CIS3931 – Intro to JAVA

Lecture Note Set 3

19-May-05

Compound Assignment Operators

• Way of shortening code.

• Assume, c = 3.

• c = c + 7;

c += 7; //same as above

Compound Operators

Operator Use Equivalent to

+= c += 7; c = c + 7;

-= d -= 4; d = d – 4;

*= e *= 5; e = e * 5;

/= f /= 3; f = f / 3;

%= g %= 9; g = g % 9;

More Shorthand

• You can also use ++ and -- if you wish to only change the integer by 1.

• For example, d++ is the same as writing d=d+1.

• It is also the same as writing d += 1

• The same holds true for the --.

Prefix vs. Postfix

• Prefix is ++b while postfix is b++.

• Prefix and postfix operators are used mainly in loops.

• Whether you use prefix or postfix will determine the outcome of your program.

Prefix Increment

• Example: ++a

• It would increment by 1, then use the new value of a in the expression in which a resides.

• int a = 5;

b = ++a;

• What is b? What is a?

Postfix Increment

• Example: a++

• It uses the current value of a in the expression in which it resides, then increments by 1.

• int a = 5;

b = a++;

• What is b? What is a?

Decrement

• The same holds true for the postfix and prefix decrement operators.

• Depending on whether or not it is pre or post depends on when it gets computed.

Ternary Operator

• Takes in 3 operands.

• The first leftmost operand is a boolean expression (it can only return true or false)

• The second (between ? and :) is what is done if it is true.

• The third is what is done if it returns false.

• Shorthand for if/else statements.

Ternary Operator 2

• An example:

System.out.println( grade>=60 ? “Passed” : “Failed” );

• If the grade is >= 60, it prints out Passed.

• Else it prints out Failed.

( grade >= 60 ? “Passed” : “Failed”)

Precedence

• Some operators execute before others.

• This is because they have a higher precedence.

• Useful if you have more than one mathematical operation taking place on one line.

Precedence Chart

++ -- (Postfix) High precedence

++ -- + - (Prefix)

* / %

+ -

< <= > >=

== !=

?:

= += -= /= */ %= Low precedence

Counter-Controlled Repetition

Counter-controlled repetition requires:

• A control variable

• Initial Value

• The increment (or decrement)

• Loop-continuation condition

Counting a while loop

int counter = 1;

while (counter <=10 )

{

System.out.println(counter);

counter++;

} //prints out integers 1 through 10 on a

// separate line

The for statement

for( int count = 1; count <=10; count++)

{

System.out.println(count);

} //same output as the previous while loop

For Brackets

• Once again, the brackets are not required if there is just one line of code below the for statement.

• If there is more than one line, they are required.

For loops

• Is this legal?

for( ; ; )

• If so, what does it do?

• What does it not do?

General For format

for ( initialization ; loopContinuationCondition ; increment) //should be 1 line

{

statement/algorithm

}

Common errors

• When a for statement’s control variable is declared in the initialization section of the for loop, editing that same variable in the for body will produce an error.

• In other words, don’t alter the variable inside the body of the for loop.

• To sidestep this problem, use temp variables and set them equal if need be.

Finding Sum

int total = 0;

for( int i=2; i<=20 ; i+2 )

total += i;

System.out.println(“Sum is “ + total );

/* Above finds the sum of all even numbers 2 through 20 and prints it out */

Do…while statement

int counter = 1;

do

{

System.out.println(counter);

counter++;

} while ( counter <= 10 ); //notice the ;

// Same as while statement a few slides ago

Braces on do…while

• Although you do not need braces on a do while statement if it is only 1 line, it is common to add them to help readability and to separate between the while and the do.

Do…While

• Every do while statement can be written as a for statement or a while statement.

• Most people opt for the for loop instead of writing a do…while loop.

Switch statement

• Can only compare integer values

• Useful if there are several different conditions.

• See the SampleSwitch.java code

switch

switch (integer value)case 1:case 2: do something here break;default: do something here break;

Switch Defaults

• Goes to default if it doesn’t match anything else.

• You should always have a default after the last case.

• The last case in a switch statement doesn’t need a default, although most programmers add it for clarity.

Break

• A break statement will “break” out of the loop it was in.

• for( int i=1; I <=10; i++) { if (i == 5) //if i == 5, break; // break out of for loop System.out.println( i ); }

Continue

• Continue statements can be avoided by using structured programming.

• Therefore, they will not be covered in this course.

Assignment #2

• Tax withholding calculator

• Due Thursday May 26th

• Description available online at course website http://www.cs.fsu.edu/~cis3931

Questions…