, repeat as needed

Source: Starting Out with Python by Tony Gaddis Correct runtime and logic errors Design the program Write the code Correct syntax errors Test the program

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Designing a Program

• Understand the task: what are the requirements?
• Define the steps: what is the algorithm?

– Methods for documenting an algorithm:

pseudocode and flowcharts Example: Calculate and display the gross pay

• f an hourly employee

Source: Starting Out with Python by Tony Gaddis

Pseudocode

• Get the number of hours worked
• Get the hourly pay rate
• Multiply number of hours worked by hourly

pay rate

• Display the result

Flowchart

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Basic Program Structure

INPUT

hours_worked = 40 hourly_pay_rate = 25.00 gross_pay = hours_worked * hourly_pay_rate print(gross_pay)

1000.0

PROCESS OUTPUT

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Variables, Values, and Types

A variable is a name that represents a value stored in the computer's memory.

• Variables are created with an assignment statement.
• Assigned values are often strings or numbers

message = “some useful text goes here” voting_age = 18 pi = 3.1415926535897932

Values have different types

• string (sequence of text characters)
• integer (numbers without a fractional portion)
• float (numbers with a decimal point that represents fraction)

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Variable Naming Conventions

• Begin with a lowercase letter, e.g. zipcode
• Can be of arbitrary length
• Should be descriptive – use multiple words, if applicable.

Two approaches for multi-word variables

– Use underscore, _, to separate multiple words, e.g.,

miles_per_gallon (snake_case)

– Use mixed case, e.g., milesPerGallon (camelCase)

• Can contain both letters and numbers, e.g.,

totals_for_2019

• Case is significant! zip_code, zipCode and zipcode

are different variables

• Use all capital letters for constants, e.g., DAYS_IN_WEEK

Do not use Python keywords as variable names

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Variables in Assignment Statements

# Create two variables: top_speed and distance top_speed = 160 distance = 300 # Display the value referenced by the variables print('The top speed is') print(top_speed) print('The distance traveled is') print(distance) # Variables can appear on both sides # of an assignment statement counter = 0 counter = counter + 1 print('The counter value is') print(counter)

variable_demo1.py The top speed is 160 The distance traveled is 300 The counter value is 1 Output

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Display multiple items with print

# Create two variables: top_speed and distance top_speed = 160 distance = 300 # Display the value referenced by the variables print('The top speed is', top_speed) print('The distance traveled is', distance) # Variables can appear on both sides of an assignment statement counter = 0 counter = counter + 1 print('The counter value is', counter)

variable_demo2.py The top speed is 160 The distance traveled is 300 The counter value is 1 Output

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Multiple Ways to Display Output with print

• print function displays a line of output, and then prints a newline

character that is not visible, but causes output to start on a new line.

• Example

print('One') print('Two') print('Three')

• Or print on same line

print('One', 'Two', 'Three')

• Specifying an item separator

print('One','Two','Three', sep=', ') One Two Three One Two Three One, Two, Three

Gaddis: Section 2.8

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Displaying Output with Escape Characters

Python documentation on Lexical Analysis

print('One\nTwo\nThree')

One Two Three

print('Three\tFour\tFive')

Three Four Five

print('Monty Python\'s Flying Circus')

Monty Python's Flying Circus

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Style Guidelines and Comments

PEP 8 - Style Guide for Python Code

• To improve readability and consistency
• Comments: Block and inline (start with '# ')

– Why important?

• Describe the purpose of the program
• Describe/clarify the purpose of complex functions and formulas;

don't over-comment

• Use for attribution

– Bad comments are worse than no comments – Also useful for debugging: comment out sections of code to try

different approaches to solving a problem

– Multi-line comments (delimited with triple quotes) ''' Very lengthy, verbose, run-on comment goes here. '''

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Topics

Part 2

• Reading input from the keyboard
• Calculations, data conversion

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Reading Input from Keyboard

input(prompt)is a built-in Python function that reads input typed by the user on the keyboard. Example

first_name = input('Enter your first name: ') last_name = input('Enter your last name: ') print('Hello', first_name, last_name)

Example output: Hello Monty Python

How it works: when the input statement is executed by Python

• the prompt is displayed, and the program waits for you to type

something

• when you press Enter, whatever you typed is assigned to the

variable as a string

string_input.py

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Gross Pay Calculation

# Prompt for hours worked hours_worked = input('Enter hours worked: ') # Prompt for hourly pay rate hourly_pay_rate = input('Enter hourly pay rate: ') # Calculate gross pay and display result gross_pay = hours_worked * hourly_pay_rate print('Gross pay: ', gross_pay)

gross_pay_calc.py Enter hours worked: 40 Enter hourly pay rate: 25 Traceback (most recent call last): File ".../gross_pay_calc.py", line 8, in <module> gross_pay = hours_worked * hourly_pay_rate

TypeError: can't multiply sequence by non-int of type 'str'

Process finished with exit code 1 Output in the PyCharm Run Window

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Converting Data

Use Python built-in functions that convert data to another type.

