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CMSC 150
CONDITIONAL
EXECUTION
CS 150: Mon 16 Jan 2012
Sequential Execution
6
import java.util.Scanner;
Java executes
public class ProgramFive
statements
{
public static void main(String[] args)
step-by-step
{
Scanner keyboard = new Scanner( System.in in
); exactly the order
you have them
System.out.println( “Enter sphere radius:“ );
double radius = keyboard.nextDouble();
double pi
= 3.14159;
double volume = (4.0 / 3.0) * pi * radius * radius * radius;
System.out.println( “The volume of a sphere with radius “
+ radius + “ is “ + volume );
}
}
Sequential Execution
7
import java.util.Scanner;
public class ProgramFive
{
public static void main(String[] args)
{
Scanner keyboard = new Scanner( System.in );
System.out.println( “Enter sphere radius:“ );
double radius = keyboard.nextDouble();
double pi
= 3.14159;
double volume = (4.0 / 3.0) * pi * radius * radius * radius;
System.out.println( “The volume of a sphere with radius “
+ radius + “ is “ + volume );
}
}
Sequential Execution
8
import java.util.Scanner;
public class ProgramFive
{
public static void main(String[] args)
{
Scanner keyboard = new Scanner( System.in );
System.out.println( “Enter sphere radius:“ );
double radius = keyboard.nextDouble();
double pi
= 3.14159;
double volume = (4.0 / 3.0) * pi * radius * radius * radius;
System.out.println( “The volume of a sphere with radius “
+ radius + “ is “ + volume );
}
}
Sequential Execution
9
import java.util.Scanner;
public class ProgramFive
{
public static void main(String[] args)
{
Scanner keyboard = new Scanner( System.in );
System.out.println( “Enter sphere radius:“ );
double radius = keyboard.nextDouble();
double pi
= 3.14159;
double volume = (4.0 / 3.0) * pi * radius * radius * radius;
System.out.println( “The volume of a sphere with radius “
+ radius + “ is “ + volume );
}
}
Sequential Execution
10
import java.util.Scanner;
public class ProgramFive
{
public static void main(String[] args)
{
Scanner keyboard = new Scanner( System.in );
System.out.println( “Enter sphere radius:“ );
double radius = keyboard.nextDouble();
double pi
= 3.14159;
double volume = (4.0 / 3.0) * pi * radius * radius * radius;
System.out.println( “The volume of a sphere with radius “
+ radius + “ is “ + volume );
}
}
Sequential Execution
11
import java.util.Scanner;
public class ProgramFive
{
public static void main(String[] args)
{
Scanner keyboard = new Scanner( System.in );
System.out.println( “Enter sphere radius:“ );
double radius = keyboard.nextDouble();
double pi
= 3.14159;
double volume = (4.0 / 3.0) * pi * radius * radius * radius;
System.out.println( “The volume of a sphere with radius “
+ radius + “ is “ + volume );
}
}
Sequential Execution
12
import java.util.Scanner;
public class ProgramFive
{
public static void main(String[] args)
{
Scanner keyboard = new Scanner( System.in );
System.out.println( “Enter sphere radius:“ );
double radius = keyboard.nextDouble();
double pi
= 3.14159;
double volume = (4.0 / 3.0) * pi * radius * radius * radius;
System.out.println( “The volume of a sphere with radius “
+ radius + “ is “ + volume );
}
}
Sequential Execution
13
import java.util.Scanner;
public class ProgramFive
{
public static void main(String[] args)
{
Scanner keyboard = new Scanner( System.in );
System.out.println( “Enter sphere radius:“ );
double radius = keyboard.nextDouble();
double pi
= 3.14159;
double volume = (4.0 / 3.0) * pi * radius * radius * radius;
System.out.println( “The volume of a sphere with radius “
+ radius + “ is “ + volume );
}
}
Sequential Execution
14
import java.util.Scanner;
public class ProgramFive
{
public static void main(String[] args)
{
Scanner keyboard = new Scanner( System.in );
System.out.println( “Enter sphere radius:“ );
double radius = keyboard.nextDouble();
double pi
= 3.14159;
double volume = (4.0 / 3.0) * pi * radius * radius * radius;
System.out.println( “The volume of a sphere with radius “
+ radius + “ is “ + volume );
}
}
How Is This Different?
15
