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 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 ); 2

3 Pass by Value Swap.java: 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

4 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

5 Pass by Value 5

6 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 =? 6

7 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

8 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

9 Overlading Methds Overlading the min Methd public static duble min(duble num1, duble num2) { if (num1 < num2) return num1; else return num2; 9

10 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

11 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 Example: OverladingMax.java Java will determine which methd t call based n the parameter list Smetimes there culd be several pssibilities: 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: an errr UC. Clrad Springs

12 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. 12

13 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; 13

14 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. 14

15 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 15

16 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;... 16

17 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; 17

18 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; 18

19 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; 19

20 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() * ( )) 20

21 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) 21

22 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)) 22

23 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)) 23

24 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'); 24

25 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 Opening Prblem Find the sum f integers frm 1 t 10, frm 20