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Chapter 4
Conditional Statements
A Programmer's Lament
I really hate this damned machine;I wish that they would sell itIt never does quite what I wantbut only what I tell it.
Dennie L Van Tassel, The Compleat Computer
Conditional Statements
• Consider:if the mouse location is contained
in the rectangle, display message “success”
• Some programming constructs can choose between blocks of code to execute
Code to Update Votes:
// Update votes and display vote countspublic void onMouseClick( Location point ) {
if ( point.getX() < MID_X ) { countA++;infoA.setText( "So far there are " + countA + " vote(s) for A." );
} else { countB++;infoB.setText( "So far there are " + countB + " vote(s) for B." );
}}
Syntax of the if Statement• The condition is evaluated. If true, the bracketed code following if is
executed; otherwise, the bracketed code after else is executed
if (condition) {//do something
} else {//do something else
}
• The else part can be omitted
if (condition) {//do something here
}
ifif-else
if Statement and 2D Objects
To check if the mouse is within a 2D object:public void onMouseClick ( Location point ) {
if ( anObject.contains ( point ) ) {
//do something here
}
}
Comparison Operators
Operator Description
> Greater than
< Less than
== Equal to
<= Less than or equal to
>= Greater than or equal to
!= Not equal to
• ExamplesIf a=25, b=30
a<b evaluates to true
a<=b evaluates to true
a==b evaluates to false
a!=b evaluates to true
a>b evaluates to false
a>=b evaluates to false
4 < 3 false
2 == 2 true
Since these expressions are evaluated to either true or false, they are called boolean expressions
The boolean Data Type
• A data type that has only two values: true or false
• Can be declared with the word boolean
Ex: boolean a=true; //holds the value true
boolean b=c>d; //holds the result of c > d
boolean k; //creates k, which can take on true or false
Dragging a Box//boolean variable to determine whether the box is grabbed
private boolean boxGrabbed;
// Save starting point and whether point was in box
public void onMousePress( Location point ) {
lastPoint = point;
boxGrabbed = box.contains( point );
}
// if mouse is in box, then drag the box
public void onMouseDrag( Location point ) {
if ( boxGrabbed ) {
box.move( point.getX() - lastPoint.getX(),
point.getY() - lastPoint.getY() );
lastPoint = point;
}
} Complete source code
More Uses of The if else Statement• Picks one choice among manyEX: Converting a score into a letter grade
if ( score >= 90 ) { gradeDisplay.setText( "The grade is A" );
} else if ( score >= 80 ) { gradeDisplay.setText( "The grade is B" );
} else if ( score >= 70 ) { gradeDisplay.setText( "The grade is C" );
} else { gradeDisplay.setText( "No credit is given");
}
Combining Multiple Conditionals
• && (and) combines adjoining conditions in a way that the final result will be true only if all are true
Ex: a && b && cis true if a,b,c are true
• || (or) combines adjoining conditions in a way that if any of them is true, the final result will be true
Ex: a || b || c
is true if any of a, b, c, is true
• ! (not) negates the value of a boolean (true false)
Logical Operators
• && (and) – true only when both sides are true
• || (or) – true whenever one or both are true
• ! (not)– makes false -> true and true -> false
Examples using &&
• broom.overlaps(goal) && box.overlaps(goal)– The broom overlaps both goals at same time
• (3<4) && ( 4<5) true
• (3>4) && ( 4<5) false
• (3>4) && ( 4>5) false
Examples Using ||
• box.overlaps( goal1 ) || box.overlaps(goal2)– box overlaps either goal1 or goal2
• (3<4) || ( 4<5) true
• (3>4) || ( 4<5) true
• (3>4) || ( 4>5) false
Examples using !
• ! broom.overlaps(goal)– The broom does not overlap the goal
• !(3>4) !false = true
Order of Operations for Boolean Logic
1. Parentheses ( )
2. Relational ops <,>,<=,>=,!=,==
3. !
4. &&
5. ||
This order is applied in processing complex expressions
Parts at the same level are handled left to right
ExampleEXAMPLE: suppose
x=3
y=10
z=20
Evaluate
!( x<y ) || (z > x)
! (3<10) || (20 > 3) first, plug in values for variables
! true || true second, evaluate parentheses
false || true third, apply ! to true to get false
true last, or the false with true to get true
Processing a Big Logic Expression
(3 < 10) && (4>10) || (3 == 3) || (3 > 6) && (4 == 2)
T && F || T || F && F
F || T || F
T || F
T
Random Number Generator
• private RandomIntGenerator rand;– rand is an object that spits out random values
• rand = new RandomIntGenerator(1,100);– Makes rand refer to a new RIG
– Will produce values from 1 to 100
• rand.nextValue();– Gets a new random number from rand
• score = rand.nextValue();– Stores the random value in an integer score
• box.moveTo(rand.nextValue(), rand.nextValue());
– Moves box to a new random location
The Craps Example• A block of code that uses || (or) to determine whether the player wins a game of
Craps
if ( roll == 7 || roll == 11 ) { // 7 or 11 wins on first throwstatus.setText( "You win!" );
} else if ( roll == 2 || roll == 3 || roll == 12 ) { // 2, 3, or 12 losesstatus.setText( "You lose!" );
} else { // Set roll to be the point to be madestatus.setText( "Try for your point!" );point = roll;…
}
Complete source code
Nesting
• Suppose we want to decide among several choices based on several conditions, such as shown by the table:
• To do this, we use conditionals inside a conditional. This is called nesting.
Sunny Not sunny
Rich Outdoor Concert Indoor Concert
Not Rich Ultimate Frisbee Watch TV
Nesting Example
if ( ( sunny ) && (rich)) { activityDisplay.setText( "outdoor concert" );
} else if ((sunny) && (!rich) ){ // not richactivityDisplay.setText( "play ultimate" );
}else { // not sunny
if ( rich ) { activityDisplay.setText( "indoor concert" );
} else { // not richactivityDisplay.setText( "watch TV" );
}}
Creating a Game of CrapsBy adding an outer conditional, we can effectively determine whether the player wins:
if ( newGame ) { //Starting a new gameif ( roll == 7 || roll == 11 ) { // 7 or 11 wins on first throw
status.setText( "You win!" );} else if ( roll == 2 || roll == 3 || roll == 12 ) { // 2, 3, or 12 loses
status.setText( "You lose!" );} else { // Set roll to be the point to be made
status.setText( "Try for your point!" );point = roll;newGame = false; // no longer a new game
}}
Complete source code