import java.util.Scanner;
public class ProgramFive
{
public static void main(String[] args)
{
Scanner keyboard = new Scanner( System.in );
double radius = keyboard.nextDouble();
double pi
= 3.14159;
double volume = (4.0 / 3.0) * pi * radius * radius * radius;
System.out.println( “Enter sphere radius:“ );
System.out.println( “The volume of a sphere with radius “
+ radius + “ is “ + volume );
}
}
How Is This Different?
16
import java.util.Scanner;
public class ProgramFive
{
public static void main(String[] args)
{
Scanner keyboard = new Scanner( System.in );
double radius = keyboard.nextDouble();
double pi
= 3.14159;
double volume = (4.0 / 3.0) * pi * radius * radius * radius;
System.out.println( “Enter sphere radius:“ );
System.out.println( “The volume of a sphere with radius “
+ radius + “ is “ + volume );
}
}
Now, How Is This Different?
17
import java.util.Scanner;
public class ProgramFive
{
public static void main(String[] args)
{
Scanner keyboard = new Scanner( System.in );
System.out.println( “Enter sphere radius:“ );
double radius = keyboard.nextDouble();
double volume = (4.0 / 3.0) * pi * radius * radius * radius;
double pi
= 3.14159;
System.out.println( “The volume of a sphere with radius “
+ radius + “ is “ + volume );
}
}
Now, How Is This Different?
18
import java.util.Scanner;
Will not compile!
public class ProgramFive
{
public static void main(String[] args)
{
Scanner keyboard = new Scanner( System.in );
Trying to use a variable
before it is declared
System.out.println( “Enter sphere radius:“ );
double radius = keyboard.nextDouble();
double volume = (4.0 / 3.0) * pi * radius * radius * radius;
double pi
= 3.14159;
System.out.println( “The volume of a sphere with radius “
+ radius + “ is “ + volume );
}
}
Find the Runtime Error
19
import java.util.Scanner;
public class ExampleProgram
{
public static void main(String[] args)
{
Scanner keyboard = new Scanner( System.in );
System.out.println( “Enter an integer:” );
int firstNumber = keyboard.nextInt();
System.out.println( “Enter another integer:” );
int secondNumber = keyboard.nextInt();
double ratio = (double) firstNumber / secondNumber;
System.out.println( “The ratio is “ + ratio );
}
}
Find the Runtime Error
20
import java.util.Scanner;
public class ExampleProgram
{
public static void main(String[] args)
{
Scanner keyboard = new Scanner( System.in );
System.out.println( “Enter an integer:” );
int firstNumber = keyboard.nextInt();
System.out.println( “Enter another integer:” );
int secondNumber = keyboard.nextInt();
double ratio = (double) firstNumber / secondNumber;
System.out.println( “The ratio is “ + ratio );
}
}
potentially…
Conditional Execution
21
// lots of your great code
Java will execute the
…
statement only if the
if ( condition )
condition is true
{
statement;
}
…
// lots more of your great code
Conditional Execution
22
// lots of your great code
a Boolean
condition
…
double ratio = 0.0;
if ( secondNumber != 0 )
{
ratio = (double) firstNumber / secondNumber;
}
…
// lots more of your great code
Conditional Execution
23
// lots of your great code
…
double ratio = 0.0;
if ( secondNumber != 0 )
{
ratio = (double) firstNumber / secondNumber;
}
…
will compute the ratio
// lots more of your great code Java
only if the condition is true
(i.e., non-zero second #)
Note The Difference…
24
// lots of your great code
// lots of your great code
…
…
double ratio = 0.0;
if ( secondNumber != 0 )
if ( secondNumber != 0 )
{
{
ratio =
double ratio =
(double) firstNumber /
(double) firstNumber /
secondNumber;
secondNumber;
}
}
…
…
// lots more of your great code
// lots more of your great code
Note The Difference…
25
// lots of your great code
// lots of your great code
…
…
double ratio = 0.0;
if ( secondNumber != 0 )
if ( secondNumber != 0 )
{
{
ratio =
double ratio =
(double) firstNumber /
(double) firstNumber /
secondNumber;
secondNumber;
}
}
…
…
// lots more of your great code
// lots more of your great code
the variable ratio
will not be accessible
outside the if statement!
Conditions
26

