Download Math. Local. Array.

SP1 Lecture 5: 9/2/11:
• Static and instance variables
• Class Math
– Random
– Loan calculator
• Local variables
• Array (if time permits)
1
Static and instance variables
Static variable: belongs to its class,
and it is shared by all class instances,
with the same value
Instance variable: a class variable
without the “static” modifier, is shared
by all class instances, but its values
can differ in different instances
Local variable:
is created within a
method or instance in a { } block. Its
scope is limited within the block.
2
Example (1)
public class TesNum {
int instVar = 1;
static int statVar = 10;
TesNum() {
System.out.println("test: " + instVar + "
and " + statVar);
instVar = 7; statVar = 5;
} \\ constructor
3
Example(2)
public static void main(String[] args) {
TesNum alpha1 = new TesNum();
alpha1.instVar = 3;
alpha1.statVar = 6; //syn. to: TesNum.statVar = 6;
TesNum alpha2 = new TesNum();
System.out.println("inst: " + alpha1.instVar + " and "
+ alpha2.instVar);
System.out.println("stat: " + alpha1.statVar + " and "
+ alpha2.statVar);
//System.out.print("mix: " + instVar + " and " + statVar); wrong
}//end of main
}//end of class
4
What’s going on in TesNum
instVar
statVar
1. With the class:
1 (in class)
10
2. At the constructor in class (virtual):
7
3. After alpha1:
5
Constructor prints:
3 (within alpha1)
4. After alpha2:
Constructor prints:
7 (within alpha2)
5. Method main prints:
1 and 10
6
1 and 6
5
3 and 7
5 and 5
5
A method added:
public int SS(int a){
int b=instVar;
int sum=0;
if (a>b){ //swap a and b
int c=b; b=a; a=c;}
for(int i=a;i<=b;i++)
sum=sum+i;
return sum;
}// computes the sum of integers from a to b
int b1=alpha1.SS(statVar);
int b2=alpha2.SS(statVar);
System.out.println("sum : " + b1 + " and " + b2);
6
Sums to be printed
From alpha1: a=5, b=3
The sum: 3+4+5=12, that is, b1=12
From alpha2: a=5, b=7
The sum: 5+6+7=18, that is, b2=18
The print:
sum: 12 and 18
7
References to object variables
and methods
Examples from TesNum
alpha1.statVar
alpha2.instVar
TesNum.statVar
from TesNMod
alpha1.SS(statVar)
8
Class Math (no need to import)
Math.pi
 =3.14…, the ratio of the
circumference to its diameter
Math.abs(a)
a if a >= 0, or -a if a < 0
Math.log(a)
the natural logarithm
(base e) of number a
square root of number a
Math.sqrt(a)
Math.pow(a,b)
ab ; if b is an integer then
ab =aa…a (b times)
9
Loan calculator I
• Deposit:
– You put £1000 with annual interest rate 5%
– Q: How much money it will be in 10 years?
– A: Run Java loop:
• int A=1000;
• for (int k=1;k<=10;k++)
• A=A*1.05;
• System.out.println(“In 10 years the value is ”+A);
• Loan
– You take £1000 with annual interest rate 5% for
10 years.
– Q: How much money to pay monthly?
10
• Loan
Loan calculator II
– You take £1000 with annual interest rate 5% for
10 years.
– Q: How much money to pay monthly?
– No simple arithmetic answer; powers needed
• Java computation
–
–
–
–
–
–
double a=1000.0; double air=0.05;
int period=10;
double mopay, totpay;
double mir=air/12; int mp=period*12;//month unit
mopay=(a*mir)/(1 – Math.pow(1/(1+mir),mp));
totpay=mopay*mp;
11
Loan calculator III
• public class Loan{
•
public static void main(String[] args)
{
•
Loan lo = new Loan();
•
double mopay=lo.pay(10000, 0.05,10);
•
double topay=mopay*12*10;
•
System.out.println("Monthly payment is "+mopay);
•
System.out.println("Total payment is "+topay);
•
}
•
double pay(double a, double air,int period){
•
double mir=air/12; int mp=period*12;//month unit
•
double mop=(a*mir)/(1 - Math.pow(1/(1+mir),mp));
•
return mop;
•
}
12
• }
Loan calculator IV (Homework)
Modify class loan in such a way that its
constructor takes from the user some or all of
the details needed to do the computation:
– Loan value
– Repayment period (in years)
– Rate
13
Math.random()
pseudorandom number: double within
interval [0.0, 1.0) (zero included, unity
not)
How to use it to generate a random integer
between 1 and 6 (inclusive), to imitate
casting a dice?