General syntax: int(item)converts item to an int (integer, or whole number)

float(item)converts item to a float (number w/decimal point)

Examples: converting strings to numeric

days_in_week = int('7')

pi = float('3.1416')

Rule: item must be a number, or a string containing a

number, otherwise an error (exception) is generated

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Converting User Input Strings to Numbers

hours_worked = input('Enter hours worked: ') hours_worked = float(hours_worked)

This can also be written as a nested function call:

hours_worked = float(input('Enter hours worked: '))

After the input function executes, it will return the value you entered, as a string. That value is saved (temporarily) by Python, and then is passed as an argument to the float function which converts it to a number.

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Exercise: Gross Pay Calculation

Enter hours worked: 40 Enter hourly pay rate: 25 Gross pay: 1000.0 Process finished with exit code 0

Output in the PyCharm Run Window

Update gross_pay_calc.py so that it converts the user inputs to float before calculating the gross pay amount.

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Performing Calculations: Math operators

Symbol Operation Description

+

Addition Adds two numbers

• Subtraction

Subtracts one number from another

*

Multiplication Multiplies one number by another

/

Division Divides one number by another and gives the result as a floating point number

//

Integer division Divides one number by another and gives the result as an integer

%

Remainder Divides one number by another and gives the remainder

**

Exponent Raises a number to a power

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Exercise: Sale Price Calculation

• Calculate the price of an item with a 20% discount
• Apply a 4.75% sales tax for the total price

# Prompt for the price

• riginal_price = float(input("Enter the item's original price: "))

# Calculate the sale_price with a 20% discount # Calculate the total_price = sale_price + tax # Display sale and total prices print('The sale price is ', sale_price) print('The total price is', total_price) sale_price.py Enter the item's original price: 100.00 The sale price is 80.0 The total price is 83.8

Sample output

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When the result looks like 83.80000000000001

• Remember that all data is represented in binary
• When a decimal fraction is converted to binary, it can't always be

converted exactly

• For example, when 1/3 is calculated in binary, the resulting

decimal version looks like 0.3333333333333333

• You can't predict whether the fractional portion will have 2 digits or

16, but you can control how many digits are displayed Specify a minimum field width with a format specifier (Section 2.8)

some_number = 12345.6789 print(format(some_number, '12.2f')) Result: 12345.68

Displays the number in a field of 12 spaces, with 2 decimal places

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Topics

Part 3

• Formatting, expressions

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Example using Format Specifier

# This program displays the following floating-point numbers # in a column with their decimal points aligned. num1 = 127.899 num2 = 3465.148 num3 = 3.776 num4 = 264.821 num5 = 88.081 num6 = 799.999 # Display each number in a field # of 7 spaces with 2 decimal places. print(format(num1, '7.2f')) print(format(num2, '7.2f')) print(format(num3, '7.2f')) print(format(num4, '7.2f')) print(format(num5, '7.2f')) print(format(num6, '7.2f')) 127.90 3465.15 3.78 264.82 88.08 800.00

Result:

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Operator Precedence

Given: result = 12.0 + 6.0 / 3.0 What is the value of result? Python uses the same rules of precedence for evaluating expressions that you learned in math class

• Operations in parentheses are performed first
• Operators with the higher precedence are applied first
• Precedence of math operators, from highest to lowest:

– Exponentiation: ** – Multiplication, division, and remainder: * / // % – Addition and subtraction: + -

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Mixed Type Expressions

• When you perform a math operation, the data type of the

result will depend on the data type of the operands

• Rules

– When both operands are int, the result is an int – When both operands are float, the result is a float – When one operand is an int and the other is a float,

the result is a float

• Examples

– 12 * 2 → 24 – 12.0 * 2.0 → 24.0 – 12 * 2.0 → 24.0

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Converting Math Formulas to Programming Statements

Algebraic Expression Python Statement

Area of a triangle

A = 1/2bh

area_of_triangle = base * height / 2 Simple interest

I = prt

simple_interest = principal * rate * time Compound interest A = P (1 + R)n amount = principal * (1 + rate)**time Distance between 2 points A = P (1 + R)n distance = ((x2 - x1)**2 + (y2 - y1)**2)**0.5

Some Math Formulas can be simplified with the use of Python built-in Math functions

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Coin Conversion Program

• For a given number of pennies, convert to:

dollars, quarters, dimes, nickels, and pennies

• For example, the prompt and output might be:

Enter number of pennies: 573 Here is the amount in dollars and coins Dollars: 5 Quarters: 2 Dimes: 2 Nickels: 0 Pennies: 3

coin_conversion.py

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Exercise:

Convert Seconds to hours, mins, secs

• For a given number of seconds, convert this number to

hours, minutes, and seconds

• For example, the prompt and output might be:

Enter number of seconds: 123456 Hours: 34 Minutes: 17 Seconds: 36

print(format(hours,'02d'), format(minutes,'02d'), format(remaining_seconds,'02d'), sep=':')

'02d' displays the number with 2 decimal digits, and inserts a leading 0

if there is only 1 digit

Another way to format the results 34:17:36