Boolean expressions
Must evaluate to true or false

Comparison operators:



==, <, >, <=, >=, !=
Compound expressions:
Logical and:
 Logical or:
 Logical not:

&&
||
!
Truth Tables
27
P
Q
P && Q
P
Q
P || Q
T
T
T
T
T
T
T
F
F
T
F
T
F
T
F
F
T
T
F
F
F
F
F
F
P
!P
T
F
F
T
P, Q can be any Boolean expression (perhaps compound)
Compound Expressions
28

firstNumber == 0 && secondNumber == 0

firstNumber < 0
&& secondNumber < 0

firstNumber > 0
&& !(secondNumber > 0)

firstNumber == 0 || secondNumber == 0

firstNumber < 0
&& firstNumber > 0

firstNumber < 0
|| firstNumber > 0

!( firstNumber == 0 || secondNumber == 0 )
If / If-Else / If-Else-If
29

if ( condition )
{
statement;
}

if ( condition )
{
statement;
}
else
{
statement;
}

if ( condition )
{
statement;
}
else if ( condition )
{
statement;
}
else
{
statement;
}
Understanding Conditionals
30
if ( secondNum != 0 )
if ( secondNum != 0 )
{
{
ratio = firstNum / secondNum;
}
ratio = firstNum / secondNum;
}
else
{
if ( secondNum == 0 )
System.out.println(
{
“Divide by zero!” );
System.out.println(
}
“Divide by zero!” );
}
Understanding Conditionals
31
String sign;
String sign;
if ( number == 0 )
if ( number == 0 )
{
{
sign = “non-negative”;
sign = “non-negative”;
}
}
else if ( number < 0 )
if ( number < 0 )
{
{
sign = “negative”;
sign = “negative”;
}
}
else if ( number > 0 )
if ( number > 0 )
{
{
sign = “positive”;
}
sign = “positive”;
}
Understanding Conditionals
32
String sign;
String sign;
if ( number == 0 )
if ( number == 0 )
{
{
sign = “non-negative”;
sign = “non-negative”;
}
}
else if ( number < 0 )
if ( number < 0 )
{
{
sign = “negative”;
sign = “negative”;
}
}
else
else
{
{
sign = “positive”;
}
sign = “positive”;
}
Understanding Conditionals
33
String sign;
String sign;
if ( number == 0 )
if ( number == 0 )
{
{
sign = “non-negative”;
sign = “non-negative”;
}
}
else if ( number < 0 )
if ( number < 0 )
{
{
sign = “negative”;
sign = “negative”;
}
}
else
else
{
{
sign = “positive”;
}
sign = “positive”;
}
Understanding Conditionals
34
String sign;
String sign;
if ( number == 0 )
if ( number == 0 )
{
{
sign = “non-negative”;
sign = “non-negative”;
}
}
else if ( number < 0 )
if ( number < 0 )
{
{
sign = “negative”;
sign = “negative”;
}
}
else
if ( number > 0 )
{
{
sign = “positive”;
}
sign = “positive”;
}
Compound Statements
35

if ( condition )
{
statement;
statement;
statement;
…
}
To Java, this is
a single statement

if ( condition )
{
statement;
statement;
statement;
…
}
else
{
statement;
statement;
statement;
…
}
Compound Statements
36

if ( condition )
{
statement;
statement;
statement;
…
}
else
{
statement;
statement;
statement;
Statements inside can be anything…
…
Even another if statement!
}
if ( condition )
{
statement;
statement;
statement;
…
}

