1. Java LESSON 2 Operators & Conditionals

2. New stuff now…….. Last Lecture: How To…. declare chars
char aCharOfMine;
char anotherChar= 'x';
read a char from the keyboard
declaring a String
aCharOfMine = UserInput.readChar();
read a String from the keyboard
String myString;
String yourString = "Hello World";
myString = UserInput.readString();
declaring constants with variables
static final int FREEZING = 36;
static final float CONV_FACTOR = 2.5f;
New stuff now……..

3. Operators Operators come in different 'flavours':
arithmetic * / % ( )
incremental
comparison < > == != >= <=
logical && || !

4. ( ) % *
ARITHMETIC OPERATORS

5. Using Arithmetic Operators
Arithmetic operators are used as we know it from maths at school.
Only difference: there may be variables or constants in the equation, too:
What gets printed ?
int a=5, b=6, c;
static final int TAX = 20;
c = a * b TAX;
System.out.println("c is: " +c); 20

6. Operator Precedence
The usual rules for "what gets calculated first" apply:
1. first everything in braces ( )
2. then * and / and %
3. finally + and -
int a=5, b=6, c;
static final int TAX = 20;
Therefore: c = a * b TAX;
is different from: c = a * (b TAX);
-20 = 5 * ( – 20 )
20 = 5 * – 20

7. Quick Reminder When you see an assignment operator in a line:
the things on the right of the = calculated first
c = a * b TAX;
… and then copied into the variable to the left
destination source

8. INCRIMENTAL OPERATORS-- ++
INCRIMENTAL OPERATORS -- ++

9. Increment Operator Programmers are lazy.
In order to add 1 to an int you could write:
int myIntegerVariable = 100; // declare and initialise
myIntegerVariable = myIntegerVariable + 1; // add one
The second line can be written in "shorthand":
++ myIntegerVariable; OR
myIntegerVariable ++;

10. NO! -- myIntegerVariable; myIntegerVariable --; Decrement Operator
Similarly in order to subtract 1 you could write
myIntegerVariable = myIntegerVariable - 1;
-- myIntegerVariable;
myIntegerVariable --;
Could you subtract 2 by writing
-- -- myIntegerVariable or
myIntegerVariable -- --?
NO!

11. Example int x = 5; // declare and initialise
System.out.println("x is " +x); // prints: x is 5
-- x; // decrement
System.out.println("x is now " +x); // prints: x is now 4
++ x; // increment
System.out.println("x back to " +x); // x back to 5

12. Inc/Dec Compatible Data Types
Originally Java only allowed incrementing and decrementing of int variables.
Java now allows the following data types to work with the ++ and -- operators.
byte
short
int
long
Whole Numbers
float
double
Decimal Numbers
char
String
boolean
Others
Strings and booleans cannot be incremented

13. Inc / Decrementing Decimal Variables
The code is the same as with ints and the behaviour is similar..
double height =2.5;
++ height;
System.out.println ( height );
float inches =3.21f;
-- inches;
3.5
2.21

14. Inc / Decrementing a Char
The code is the same as with ints and the but the behaviour is very different..
char letter = ‘a’;
char letter2 = ‘Z’;
++ letter;
System.out.println ( letter);
-- letter2;
System.out.println ( letter2 ); b Y

15. Inc/ Decrimenting By More Than One
It is possible to increment by more than one by using += or -= operators.
x = x + 2
X += 2

16. Inc/ Decrimenting By More Than One
It is possible to increment by more than one by using += or -= operators.
x = x - 2
X -= 2

17. Examples int x = 5; x += 5; System.out.println("x is " +x);
double y = 2.2;
x += 0.8;
System.out.println(“y is " + y);
double z = 0.5;
z -= 2.0;
System.out.println(“z is " +z);
x is 10
y is 3.0
z is -1.5

18. More Examples char y = ‘a’; x += 2; System.out.println(“y is " + y);
int z = 1;
int a = 5;
z += a;
System.out.println(“z is " +z);
y is c
z is 6

19. Check your knowledge now:
Spot the mistakes in the code:
static final float tax = 0.78f;
float income = £50.00;
float netIncome;
netIncome = income * TAX;
++ ++netIncome;
TAX should be uppercase
should be just 50.0
correct
Can’t use two inc operators

20. Conditional Operators
arithmetic * / % ( )
incremental
comparison < > == != >= <=
logical && || !

21. Conditions Happen in our day to day lives They are logical arguments
Have true or false answers
If your age is 17 or older then
You can drive a car.
If your licence points are more than 12 then
You cannot drive a car.
If thirsty then
Take a drink

22. Conditions If age 18 or older then Allow night club entry.

23. Conditions If age 18 or older then { Allow night club entry.
If thirsty
get a drink }
Allow Nightclub Entry
Thirsty?
Get a drink

