SP1 Lecture 5: 9/2/11:

1. SP1 Lecture 5: 9/2/11: • Static and instance variables • Class Math • Random • Loan calculator • Local variables • Array (if time permits)

2. 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.

3. 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

4. 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

5. What’s going on in TesNum instVar statVar 1. With the class: 1 (in class) 10 2. At the constructor in class (virtual): 7 5 3. After alpha1: Constructor prints: 1 and 10 3 (within alpha1) 6 4. After alpha2: Constructor prints: 1 and 6 7 (within alpha2) 5 5. Method main prints: 3 and 7 5 and 5

6. 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);

7. 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

8. References to object variables and methods Examples from TesNum alpha1.statVar alpha2.instVar TesNum.statVar from TesNMod alpha1.SS(statVar)

9. 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 Math.sqrt(a) square root of number a Math.pow(a,b) ab ; if b is an integer then ab =aa…a (b times)

10. 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?

11. Loan calculator II • Loan • 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;

12. 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; • } • }

13. 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

14. 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?

15. 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);

16. 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

17. 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.

18. 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)); }

19. 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

20. Actual printout Working from main method: Print Why 3 First line executed, static i=3 4 Second line executed, static i=3 8 Third line executed, method1 at which i=8 6 Fifth line executed with i=method2(3), that is, i=6 8 Sixth line executed, method3(9),two printings 18

21. 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

22. 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

23. 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];

24. 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

25. 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)

26. 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]);

27. 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);