Nested Conditionals
37

if ( condition_1 )
{
if ( condition_2 )
{
statement_1;
}
else
{
statement_2;
}
}
Nested Conditionals
38

if ( condition_1 )

if ( condition_1 && condition_2 )
{
{
statement_1;
if ( condition_2 )
}
{
statement_1;
}
if ( !condition_2 )
else
{
statement_2;
{
statement_2;
}
}
}
Nested Conditionals
39

if ( condition_1 )

if ( condition_1 && condition_2 )
{
{
statement_1;
if ( condition_2 )
}
{
statement_1;
}
if ( !condition_2 )
else
{
statement_2;
{
statement_2;
}
}
}
Nested Conditionals
40

if ( condition_1 )

if ( condition_1 && condition_2 )
{
{
statement_1;
if ( condition_2 )
}
{
statement_1;
}
if ( condition_1 && !condition_2 )
else
{
statement_2;
{
statement_2;
}
}
}
Changing The Flow
41
...
firstNumber
21
secondNumber
7
System.out.println( “Enter another integer:” );
int secondNumber = keyboard.nextInt();
if ( secondNumber != 0 )
{
double ratio = (double) firstNumber / secondNumber;
System.out.println( “The ratio is “ + ratio );
}
else
{
System.out.println( “Divide by zero error!” );
}
...
Changing The Flow
42
...
firstNumber
21
secondNumber
7
System.out.println( “Enter another integer:” );
int secondNumber = keyboard.nextInt();
if ( secondNumber != 0 )
{
double ratio = (double) firstNumber / secondNumber;
System.out.println( “The ratio is “ + ratio );
}
else
{
System.out.println( “Divide by zero error!” );
}
...
Changing The Flow
43
...
firstNumber
21
secondNumber
7
ratio
3
System.out.println( “Enter another integer:” );
int secondNumber = keyboard.nextInt();
if ( secondNumber != 0 )
{
double ratio = (double) firstNumber / secondNumber;
System.out.println( “The ratio is “ + ratio );
}
else
{
System.out.println( “Divide by zero error!” );
}
...
Changing The Flow
44
...
firstNumber
21
secondNumber
7
ratio
3
System.out.println( “Enter another integer:” );
int secondNumber = keyboard.nextInt();
if ( secondNumber != 0 )
{
double ratio = (double) firstNumber / secondNumber;
System.out.println( “The ratio is “ + ratio );
}
else
{
System.out.println( “Divide by zero error!” );
}
...
Changing The Flow
45
...
firstNumber
21
secondNumber
0
System.out.println( “Enter another integer:” );
int secondNumber = keyboard.nextInt();
if ( secondNumber != 0 )
{
double ratio = (double) firstNumber / secondNumber;
System.out.println( “The ratio is “ + ratio );
}
else
{
System.out.println( “Divide by zero error!” );
}
...
Changing The Flow
46
...
firstNumber
21
secondNumber
0
System.out.println( “Enter another integer:” );
int secondNumber = keyboard.nextInt();
if ( secondNumber != 0 )
{
double ratio = (double) firstNumber / secondNumber;
System.out.println( “The ratio is “ + ratio );
}
else
{
System.out.println( “Divide by zero error!” );
}
...
Changing The Flow
47
...
firstNumber
21
secondNumber
0
System.out.println( “Enter another integer:” );
int secondNumber = keyboard.nextInt();
if ( secondNumber != 0 )
{
double ratio = (double) firstNumber / secondNumber;
System.out.println( “The ratio is “ + ratio );
}
else
{
System.out.println( “Divide by zero error!” );
}
...
Let’s Hunt The Wumpus
48
Our Maze
49
PIT
Our Rules
PIT
50

Player can move one step per turn


up, down, left, or right
cannot move off the maze (grid)
 win
 lose
 lose a turn

Find the gold
Find the wumpus
Find the pit

Player gets only four turns, otherwise draw


Location In The Maze
51
0
1
2
0
1
2
PIT
Handle Player Movement First…
52
0
0
1
2
1
2