CS1150 Principles of Computer Science Methods

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1 CS1150 Principles f Cmputer Science Methds Yanyan Zhuang Department f Cmputer Science CS1150 UC. Clrad Springs

2 Opening Prblem Find the sum f integers frm 1 t 10, frm 20 t 30, and frm 35 t 45, respectively. 2

3 Prblem int sum = 0; fr (int i = 1; i <= 10; i++) sum += i; System.ut.println("Sum frm 1 t 10 is " + sum); sum = 0; fr (int i = 20; i <= 30; i++) sum += i; System.ut.println("Sum frm 20 t 30 is " + sum); sum = 0; fr (int i = 35; i <= 45; i++) sum += i; System.ut.println("Sum frm 35 t 45 is " + sum); 3

4 Prblem int sum = 0; fr (int i = 1; i <= 10; i++) sum += i; System.ut.println("Sum frm 1 t 10 is " + sum); sum = 0; fr (int i = 20; i <= 30; i++) sum += i; System.ut.println("Sum frm 20 t 30 is " + sum); sum = 0; fr (int i = 35; i <= 45; i++) sum += i; System.ut.println("Sum frm 35 t 45 is " + sum); 4

5 Slutin public static int sum(int i1, int i2) { int sum = 0; fr (int i = i1; i <= i2; i++) sum += i; return sum; public static vid main(string[] args) { System.ut.println("Sum frm 1 t 10 is " + sum(1, 10)); System.ut.println("Sum frm 20 t 30 is " + sum(20, 30)); System.ut.println("Sum frm 35 t 45 is " + sum(35, 45)); 5

6 Defining Methds A methd is a cllectin f statements that are gruped tgether t perfrm an peratin. public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; Define a methd Invke a methd int z = max(x, y); actual parameters (arguments) 6

7 Defining Methds A methd is a cllectin f statements that are gruped tgether t perfrm an peratin. methd header methd bdy public static int max(int num1, int num2) { mdifier int result; Define a methd return value type if (num1 > num2) result = num1; else result = num2; return result; methd name return value frmal parameters parameter list methd signature Invke a methd int z = max(x, y); actual parameters (arguments) 7

8 Methd Signature Methd signature: the methd name and the parameter list Methd name: a prgrammer defined name Parameter list: infrmatin that is cming int the methd methd header methd bdy public static int max(int num1, int num2) { mdifier int result; Define a methd return value type if (num1 > num2) result = num1; else result = num2; return result; methd name return value frmal parameters parameter list methd signature Invke a methd int z = max(x, y); actual parameters (arguments) 8

9 Frmal Parameters The variables defined in the methd header are knwn as frmal parameters. methd header methd bdy public static int max(int num1, int num2) { mdifier int result; Define a methd return value type if (num1 > num2) result = num1; else result = num2; return result; methd name return value frmal parameters parameter list methd signature Invke a methd int z = max(x, y); actual parameters (arguments) 9

10 Actual Parameters When a methd is invked, yu pass a value t the parameter. This value is referred t as actual parameter r argument. methd header methd bdy public static int max(int num1, int num2) { mdifier int result; Define a methd return value type if (num1 > num2) result = num1; else result = num2; return result; methd name return value frmal parameters parameter list methd signature Invke a methd int z = max(x, y); actual parameters (arguments) 10

11 Mdifiers Bth public and static are the methd s mdifier public: access t the methd is public (visible t all ther classes) static: the methd belng t the class, nt an individual instance Define a methd Invke a methd methd header methd bdy public static int max(int num1, int num2) { mdifier int result; return value type if (num1 > num2) result = num1; else result = num2; return result; methd name return value frmal parameters parameter list methd signature int z = max(x, y); actual parameters (arguments) 11

12 Return Value Type A methd may return a value. The returnvaluetype is the data type f the value the methd returns. If the methd des nt return a value, the returnvaluetype is the keywrd vid. Fr example, the returnvaluetype in the main methd is vid. Define a methd Invke a methd methd header methd bdy public static int max(int num1, int num2) { mdifier int result; return value type if (num1 > num2) result = num1; else result = num2; return result; methd name return value frmal parameters parameter list methd signature int z = max(x, y); actual parameters (arguments) 12

13 Rules fr Methds A methd may r may nt return a value A methd must declare a return type! If a methd returns a value Return type is the data type f the value being returned The return statement is used t return the value If a methd des nt return a value Return type in this case is vid N return statement is needed (lk at max n return example) The values yu pass in much match the rder and type f the parameters declared in the methd UC. Clrad Springs

14 Calling Methds pass the value f i pass the value f j public static vid main(string[] args) { int i = 5; int j = 2; int k = max(i, j); System.ut.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; The main methd is where the cde starts 14

