1. Chapter 2 More on Math More on Input
Math and I/O
Chapter 2
More on Math
More on Input

2. Math Math operators different for ints & doubles Addition: +
7 + 3   10.0
Subtraction: –
7 – 3  – 3.0  4.0
Multiplication: *
7 * 3  * 3.0  21.0

3. Division Operators Division: / Modulus (Division Remainder): %
7 / 3  / 3.0 
integer division for ints; floating-point division for doubles
(mixed expressions – later)
Modulus (Division Remainder): %
7 % 3  % 3.1  1.5

4. Math Expressions
int m; m int n; ? n m = 3 + 5; 8 ? n = 4 * m; 8 32 m = n % 5; 2 32 n = n / 5; 2 6 n = m + 8 / m + 3; 2 __

5. Order of Operations Precedence: the order things get done
Left to right except:
Parentheses ( ... ) done before
Multiplying * / % done before
Adding + – done before
PEMDAS / PEDMAS
parentheses, (exponents), mult/div, add/sub
Java doesn’t do exponents

6. Common Mistake x + 8 / x + 3, where x = 2 BUT (x + 8) / (x + 3)
2 + 8 / 2 + 3
6 + 3 = 9
BUT
(x + 8) / (x + 3)
10 / 5 = 2

7. Exercises 2 * 2 + 3 * 4 = _____ 5 / 3 – 1 = _____
4 + 3 * 4 = 7 * 4 = 28
4 + 3 * 4 = = 16
2 * 5 * 4 = 10 * 4 = 40
5 / 3 – 1 = _____
1.667 – 1 = 0.667
5 / 2 = 2.5
5 / 2 = 2
1 – 1 = 0

8. Mixed Mode Expressions
Expressions with both doubles and ints
= ?
Integers are converted to double
= 5.5
Type Coercion
Just what you’d expect
usually!

9. When Coercion Happens Not before needed
int sub-expressions use int math
/ / 2 ( ) / 2
( ) / 2
/ 2
/ 2.0
4.3

10. Mixed Mode Assignments
Assign an int value to a double variable…
double x;
x = 0;
…it gets automatically changed to double
assignment above is same as
x = 0.0;
Use the second way, anyway!
helps prevent confusion

11. Mixed Mode Assignments
Assign an double value to an int variable…
int n;
n = 0.0;
Java compiler objects!
can’t assign a double value to an int variable!
even tho’ 0.0 is equal to 0, which is an integer!
“possible loss of precision” error

12. Type Casting
You can tell the computer to change a number from one type to another
used especially with variables
int count = 0;
int sum = 0; …
double average = (double)sum / (double)count;
45 / 4 = 11
45.0 / 4.0 = 11.25
11.25
average
count 45
count 4
sum
sum
45.0
4.0

13. Changing Double to Int When change double to int…
double price;
int intPrice;
price = 19.95; // price in $ and ¢
intPrice = (int)price; // just $
…everything after the decimal point is lost
it is not “rounded off”!
we’ll see how to round off later
19.95
price 19
intPrice

14. Exercises Say what value gets assigned int j; double f; j f
f = f + 2.6; __ __
j = (int)f; __ __
f = (double)(j / 2) + 1.1; __ __

15. Doing Input with Scanners
System.out is useful for doing output
There is also a System.in
it’s connected (usually) to our keyboard
it’s not nearly so useful (for beginners)
it doesn’t know how to read a number
So for input we will use Scanners
need to tell the computer we’ll be using them
need to create the Scanners we use

16. Getting Ready for Input
Tell the computer we’ll be using Scanners
import java.util.Scanner;
Scanner is a class (a piece of software)
Create the scanner we will use
Scanner keyboard = new Scanner(System.in);
keyboard is an object (a piece of s/w in use)
it uses System.in (it knows how)
this command is a(n object) variable declaration

