CSCI 144 - Introduction to Computer Science Instructor: John - - PowerPoint PPT Presentation

CSCI 144 - Introduction to Computer Science

Instructor: John Goettsche Computer Science Department Pacific Lutheran University Spring 2018

1

Using The Scanner for Input

https://www.cs.plu.edu/~caora

About the questions website Website names? Like it or not? Change to google or top hat

Conversions & Casting

Data Conversions

25 25

byte int

1289 1289.0

int double

Widening Conversions

123456.789 123456

double int int

123456 123

byte

Narrowing Conversions

SO WHAT?

“The failure occurred because the horizontal velocity exceeded the maximum value for a 16 bit unsigned integer when it was converted from it's signed 64 bit representation.”



Ariane 5 was a $500 million unmanned space craft designed by the European Space Agency



On June 4, 1996 it was launched for the first time



Here’s what happened

Source: http://www.vuw.ac.nz/staff/stephen_marshall/SE/Failures/SE_Ariane.html

Conversions can occur in three ways

1.

Assignment conversion

2.

Arithmetic conversion

3.

Casting

Assignment conversion

int x = 1234; double y; y = x;

1234

x

memory ?

y

1234.0 Only widening conversions can happen via assignment

Arithmetic conversion

/ 15 4.0 15.0 3.5

int gets promoted to a double

Operands are always promoted to the “wider” type

Casting

int x; double y 12345.67; x = (int) y;

?

x

memory 12345.67

y

12345

Type is put in parentheses in front of value being converted

avg = (double) sum / count;

Common use is when we are dividing two ints but want a floating point result

Casting Practice

• 1. (double) 10 / 4
• 2. (double) (125 / 10) + 1
• 3. (int) ((double) 10 / (double) 4 ) + 1.0
• 4. (double) (10 / 4)

Casting Practice

(double) 10 / 4 10.0 / 4 10.0 / 4.0 2.5

casting arithmetic promotion double division

Casting Practice

(double) (10 / 4) (double) 2 2.0

int division casting

Casting Practice

(int) ((double) 10 / (double) 4 ) + 1.0 (int) ( 10.0 / 4.0 ) + 1.0 (int) 2.5 + 1.0 2 + 1.0

casting double division casting

2.0 + 1.0

arithmetic promotion

3.0

double addition

So what?

double gpa; int totalPoints = 114, totalHours = 30; gpa = totalPoints / totalHours; gpa = 114 / 30

integer division

gpa = 3

assignment conversion

Note: 114.0 / 30.0 = 3.8 3.0

So what?

Peer Instruction questions

Pair Practice

practice…

Carbon footprint for air travel

5000 2500 800 250 extended long medium short

Source: http://www.climatecrisis.net/takeaction/carboncalculator/

Carbon footprint for air travel

5000 2500 800 250 extended long medium short

Emissions factors

.64 lbs/mile .45 lbs/mile .39 lbs/mile .39 lbs/mile Source: http://www.climatecrisis.net/takeaction/carboncalculator/

Together we will complete a Java program to calculate carbon emissions for short and medium flights Algorithm assign the number of short flights taken (numShort) assign the number of medium flights (numMedium) calculate total carbon emissions (totalEmissions)

numShort * shortDist * shortEmission + numMedium * medDist * medEmission

display total carbon emissions

Practice Exercise (10 mins pair and print out)

download CO2AirTravel.java from the csci144 blog page http://www.cs.plu.edu/144 -> Class blog Section 1&2

Using the Scanner class to make programs more flexible

1.

import the Scanner class

2.

instantiate a Scanner object

3.

prompt user for input value

4.

scan user input into a variable

To use the Scanner class:

import

import java.util.Scanner; public class CO2Calculator2 { public static void main(String[] args) { int milesDriven; double mpg; double gallonsUsed, co2Used; double emissionsFactor = 19.6; Scanner keyboard = new Scanner(System.in); System.out.print("Enter total miles driven: "); milesDriven = keyboard.nextInt(); System.out.print("Enter miles per gallon: "); mpg = keyboard.nextDouble(); gallonsUsed = milesDriven/mpg; co2Used = gallonsUsed * emissionsFactor; System.out.println("You used " + co2Used + " pounds of C02”); } }

instantiate import

import java.util.Scanner; public class CO2Calculator2 { public static void main(String[] args) { int milesDriven; double mpg; double gallonsUsed, co2Used; double emissionsFactor = 19.6; Scanner keyboard = new Scanner(System.in); System.out.print("Enter total miles driven: "); milesDriven = keyboard.nextInt(); System.out.print("Enter miles per gallon: "); mpg = keyboard.nextDouble(); gallonsUsed = milesDriven/mpg; co2Used = gallonsUsed * emissionsFactor; System.out.println("You used " + co2Used + " pounds of C02”); } }

prompt instantiate import

import java.util.Scanner; public class CO2Calculator2 { public static void main(String[] args) { int milesDriven; double mpg; double gallonsUsed, co2Used; double emissionsFactor = 19.6; Scanner keyboard = new Scanner(System.in); System.out.print("Enter total miles driven: "); milesDriven = keyboard.nextInt(); System.out.print("Enter miles per gallon: "); mpg = keyboard.nextDouble(); gallonsUsed = milesDriven/mpg; co2Used = gallonsUsed * emissionsFactor; System.out.println("You used " + co2Used + " pounds of C02”); } }

scan prompt instantiate import

import java.util.Scanner; public class CO2Calculator2 { public static void main(String[] args) { int milesDriven; double mpg; double gallonsUsed, co2Used; double emissionsFactor = 19.6; Scanner keyboard = new Scanner(System.in); System.out.print("Enter total miles driven: "); milesDriven = keyboard.nextInt(); System.out.print("Enter miles per gallon: "); mpg = keyboard.nextDouble(); gallonsUsed = milesDriven/mpg; co2Used = gallonsUsed * emissionsFactor; System.out.println("You used " + co2Used + " pounds of C02”); } }

Revise the Java program to calculate carbon emissions so it asks the user for the number of short and medium flights Revised Algorithm ask user for number of short flights taken scan number of short flights (numShort) ask user for number of medium flights taken scan number of medium flights (numMedium) calculate total carbon emissions (totalEmissions)

numShort * shortDist * shortEmission + numMedium * medDist * medEmission

display total carbon emissions

More Practice (5 mins in pairs)

Revise CO2AirTravel.java from the previous exercise

Revise the Java program to calculate carbon emissions so it can convert user’s floating number input to the correct int data

• type. Print it out with both your names and submit the paper to

me. Revised Algorithm ask user for number of short flights taken scan number of short flights (numShort) Convert user’s float input to int type ask user for number of medium flights taken scan number of medium flights (numMedium) calculate total carbon emissions (totalEmissions)

numShort * shortDist * shortEmission + numMedium * medDist * medEmission

display total carbon emissions

More and more Practice

Revise CO2AirTravel.java from the previous exercise

Using The Scanner for Input

Check the website for announcement and lab1!