24. Three Steps to Conditional Heaven
Step 1: Comparison Operators (<, >, etc)
Step 2: Logical Operators ( &&, ||, ! )
Step 3: 'if' statement structure

25. An expression with these operators is called a boolean expression
Comparison Operators
An expression with these operators is called a boolean expression
List of 6 operators:
> greater than
< smaller than
>= greater or equal than
<= smaller or equal than
== equal to
!= not equal to 25

26. Comparison Operators cont..
Examples:
int a = 5, b = 10, c = 5;
Code
English
a > b a greater than b
a < b a less than b
a >= b a greater or equal to b
a <= b a less or equal to b
a == c a equal to c
a != b a not equal to b
False
True
False
True
True
True

27. Comparison Operators Example (using an int named age):
Used in decisions and loops
Compares two expressions
Outcome is either true or false
Example (using an int named age):
if ( age >= 18 ) {
System.out.println(“Welcome to the club“); }
Age >= 18
“Welcome to the club”

28. Notes on '==' If ( x == 3.0 ) could be false!
Try to avoid using == on decimal variables ?
a conditional expression
That's an assignment, remember ?
Notes on '=='
It is a double '==' e.g. if (x == 10)
Don't confuse with single '=' e.g. x=10;
The trouble with using '==' on floats :
The float x=3.0 may internally be stored as
or ,
therefore the outcome of
If ( x == 3.0 ) could be false!

29. Three Steps to Conditional Heaven
Step 1: Comparison Operators
Step 2: Logical Operators ( &&, ||, ! )
Step 3: 'if' statement structure 

30. Logical Operators With comparison operators alone we can say:
" if a is greater than b, then do …."
But how can we say:
" if a is greater than b AND c is greater than d, then do …. "
Here is how :
if ( ( condition ) && ( condition ) ) {
// then do stuff here }
Here is how :
if ( ( a>b ) && ( c>d ) ) {
// then do stuff here }

31. Logical Operators cont.
&& and
|| or
! not
List of 3 operators:
|| is two of these

32. Logical Operators cont.
Logical Truth Tables:
True && True = True
False && True = False
True && False = False
False && False = False
AND
True || True = True
False II True = True
True II False = True
False || False = False OR
! Adding a NOT Will invert the result.
XOR study this in your own time 32

33. Logical Operators cont.
&& is evaluated before ||
Example:
if ( (a>b) || (c>d) && (x==y) ) {
// then do stuff here }
1 tab indentation

34. Logical Operators cont.
Another Example:
if ( !(a>b) && !(x==y) ) {
// then do stuff here }
This is identical to:
if ( (a<=b) && (x!=y) ) {
// then do stuff here }

35. Three Steps to Conditional Heaven
Step 1: Comparison Operators
Step 2: Logical Operators ( &&, ||, ! )
Step 3: 'if' statement structure  

36. 'if' Statements, version 1 of 4
simple ‘if’
'if' statements can come in different forms
The simplest form is this:
Condition
Go here only if true
Continue here
// program comes from here
if (condition here ) {
// go here if only true
// execute lines coded here }
// Continue here
LINK

37. 'if' Statements, version 2 of 4
‘if’ ’else’
The second version looks like this:
// program comes from here
if (condition here ) {
// Go here if true
// execute code written here }
else
// Go here if false
// Continue here
Condition
Go here if true
Go here if false
Continue here
LINK

38. 'if' Statements, version 3 of 4
‘else if’
’else’
The third version looks like this:
Condition1
Condition2
Go here if cond1 true
Else go here if cond2 true
Else go here
// program comes from here
if (condition here ) {
// Go here if true }
else if ( another condition here )
//Go here if 1st condition false and 2nd true
else
// Go here if none of the above expressions true
//Continue here

39. 'if' Statements, version 4 of 4
‘else if’ :
’else’
'if' Statements, version 4 of 4
1st Condition
2nd Condition
3rd Condition
If 1st cond true do this
If 2nd cond true do this
If 3rd cond true do this
else to this
if (condition here ) {
// Go here if first condition is true }
else if (2nd condition here ) {
// only if 1st condition is false
// and the 2nd condition one is true
else if (3rd condition here ) {
// only if 1st and 2nd conditions are false
// and the 3rd one true
else {
// if all the above conditions are false do this.
//Continue here

40. Three Steps to Conditional Heaven
Step 1: Comparison Operators
Step 2: Logical Operators ( &&, ||, ! )
Step 3: 'if' statement structure  
Done ! 

