Java I

Control Structures: Part I Chapter 4 Control Structures: Part I

Structured Programming “ There is No goto in Java ” • Structured programming: the building blocks • There are 3 different kinds of operations in a program: perform a sequence of actions, perform a selection between alternative actions, or perform a repetition or iteration of the same action. Sequence, Selection, Iteration White Space Characters

Structured Programming • Sequence: one thing after another task1 task2 White Space Characters task3

? Structured Programming • Selection: making choices YES taskA NO only one entrance, only one exit. White Space Characters taskB

“A while loop repeats as long as a condition is true.” Structured Programming • Repetition, Part I: doing the same thing again until there’s a reason to stop. expression ? TRUE taskA FALSE White Space Characters Do while: maybe won’t ever do taskA even once. “A while loop repeats as long as a condition is true.”

? Structured Programming • Repetition, Part II: doing the same thing again until there’s a reason to stop. taskA ? TRUE White Space Characters FALSE Do until: will always do taskA at least once. “A Do Until loop repeats as long as a condition is false.”

Procedural Structured Programming • Begin at the top, move to the bottom. • Each program unit has only one entrance and only one exit. White Space Characters

Selection in Java Object Oriented Programming • Within a method, procedural code. • Simple ‘if’ with or without brackets. if( expression ) statement; { } White Space Characters

Object Oriented Programming Selection in Java Object Oriented Programming • Simple ‘if’ with or without brackets. if( expression ) statement; { } White Space Characters • Within brackets, a “block.”

• Without brackets, limit of only one statement per branch. Selection in Java Object Oriented Programming • Simple ‘if’ / ‘else’ without brackets. if( expression ) statement; else White Space Characters • Without brackets, limit of only one statement per branch.

Object Oriented Programming Selection in Java w Object Oriented Programming • Simple ‘if’ / ‘else’ with brackets. if( expression ) { statement; } else White Space Characters

Selection in Java • Compound ‘if’ / ‘else if’ / ‘else if’ / ‘else’. if( expression ) { statement; } else if( expression ) else White Space Characters

System.out.print(expression ? “True” : “False”) Selection in Java • Special “Ternary” ? : Operator—shorthand ‘if’ / ‘else’ System.out.print(expression ? “True” : “False”) 1.) expression must evaluate to True or False. 2.) If expression is True, execute the command before the colon. 3.) If expression is False, execute the command after the colon. Do this if expression is False White Space Characters Do this if expression is True

Repetition: while Part I while—“the Do While” • The Test is First while( expression ) statement; White Space Characters

Repetition: while Part I while—“the Do While” • The Test is First The while { “Do While” } is used when you can’t predict exactly how many times your loop will be executed. The while may not be executed even once. It executes the loop while the expression is still true. while( expression ) { statement; } White Space Characters

// DoUntil.java // Even though "c" begins the loop false, // it still executes at least once. public class DoUntil { public static void main( String args[] ) boolean c = false; do System.out.println( ”Execute DoUntil at least once " ); } while( c ); System.exit( 0 );

while—“the Do Until” Repetition: while Part II • The Test is Last do This do/while {“Do Until”} is also used when you can’t predict exactly how many times your loop will be executed. It executes at least once. It executes Until the expression becomes false. do { statement; } while( expression ); White Space Characters

// WhileTest.java // Since "c" is already false when it reaches the // test, the loop never executes. public class DoWhile { public static void main( String args[] ) boolean c = false while( c ) System.out.println( ”Execute DoWhile while c is true" ); } System.exit( 0 );

• Test Trinary Operator // SelectionTest.java import javax.swing.*; public class SelectionTest { public static void main( String args[] ) int b, s; String big, small, out; big = JOptionPane.showInputDialog( "Big Number" ); small = JOptionPane.showInputDialog( "Small Number" ); b = Integer.parseInt( big ); s = Integer.parseInt( small ); out = ( b > s ? "Big was larger" : "Small was larger" ); JOptionPane.showMessageDialog( null, out, "Results", JOptionPane.INFORMATION_MESSAGE); System.exit( 0 ); } • Test Trinary Operator White Space Characters