15 Trace Methd Invcatin i is nw 5 public static vid main(string[] args) { int i = 5; int j = 2; int k = max(i, j); System.ut.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; 15

16 Trace Methd Invcatin j is nw 2 public static vid main(string[] args) { int i = 5; int j = 2; int k = max(i, j); System.ut.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; 16

17 Trace Methd Invcatin invke max(i, j) public static vid main(string[] args) { int i = 5; int j = 2; int k = max(i, j); System.ut.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; 17

18 Trace Methd Invcatin invke max(i, j) Pass the value f i t num1 Pass the value f j t num2 public static vid main(string[] args) { int i = 5; int j = 2; int k = max(i, j); System.ut.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; 18

19 Trace Methd Invcatin declare variable result public static vid main(string[] args) { int i = 5; int j = 2; int k = max(i, j); System.ut.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; 19

20 Trace Methd Invcatin (num1 > num2) is true since num1 is 5 and num2 is 2 public static vid main(string[] args) { int i = 5; int j = 2; int k = max(i, j); System.ut.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; 20

21 Trace Methd Invcatin result is nw 5 public static vid main(string[] args) { int i = 5; int j = 2; int k = max(i, j); System.ut.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; 21

22 Trace Methd Invcatin return result, which is 5 public static vid main(string[] args) { int i = 5; int j = 2; int k = max(i, j); System.ut.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; 22

23 Trace Methd Invcatin return max(i, j) and assign the return value t k public static vid main(string[] args) { int i = 5; int j = 2; int k = max(i, j); System.ut.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; 23

24 Trace Methd Invcatin Execute the print statement public static vid main(string[] args) { int i = 5; int j = 2; int k = max(i, j); System.ut.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; 24

25 CAUTION A return statement is required fr a value-returning methd. The methd shwn belw in (a) is lgically crrect, but it has an errr because Java thinks it pssible that this methd des nt return any value. public static int sign(int n) { if (n > 0) return 1; else if (n == 0) return 0; else if (n < 0) return 1; Shuld be public static int sign(int n) { if (n > 0) return 1; else if (n == 0) return 0; else return 1; (a) T fix this prblem, delete if (n < 0) in (a), s that Java will see a return statement t be reached regardless f hw the if statement is evaluated. (b) 25

26 Reuse Methds frm Other Classes NOTE: One f the benefits f methds is fr reuse. The max methd can be invked frm any class besides FindingMax. If yu create a new class Test, yu can invke the max methd using ClassName.methdName (e.g., FindingMax.max). 26

27 Value-Returning Methds Return a value t the caller Call t a value returning methd can be treated as a value r a statement returning a value SumIntegers example The main methd is where the cde starts The methd sum is called: The value entered by the user is the actual parameter This means the value stred in number (5) is sent t the methd Cntrl enters the sum methd UC. Clrad Springs

28 Value-Returning Methds SumIntegers example Substitute actual parameter (i.e. number1) int the frmal parameter (i.e. i1) The methd nw has a value fr i1" that can be used within the methd Prcess fr lp The return statement frces cntrl t return t the caller (main) & sends t caller an integer value UC. Clrad Springs

29 Vid Methds Des nt return a value t the caller The return type must be set t vid N return statement is used Cntrl returns t caller when the methds clsing curly brace is reached Call t a vid methd must be made as a statement See SumIntegersVid example UC. Clrad Springs

30 Call Stacks 30

31 Call Stacks The call stack is used t keep track f active methds Stack is a data structure Like.plate stacks at the buffet line: pp a plate ff the stack, push a plate nt the stack Stacks are last in, first ut data structure (last item pushed nt stack is 1st item ppped ff) UC. Clrad Springs

32 Call Stacks When methd is called (invked) Infrmatin fr that methd is pushed nt the stack Values f parameters and variables Current line f cde executing This infrmatin is called a "frame" When the methd returns (cmpletes) Infrmatin fr that methd is ppped ff f the stack (frame is remved) Methd n tp f stack is the current executing methd Methd stays n tp until the ending curly brace is reached UC. Clrad Springs

33 Trace Call Stack i is declared and initialized public static vid main(string[] args) { int i = 5; int j = 2; int k = max(i, j); System.ut.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; The main methd is invked. i: 5 return result; 33

34 Trace Call Stack j is declared and initialized public static vid main(string[] args) { int i = 5; int j = 2; int k = max(i, j); System.ut.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; The main methd is invked. j: 2 i: 5 return result; 34

35 Trace Call Stack Declare k public static vid main(string[] args) { int i = 5; int j = 2; int k = max(i, j); System.ut.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; Space required fr the main methd k: j: 2 i: 5 The main methd is invked. return result; 35

36 Trace Call Stack Invke max(i, j) public static vid main(string[] args) { int i = 5; int j = 2; int k = max(i, j); System.ut.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; Space required fr the main methd k: j: 2 i: 5 The main methd is invked. return result; 36

