Chapter 4
Conditional Statements
A Programmer's Lament
I really hate this damned machine;
I wish that they would sell it
It never does quite what I want
but 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
A Vote Counting Example
Complete source code
Code to Update Votes:
// Update votes and display vote counts
public 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
}
if-else
if
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
• Examples
If 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 many
EX:
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 && c
is 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)
(3>4) && ( 4<5)
(3>4) && ( 4>5)
true
false
false
Examples Using ||
•
box.overlaps( goal1 ) || box.overlaps(goal2)
– box overlaps either goal1 or goal2
•
•
•
(3<4) || ( 4<5)
(3>4) || ( 4<5)
(3>4) || ( 4>5)
true
true
false
Examples using !
• ! broom.overlaps(goal)
– The broom does not overlap the goal
• !(3>4)
!false = true
Order of Operations for
Boolean Logic
1.
2.
3.
4.
5.
Parentheses ( )
Relational ops <,>,<=,>=,!=,==
!
&&
||
This order is applied in processing complex
expressions
Parts at the same level are handled left to right
Example
EXAMPLE: suppose
x=3
y=10
z=20
Evaluate
!( x<y ) || (z > x)
! (3<10) || (20 > 3)
! true || true
false || true
true
first, plug in values for variables
second, evaluate parentheses
third, apply ! to true to get false
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
F
||
T
||
||
T
T
||
||
T
F
&&
F
F
F
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 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;
…
}
Complete source code
Nesting
• Suppose we want to decide among several
choices based on several conditions, such as
shown by the table:
Sunny
Not sunny
Rich
Outdoor Concert
Indoor Concert
Not Rich
Ultimate Frisbee
Watch TV
• To do this, we use conditionals inside a
conditional. This is called nesting.
Nesting Example
if ( ( sunny ) && (rich)) {
activityDisplay.setText( "outdoor concert" );
} else if ((sunny) && (!rich) ){ // not rich
activityDisplay.setText( "play ultimate" );
}
else { // not sunny
if ( rich ) {
activityDisplay.setText( "indoor concert" );
} else { // not rich
activityDisplay.setText( "watch TV" );
}
}
Creating a Game of Craps
By adding an outer conditional, we can effectively determine whether the player
wins:
if ( newGame ) {
//Starting a new game
if ( 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