14
Casting a dice
double aa=Math.random();
//aa, a real number between 0 and 1
int an= 6*aa; //a real number between 0 and 6
int rand=(int) an;
// whole number between 0 and 5
int randw=rand+1;
// whole number between 1 and 6
The same in one line:
int randw= (int) (6*Math.random()+1);
15
Casting a dice question
How to generate a random integer
between 10 and 20 inclusive?
Answer:
int rdt= (int) (11*Math.random()+10);
Another possibility: using class Random
with
import java.util.Random
16
Local variables: Definition
Local variable:
is created within a method or instance
in a { } (curly brace) block.
Its scope is limited within the block.
Therefore, same name can be used in
different blocks for different variables.
17
Local variables(1)
public class Prog03{ private static int i=3;
public static void method1(){
int i=2;
i+=6;
System.out.println(i); }
public static int method2(int a){
a=a+3;
int i=2*a-6;
return i; }
public static void method3(int i){
System.out.println(i+1);
i=i+2;
System.out.println(method2(i)); }
18
Local variables(2)
public static void main(String[] args){
System.out.println(i);
System.out.println(i+1);
method1();
i = method2(i);
System.out.println(i);
method3(i+1);
}
} //end of class
19
Actual printout
Working from main method:
Print
3
4
8
6
8
18
Why
First line executed, static i=3
Second line executed, static i=3
Third line executed, method1 at which i=8
Fifth line executed with i=method2(3), that is, i=6
Sixth line executed, method3(9), two printings
20
Array (1)
Array is an indexed list of elements of the same
type; the index is supplied by default (!)
A string array nam[ ]: contains both entries and
index.
String nam[] ={“John”,“Paul”,“George”,“Ringo”};
Index:
0
1
2
3
Length (the number of entries) is 4
An integer array age[ ]:
int age[ ]= {23, 32, 19, 30, 25, 25, 23, 30};
Index:
0
1 2 3 4 5 6 7
Length is 8
21
Array (2)
Not an array: abc[ ]={8, Ringo, +} - WHY?
(different types)
[ ] - on the array name's right is used to indicate
arrays
2. Declaring arrays
Both,
int ages[ ];
and
int[ ] ages;
is OK
22
Array (3)
Initialisation of an array: either
ages = new int[8]; // array with 8 zeros
or
ages[ ] = {23, 32, 19, 30, 25, 25, 23, 30};
//specify what is needed
Simultaneously declaring & initialising (with zeros)
int ages[] = new int[8];
23
Array (4)
ages[ ] = {23, 32, 19, 30, 25, 25, 23, 30};
Accessing array elements
ages [1] is 32
int i=4;
int j = ages [i]; // assigning j with 25
24
Work with arrays(1)
Data of 5 students:
double height[ ]={1.56, 1.72, 1.80, 1.85, 1.90};
//in m
double weight[ ]={65.3,80.0,78.1,76.5,112.8};
// in kg
Problem: compute the body mass index for all
the students, bmi=weight/height2
(in the US, those with bmi between 20 and 25
are considered of normal weight)
25
Work with arrays(2)
Loop for is natural with arrays: the index used as the
counter
bmi[ ]=new double[5];
for (int I = 0; I < 5; I + +)
bmi[I]=weight[I] / (height[I]height[I]);
If length of student arrays is not known or is
variable, put array’s length whatever it is:
bmi[ ]=new double[height.length];
for (int I = 0; I < height.length; I + +)
bmi[I]=weight[I] / (height[I]height[I]);
26
Work with arrays(3)
The same result with a method for the bmi:
double[ ] bmiindex(double h[ ], double w[ ]){
double in[ ];
for (int ii = 0; ii < h.length; ii = ii+1)
in[ii]=h[ii]/(w[ii]w[ii]);
return in; }
Method bmiindex is just a frame box; to make it
work, one needs to put within a class this:
double[ ] bmi=bmiindex(weight, height);
27