41. Check your knowledge now:
What is wrong here ?
float premium, carValue=500.0;
int driverAge=30;
if ( (driverAge>=18) & (driverAge <30 ) ) {
premium = 0.20; }
else if (driverAge>30) && (driverAge <40) ) {
premium = 0.10;
else if {
premium = 0.05;
System.out.println("Premium is " +(premium*carValue) );
correct
& should be &&
'(' missing
what if driver is precisely 30?
'else if' but no boolean expr.

42. A Little Task Now…(5 min)
Write a “pub crawl” program:
If you have more than 50 pounds:
visit 1 pub if you have 1 hour or less
visit 2 pubs if you have more than 1 hour
but less than 2
visit 3 pubs if you have even more time
If you have only 50 pounds or less you won't visit any pub.
Use these variables:
int pounds=55, hours=2, pubVisits;

43. int pounds=55, hours=2, pubVisits;
if ( pounds > 50 ) { // if you have more than 50 pounds
// do one
// of many
// possible
// pub visit
// scenarios }
else { // if you have 50 pounds or less
pubVisits = 0; // forget it
if( hours <= 1) { // if you have 1 hour or less
pubVisits = 1; // visit 1 pub }
else if ( hours < 2 ) { // if you have less than 2
pubVisits = 2; // visit 2 pubs
else { // even more time
pubVisits = 3; // visit 3 pubs

44.  And Now Something New: Two ways of implementing program branches:
'if' statements
'switch' - 'case' statements 

45. Structure of 'switch' - 'case'
int in;
int x=5, y=2, z;
in = UserInput.readInt();
switch( in ) {
case 1 : z = x+y;
break;
case 2: z = x-y;
default: System.out.println("Invalid input"); }
The 'switch' - 'case' statement uses 4 keywords:
case
switch
break
default

46. Usage of 'switch' - 'case' Usually in 'menu' type applications:
This is a calculator program. Type:
'A' to add two numbers
'S' to subtract two numbers
'M' to multiply
'D' to divide

47. Implementation of the 'Calculator'
char in;
in = UserInput.readChar();
switch( in ) {
case 'A' : // code to prompt, read and add here
break;
case 'S': // prompt, read and subtract here
case 'M': // prompt, read and multiply here
case 'D': // prompt, read and divide here
default: System.out.println("Invalid input"); }
What happens if a lowercase letter is entered?

48. Catching Multiple Cases
If you want to catch multiple inputs write:
char in;
in = UserInput.readChar();
switch( in ) {
case '+' :
case 'a' :
case 'A' : // prompt, read and add here
break;
case '-' :
case 's' :
case 'S': // prompt, read and subtract here

49. Do we Always Need a 'break' ?
Consider a programming IDE:
step 1: edit
step 2: compile
step 3: run
if you have just started you want the sequence:
edit-compile-run
if you already have an edited file you want:
compile-run
if you already have edited and compiled you just:
run

50. Implementing the IDE 'menu'
char in;
in = UserInput.readChar();
switch( in ) {
case 'e' ;
case 'E' : // start the editor here
case 'c' :
case 'C' : // start the compiler here
case 'r' :
case 'R': // run the programme here
break; // this ‘break’ is required!
default: System.out.println("Invalid input"); }

51. Check your knowledge now:
What is wrong here ?
char input;
input = UserInput.readString();
switch ( input )
case "+" :
case "A" : // add here
case '-' :
case 'S': // subtract
default System.out.println("wrong input"); }
should be .readChar();
opening '{' missing
char is in single quotes 'A', '+'
'possibly a 'break' missing
definitely a 'break' missing
':' after default missing

52. Exercise Write a menu for a student lifestyle simulator program:
Menu points are: (G) Get up;
(H) Have breakfast; (S) Skip lecture;
start the sequence get up - have breakfast, if the
user types a ‘g’ or a ‘G’
have only breakfast if ‘h’ or a ‘H’ is typed in,
simulate skipping a lecture if a ‘s’ or ‘S’ has been
entered.
default behaviour (in the event of an unrecognised
input ,i.e. no 'g', 'h' or 's' ) is a pub crawl.

53. Solution char usrIn = UserInput.readChar(); switch( usrIn ) {
case ‘g’:
case ‘G’: // getting up code here
case ‘h’:
case ‘H’: // breakfast code here
break;
case ‘s’:
case ‘S’: // lecture skipping code here
default: // pub visit code here }

54. Summary 1 Today we have learned how to...
use arithmetic operators (+, -, %, *, / )
use incremental operators (++, --)
build boolean expressions in conditional statements ('if') by
using comparison operators (<,>, ==,…)
using logical operators (&&, ||, ! )
apply the structure of the 'if' statement

55. Summary 2 Today we have ... revised 'if' statements
introduced 'switch' - 'case'
the 4 keywords: switch, case, default, break
catching multiple inputs: 'A', 'a', '+'
where to 'break' and where not
designing a 'menu' structure

56. END OF LESSON2