• Test DoWhile // DoWhileTest.java import javax.swing.*; public class DoWhileTest { public static void main( String args[] ) int b = 2, s = 1; String big, small, out = “Big is still Bigger”; while( b > s ) JOptionPane.showMessageDialog( null, out, "Results", JOptionPane.INFORMATION_MESSAGE); big = JOptionPane.showInputDialog( "Big Number" ); small = JOptionPane.showInputDialog( "Small Number" ); b = Integer.parseInt( big ); s = Integer.parseInt( small ); } System.exit( 0 ); • Test DoWhile White Space Characters

• Test DoUntil // DoUntilTest.java import javax.swing.*; public class DoUntilTest { public static void main( String args[] ) int b = 2, s = 1; // preload variables. String big, small, out = “Big is still Bigger”; do JOptionPane.showMessageDialog( null, out, "Results", JOptionPane.INFORMATION_MESSAGE); big = JOptionPane.showInputDialog( "Big Number" ); small = JOptionPane.showInputDialog( "Small Number" ); b = Integer.parseInt( big ); s = Integer.parseInt( small ); } while( b > s ); System.exit( 0 ); • Test DoUntil White Space Characters

Repetition: while Loops • The majority of applications use the plain while loop. • Choose either while loop when you can’t know in advance how many times the loop will be executed. • The loop is repeated until it encounters a sentinel value, that announces that the loop has finished. White Space Characters

Assignment Operators • We are already familiar with this statement: int x; x = 15; • This means the value 15 is placed into the variable x. • We say, “15 is assigned to x.” • In Java, a single equals sign is the assignment operator.

Assignment Operators • Another common bit of code is this: int x; Declares x as an int. x = 15; After this assignment, x contains 15. x = x + 5; x contains 20. First the addition on the right is done. Then, the result is assigned to the the variable on the left.

+= Assignment Operators • Java offers a shortcut to the statement below: int x = 15; x = x + 5; After this assignment, x contains 20. x += 5; After this assignment, x contains 20.

-= Assignment Operators • Java offers a shortcut to the statement below: int x = 15; x = x - 5; After this assignment, x contains 10. x -= 5; After this assignment, x contains 10.

*= Assignment Operators • Java offers a shortcut to the statement below: int x = 15; x = x * 5; After this assignment, x contains 75. x *= 5; After this assignment, x contains 75.

/= Assignment Operators • Java offers a shortcut to the statement below: int x = 15; x = x / 5; After this assignment, x contains 3. x /= 5; After this assignment, x contains 3.

%= Assignment Operators • Java offers a shortcut to the statement below: int x = 15; x = x % 5; After this assignment, x contains 0. x %= 5; After this assignment, x contains 0.

Increment/Decrement Operators • There is one addition statement in Java that is so common, it gets its own operator. • It means, simply, “Add one to the variable.” x++ x--

x++ Increment/Decrement Operators • Java offers a shortcut to the statement below: int x = 2; x = x + 1; After this assignment, x contains 3. x++; After this assignment, x contains 3. x++

x-- Increment/Decrement Operators • Java offers a shortcut to the statement below: int x = 2; x = x - 1; After this assignment, x contains 1. x--; After this assignment, x contains 1. x--

Increment/Decrement Operators • You can do a pre-increment, or a post-increment. Pre Post ++x; x++; --x; x--; • If each of these statements is on a line by itself, there is no difference in the effect of doing a pre- or post-increment.

Increment/Decrement Operators • If each of these statements is on a line by itself, there is no difference in the effect of doing a pre- or post-increment. • However, if the variable—which is having the pre- or post-increment applied to it—is used within another statement, your choice of pre- or post-increment can alter your results.

w Baseline, x = 0 Pre-increment = ++x = 1 After increment, x = 1 // PrePostIncrement.java public class PrePostIncrement { public static void main( String args[] ) int x=0; System.out.println( " Baseline, x = " + x ); System.out.println( "\n Pre-increment = ++x = " + ++x ); System.out.println( "\n After increment, x = " + x ); x = 0; System.out.println( "\n 2nd Baseline, x = " + x ); System.out.println( "\n Post-increment = x++ = " + x++ ); System.exit( 0 ); } w Baseline, x = 0 Pre-increment = ++x = 1 After increment, x = 1 2nd Baseline, x = 0 Post-increment = x++ = 0