37 Trace Call Stack pass the values f i and j t num1 and num2 public static vid main(string[] args) { int i = 5; int j = 2; int k = max(i, j); System.ut.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; num2: 2 num1: 5 Space required fr the main methd k: j: 2 i: 5 return result; The max methd is invked. 37

38 Trace Call Stack Declare result public static vid main(string[] args) { int i = 5; int j = 2; int k = max(i, j); System.ut.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; result: num2: 2 num1: 5 Space required fr the main methd k: j: 2 i: 5 return result; The max methd is invked. 38

39 Trace Call Stack (num1 > num2) is true public static vid main(string[] args) { int i = 5; int j = 2; int k = max(i, j); System.ut.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; result: num2: 2 num1: 5 Space required fr the main methd k: j: 2 i: 5 return result; The max methd is invked. 39

40 Trace Call Stack Assign num1 t result public static vid main(string[] args) { int i = 5; int j = 2; int k = max(i, j); System.ut.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; Space required fr the max methd result: 5 num2: 2 num1: 5 Space required fr the main methd k: j: 2 i: 5 return result; The max methd is invked. 40

41 Trace Call Stack Return result and assign it t k public static vid main(string[] args) { int i = 5; int j = 2; int k = max(i, j); System.ut.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; Space required fr the max methd result: 5 num2: 2 num1: 5 Space required fr the main methd k:5 j: 2 i: 5 return result; The max methd is invked. 41

42 Trace Call Stack Execute print statement public static vid main(string[] args) { int i = 5; int j = 2; int k = max(i, j); System.ut.println( "The maximum between " + i + " and " + j + " is " + k); public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; Space required fr the main methd k:5 j: 2 i: 5 The main methd is invked. return result; 42

43 Passing Parameters public static vid nprintln(string message, int n) { fr (int i = 0; i < n; i++) System.ut.println(message); Suppse yu invke the methd using nprintln( Welcme t Java, 5); What is the utput? Suppse yu invke the methd using nprintln( Cmputer Science, 15); What is the utput? Can yu invke the methd using nprintln(15, Cmputer Science ); 43

44 Pass by Value Caller passes arguments (actual parameters) swap(number1, number2); Methd uses parameters (frmal parameters) public static vid swap (int num1, int num2) Actual and frmal parameters match in rder, number and type UC. Clrad Springs

45 Pass by Value Pass by value means The actual parameter is fully evaluated A cpy f that value is placed int the frmal parameter variable Because nly a cpy is sent, the actual value f the argument is NOT changed by the methd! Values f number1 and number2 are NOT changed Only a cpy f the variables sent t the methd Whatever happens t variables inside the methd des nt affect variables utside the methd But, what happens if we make the frmal parameter name match actual parameter name? Will still nly change the values f variables utside the methd UC. Clrad Springs

46 Pass by Value 46

47 Cnverting Hexadecimals t Decimals Write a methd that cnverts a hexadecimal number int a decimal number. ABCD => A*16^3 + B*16^2 + C*16^1+ D*16^0 = ((A*16 + B)*16 + C)*16+D = ((10* )* )*16+13 =? 47

48 Cnverting Hexadecimals t Decimals int decimalvalue = 0; fr (int i = 0; i < hex.length(); i++) { char hexchar = hex.charat(i); decimalvalue = decimalvalue * 16 + hexchartdecimal(hexchar); UC. Clrad Springs

49 Overlading Methds Tw r mre methds with the same name but different frmal parameters (signature) Gives the ability t create multiple versins f a methd Why? Because methds that perfrm the same task but n different data shuld be named the same (max, min) UC. Clrad Springs

50 Overlading Methds Overlading the min Methd public static duble min(duble num1, duble num2) { if (num1 < num2) return num1; else return num2; 50

51 Overlading Methds In the Math class the min and max methds are verladed In the example, min can take 2 ints 2 dubles 2 flats 2 lngs UC. Clrad Springs

52 Overlading Methds -- Rules T be cnsidered an verladed methd Name - must be the same Return type - can be different - but yu cannt change nly the return type Frmal parameters - must be different Java will determine which methd t call based n the parameter list Smetimes there culd be several pssibilities Cmplier will pick the "best match" It is pssible that the methds are written in way that the cmplier cannt decide best match This is called ambiguus invcatin This results in an errr UC. Clrad Springs

53 Ambiguus Invcatin Smetimes there may be tw r mre pssible matches fr an invcatin f a methd, but the cmpiler cannt determine the mst specific match. This is referred t as ambiguus invcatin. Ambiguus invcatin is an errr. 53