17. Reading Data Read a variable value by assigning into it
n1 = keyboard.nextInt();
nextInt is the method a Scanner uses to get the next integer
keyboard is the Scanner we’re using (remember that it’s connected to our keyboard)
keyboard.nextInt() returns the next integer
the assignment command (=) puts that number into the variable (in this case n1)

18. Exercise nextInt read next int value
What is the command for reading a double value?
write a Java command to prompt for and read the length and width of a table (in metres) …
System.out.println("The table’s area is " + (length * width) + " square metres.");

19. Prompting for Input Prompt for input
otherwise user just staring at a blank screen!
System.out.print(" Enter a number: ");
n1 = keyboard.nextInt();
System.out.print(" Enter another: ");
n2 = keyboard.nextInt();
Enter a number: 32
Enter another: 47
Enter a number: 32
Enter another:
Enter a number:
Enter a number: 32

20. Input & Output on the Monitor
Input & output mixed together on the screen
Black = produced by System.out
Blue = typed by user, read by keyboard
Note: two separate “streams”!
computer will not read the 79—it’s output
Enter a number: 32
Enter another number: 47
Their sum is 79.

21. The Two Streams Computer prints User types
Enter a number: (pause) 32
Enter another: (pause) 47
Their sum is 79.
Computer pauses when it needs more input
End of a number indicated by
enter/return ( on diagram above)
or anything that’s not part of a number

22. Anticipating the Computer
Computer prints User types
Enter a number: (pause) 32 47
Enter another: Their sum is 79.
Two numbers on first line (space between)
Second number already in stream when computer gets to second input command
Computer will not pause before reading 2nd #
Don’t need/get to type the 2nd 

23. Preventing Anticipation
nextLine method reads everything up to and including the next  character
System.out.print(“Enter a number: ”);
n1 = keyboard.nextInt();
keyboard.nextLine();
System.out.print(“Enter another: ”);
n2 = keyboard.nextInt();
Enter a number: (pause) 32 47
Enter another: (pause) 55
Their sum is 87.

24. Question Where did the 47 go?
we read it, but we didn’t put it anywhere (no assignment command)
computer read it, then immediately forgot it
What did the second nextLine read & forget?
Enter a number: (pause) 32 47
Enter another: (pause) 55
Their sum is 87.

25. Pausing for ENTER Can use nextLine to pause input Except….
wait for user to press ENTER
System.out.print(“Press ENTER to continue”);
keyboard.nextLine();
Press ENTER to continue(pause) 
Except….
What if there’s already a  in the input stream?

26. Pausing for ENTER What if there’s already a  in the input stream?
System.out.print(“Enter a number: ”);
n2 = keyboard.nextInt();
System.out.print(“Press ENTER to continue”);
keyboard.nextLine();
Enter a number: (pause) 32
Press ENTER to continue:
Computer does not pause. WHY NOT?

27. Tidying Up As You Go Along
Use nextLine() whenever you expect the user to press the ENTER key
gets rid of the  and everything before it
makes sure that your next input statement gives the user a chance to type something

28. Exercise Write commands to Make sure numbers entered separately
prompt for length
read length (double value)
prompt for width
read width (double value)
echo back length and width
pause for user to press enter key
Make sure numbers entered separately
then change so numbers entered on one line

29. Reading Strings You can save the lines you read
String line;
System.out.print("Enter a line: ");
line = keyboard.nextLine();
System.out.print("You said \"" + line + "\"");
here we put the stuff we read into a variable (Note: the  character doesn’t get saved)

30. Reading Strings You can also read Strings word by word String w1, w2;
System.out.print("Enter two words: ");
w1 = keyboard.next();
w2 = keyboard.next();
keyboard.nextLine(); // tidy up!
Note: NOT nextWord()!
Enter two words: two words ? w1
“two” w1
“words” w2 ? w2

31. Exercise Write code to read in a person’s name…
first name (one word) and last name (one word)
…then print out their name in this format:
Last, First
…then pause for user to press Enter
What is your full name? Mark Young
Young Mark
Press ENTER to continue...

32. Questions?