Transcript ppt - AD Book Enterprises

11/2: Math.random, more methods
• About DrawLine.java modifications
– allow user input
– draw a curve
• Method definitions
• Math.random()
Allow User Input (1 of 2)
• Add in an init method for two input dialog boxes.
double coeff;
double yInt;
//note that I declared these
//two variables as global.
public void init ()
{
cText = JOptionPane.showInputDialog
(“Coefficient, please.”);
yText = JOptionPane.showInputDialog
(“Y-intercept, please.”);
coeff = Double.parseDouble ( cText );
yInt = Double.parseDouble ( yText );
}
Allow User Input (2 of 2)
• Use coeff & yInt in the drawDot method
int drawDot ( int a )
{
int b;
//must cast to int below
b = (int) (coeff * a + yInt) ;
return b;
}
Draw a Curve
• Change the formula in the drawDot method from:
int drawDot ( int a )
{
int b;
b = 2 * a + 10 ;
return b;
}
• to:
int drawDot ( int a )
{
int b;
b = 2 * ( a * a ) + 10 ;
return b;
}
Method Definitions
• in SquareInt.java (simple version below), we see
a method called square:
public void paint ( Graphics g )
{
g.drawString(square(x) + “ “,25,25);
}
public int square ( int y )
{
return y * y ;
}
Method Definitions
public void paint ( Graphics g )
{
g.drawString(square(x) + “ “,25,25);
}
the method square is called in this line. Because it is
in parentheses, x is the value to use as the input.
public int square ( int y )
{
return y * y ;
}
Method Definitions
public void paint ( Graphics g )
{
g.drawString(square(x) + “ “,25,25);
}
x fits the requirement that it must be an integer.
public int square ( int y )
{
return y * y ;
}
Method Definitions
public void paint ( Graphics g )
{
g.drawString(square(x) + “ “,25,25);
}
using x as the input, the method calculates a return value.
public int square ( int y )
{
return y * y ;
}
Method Definitions
public void paint ( Graphics g )
{
x * x
g.drawString(square(x) + “ “,25,25);
} the returned value is what replaces the square(x).
This value is of type int.
public int square ( int y )
{
return y * y ;
}
Example: Maximum.java (p. 212)
public void init ()
{
(stuff left out…)
double max = maximum ( num1, num2, num3 );
outputArea.setText ( “Max is “ + max );
}
public double maximum ( double x, double y, double z )
{
return Math.max ( x, Math.max ( y, z ) );
}
Important: Coercion of Arguments
• Java will automatically promote variables to
larger types when necessary; EX: int to
double.
• In Math.pow, the requested types of input are
double. However, Java will create a double
version of an int variable to use in that method
automatically.
• go to Java class index for Math.pow
• read p. 213-214 carefully
Math.random()
• generates a random double number in the range
0.0 <= x < 1.0
• To utilize this function, we use scaling & shifting.
• scaling: multiplying the function by a number to
get the range we want.
• shifting: adding a number to the function to get
the beginning point of the range that we want.
Math.random()
• EX: Math.random ()
• shift it by 1: Math.random() + 1 makes 1 to <2
• scale it by 6: Math.random()* 6 makes 0 to <5
• to simulate rolling a die, we want 6 possibilities
(or a range of 6), from 1 to 6. They have to be
integers.
(int) ( Math.random() * 6 + 1 );
1st Program of the day
• pg. 220 RollDie.java
• Modify the program to allow the user to input
how many times to roll the die.
About RollDie.java – pt. 1
//RollDie.java
//Roll a six-sided die 6000 times
import javax.swing.*;
public class RollDie {
public static void main ( String args[] )
{
int freq1 = 0 , freq2 = 0 ,
freq3 = 0 , freq4 = 0 ,
freq5 = 0 , freq6 = 0 ;
int face;
About RollDie.java – pt. 2
for ( int roll = 1; roll <= 6000 ; roll++ ) {
face = 1 + (int) ( Math.random() * 6 );
switch ( face ) {
case 1:
++freq1; break;
case 2:
++freq2; break;
case 3:
++freq3; break;
case 4:
++freq4; break;
case 5:
++freq5; break;
case 6:
++freq6; break;
}
}
About RollDie.java – pt. 3
JTextArea outputArea = new JTextArea ( 7, 10 );
outputArea.setText ( "Face\tFrequency" +
"\n1 \t" + freq1 + "\n2 \t" + freq2 +
"\n3 \t" + freq3 + "\n4 \t" + freq4 +
"\n5 \t" + freq5 + "\n6 \t" + freq6 );
JOptionPane.showMessageDialog ( null , outputArea ,
"Rolling a die 6000 times" ,
JOptionPane.INFORMATION_MESSAGE);
System.exit (0);
}
}
Modifying RollDie.java
we need to replace the 6000 in the for loop with a
user-defined value, so we add in
int rolls;
rolls = Integer.parseInt (
JOptionPane.showInputDialog (
“How many rolls?” ) );
and replace the 6000 with rolls.
for ( int roll = 1; roll <= rolls ; roll++ ) {
face = 1 + (int) ( Math.random() * 6 );
Automatic Variables: Duration
• Identifiers are used for variable names.
• Identifiers have a specific duration (or lifetime):
– How long the identifier exists in memory (RAM).
– Identifiers representing local variables exist ONLY
while the program is inside that block.
– A block is a set of compound statements in a program
that contains declarations.
Automatic Variables: Duration
• Identifiers have a specific duration (or lifetime):
– They are automatically created in memory upon
entering the block and automatically destroyed in
(deleted from) the computer’s memory when that
block is exited.
– They are said to have automatic duration.
– They are called automatic duration variables, or
simply automatic variables.
Instance Variables & Initialization
• Instance (class-wide) variables are initialized by
default:
– primitive data types to zero (0).
– boolean types to false.
– references (names given to objects) to null.
• Automatic (block-local) variables must be
initialized manually.
– you must initialize them or get a compiler error.
Scope Rules
• Identifiers have a specific scope:
– where the identifier can be referenced in the program.
– Class scope: accessible throughout the body of the
class (between the class {}’s)
• EX: instance variables, methods
– Block scope: accessible inside their block.
• EX: local variables, method parameters
Hidden Instance Variables
• Instance variables can be “hidden” if a local
variable has the same name.
• While the program is in the local variable’s block,
the instance variable is “hidden”.
• Scoping.java gives a taste of this. (pg. 192)
2nd Program of the day
• Scoping.java
• After getting it to run correctly, analyze why the
Container c is there.