54 Ambiguus Invcatin public class AmbiguusOverlading { public static vid main(string[] args) { System.ut.println(max(1, 2)); // Errr here! The methd max(int,duble) is ambiguus public static duble max(int num1, duble num2) { if (num1 > num2) return num1; else return num2; public static duble max(duble num1, int num2) { if (num1 > num2) return num1; else return num2; 54

55 Scpe f Lcal Variables A lcal variable: a variable defined inside a methd/blck Scpe: the part f the prgram where the variable can be referenced The scpe f a lcal variable starts frm its declaratin and cntinues t the end f the blck that cntains the variable A lcal variable must be declared befre it can be used. 55

56 Scpe f Lcal Variables, cnt. Can declare a lcal variable with the same name multiple times in different nnnesting blcks in a methd Cannt declare a lcal variable twice in nested blcks Frmal parameters are cnsidered lcal variables 56

57 Scpe f Lcal Variables, cnt. A variable declared in the initial actin part f a fr lp header has its scpe in the entire lp. But a variable declared inside a fr lp bdy has its scpe limited in the lp bdy frm its declaratin and t the end f the blck that cntains the variable. Questin: inner lps? The scpe f i The scpe f j public static vid methd1() {.. fr (int i = 1; i < 10; i++) {.. int j;... 57

58 Scpe f Lcal Variables, cnt. It is fine t declare i in tw nn-nesting blcks public static vid methd1() { int x = 1; int y = 1; fr (int i = 1; i < 10; i++) { x += i; fr (int i = 1; i < 10; i++) { y += i; It is wrng t declare i in tw nesting blcks public static vid methd2() { int i = 1; int sum = 0; fr (int i = 1; i < 10; i++) sum += i; 58

59 Scpe f Lcal Variables, cnt. // Fine with n errrs public static vid crrectmethd() { int x = 1; int y = 1; // i is declared fr (int i = 1; i < 10; i++) { x += i; // i is declared again fr (int i = 1; i < 10; i++) { y += i; 59

60 Scpe f Lcal Variables, cnt. // With errrs public static vid incrrectmethd() { int x = 1; int y = 1; fr (int i = 1; i < 10; i++) { int x = 0; x += i; 60

61 Case Study: Generating Randm Characters Cmputer prgrams prcess numerical data and characters. Each character has a unique Unicde between 0 and FFFF in hexadecimal (65535 in decimal). T generate a randm character is t generate a randm integer between 0 and using the fllwing expressin: (nte that since 0 <= Math.randm() < 1.0, yu have t add 1 t ) (int)(math.randm() * ( )) 61

62 Case Study: Generating Randm Characters, cnt. Nw cnsider hw t generate a randm lwercase letter. The Unicde fr lwercase letters are cnsecutive integers starting frm the Unicde fr 'a', then fr 'b', 'c',..., and 'z'. The Unicde fr 'a' is (int)'a' S, a randm integer between (int)'a' and (int)'z' is (int)((int)'a' + Math.randm() * ((int)'z' - (int)'a' + 1) 62

63 Case Study: Generating Randm Characters, cnt. As discussed in Chapter 4, all numeric peratrs can be applied t the char perands. S, the preceding expressin can be simplified as fllws: 'a' + Math.randm() * ('z' - 'a' + 1) S a randm lwercase letter is (char)('a' + Math.randm() * ('z' - 'a' + 1)) 63

64 Case Study: Generating Randm Characters, cnt. T generalize the freging discussin, a randm character between any tw characters ch1 and ch2 with ch1 < ch2 can be generated as fllws: (char)(ch1 + Math.randm() * (ch2 ch1 + 1)) 64

65 The RandmCharacter Class // RandmCharacter.java: Generate randm characters public class RandmCharacter { /** Generate a randm character between ch1 and ch2 */ public static char getrandmcharacter(char ch1, char ch2) { return (char)(ch1 + Math.randm() * (ch2 - ch1 + 1)); /** Generate a randm lwercase letter */ public static char getrandmlwercaseletter() { return getrandmcharacter('a', 'z'); /** Generate a randm uppercase letter */ public static char getrandmuppercaseletter() { return getrandmcharacter('a', 'Z'); /** Generate a randm digit character */ public static char getrandmdigitcharacter() { return getrandmcharacter('0', '9'); /** Generate a randm character */ public static char getrandmcharacter() { return getrandmcharacter('\u0000', '\uffff'); 65

66 Summary Value-returning methds Vid methds Call stack Overlading methds Scpe f lcal variables CS1150 UC. Clrad Springs

CS1150 Principles of Computer Science Methods

CS1150 Principles of Computer Science Methods CS1150 Principles f Cmputer Science Methds Yanyan Zhuang Department f Cmputer Science http://www.cs.uccs.edu/~yzhuang CS1150 UC. Clrad Springs Passing Parameters public static vid nprintln(string message,