1. Software Development I/O and Numbers
Computer Science 209
Software Development
I/O and Numbers

2. Example: Circle Area
Input the radius
Compute the area
Output the area

3. Python Version import math def main():
radius = input("Enter the radius: ")
area = math.pi * radius ** 2
print("The area is", area)
print("The area is %.2f" % area)
main()

4. Java Version import java.util.Scanner; public class CircleArea{
public static void main(String[] args){
Scanner input = new Scanner(System.in);
System.out.print("Enter the radius: ");
double radius = input.nextDouble();
double area = Math.PI * Math.pow(radius, 2);
System.out.println("The area is " + area);:
System.out.printf("The area is %.2f\n", area); }

5. Import Statements Some classes are not built in
import java.util.Scanner;
public class CircleArea{
public static void main(String[] args){
Scanner input = new Scanner(System.in);
Some classes are not built in
They must be imported from packages (like modules)
Class names are capitalized; package names are lowercase

6. Object Instantiation
import java.util.Scanner;
public class CircleArea{
public static void main(String[] args){
Scanner input = new Scanner(System.in);
New instances or objects of a class are created with the new operator
As in Python, the constructor is the class name, which is called as if it were a method, with any expected arguments

7. Variable Declaration
import java.util.Scanner;
public class CircleArea{
public static void main(String[] args){
Scanner input = new Scanner(System.in);
When new variable, parameter, or method names are introduced, they must be prefixed with an explicit type name
This allows the compiler to do type checking at compile time, so type incompatibility errors will not occur at runtime

8. Numeric Types
import java.util.Scanner;
public class CircleArea{
public static void main(String[] args){
Scanner input = new Scanner(System.in);
System.out.print("Enter the radius: ");
double radius = input.nextDouble();
double area = Math.PI * Math.pow(radius, 2);
double, float, long, and int are the most commonly used numeric types

9. Java vs Python Syntax error caught at compile time No problem!
double radius = 25.5; .
radius += 2;
radius = "Hi there!";
radius = 25.5 .
radius += 2
radius = "Hi there!"
Syntax error caught at compile time
No problem!
All variables in Java have a type associated with them

10. Explicit vs Implicit Casting
int radius = 25; .
radius += 2.5;
System.out.println(radius);
radius = 25 .
radius += 2.5
print(radius);
Syntax error caught at compile time
No problem!
All variables in Java have a type associated with them

11. Numeric Input
import java.util.Scanner;
public class CircleArea{
public static void main(String[] args){
Scanner input = new Scanner(System.in);
System.out.print("Enter the radius: ");
double radius = input.nextDouble();
double area = Math.PI * Math.pow(radius, 2);
The Scanner method nextDouble automatically converts the user’s input to a number

12. static Variables and Methods
import java.util.Scanner;
public class CircleArea{
public static void main(String[] args){
Scanner input = new Scanner(System.in);
System.out.print("Enter the radius: ");
double radius = input.nextDouble();
double area = Math.PI * Math.pow(radius, 2);
Math.PI is a static variable and Math.pow is a static method
Many math constants and methods are in the Math class

13. The + Operator
import java.util.Scanner;
public class CircleArea{
public static void main(String[] args){ .
double area = Math.PI * Math.pow(radius, 2);
System.out.println("The area is " + area);
The + operator works with a string and a number as operands, converting the number to a string before performing the concatenation

14. Example: Guess the Number
The computer thinks of a random number between 1 and 100
The user guesses a number until she hits it or exceeds the maximum number of guesses needed
The maximum is log2(100), or 7

15. Python Version import math import random def main():
myNumber = random.randint(1, 100)
count = 0
print("I'm thinking of a number between 1 and 100.")
while True:
count += 1
userNumber = input("Enter your guess: ")
if userNumber == myNumber:
print("You guessed it in", count, "attempts!")
break
elif userNumber < myNumber:
print("Too small!")
else:
print("Too large!")
if count == round(math.log(100, 2)):
print("Exceeded maximum allowable guesses!”)

16. Math Class Methods Very much like the math module functions in Python
No need to import anything
>>> import math
>>> print(math.log(256, 2))
8.0
System.out.println(Math.log(256) / Math.log(2));
// Prints 8.0

17. Rounding vs Truncating
>>> import math
>>> x = math.log(100, 2))
>>> x
>>> int(x) 6
>>> round(x) 7
double x = Math.log(100) / Math.log(2);
System.out.println(x); //
System.out.println((int) x);
// Prints 6
System.out.println(Math.round(x));
// Prints 7

18. Get started on Programming Project 1 (on Sakai)
For Wednesday
Get started on Programming Project 1
(on Sakai)