Agenda & Reading. Class Exercise. COMPSCI 105 SS 2012 Principles of Computer Science. Arrays

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1 COMPSCI 5 SS Principles of Computer Science Arrys & Multidimensionl Arrys Agend & Reding Agend Arrys Creting & Using Primitive & Reference Types Assignments & Equlity Pss y Vlue & Pss y Reference Copying & Cloning Arrys Arrys of Ojects Creting & Using Copying & Cloning Arrys of Ojects Multidimensionl Arrys Rgged Arrys Reding Textook: Dt Astrction & Prolem Solving with Jv Chpter : Review of Jv Fundmentls (..3) : crrno_ppt.ppt (- 8) The Jv Tutoril: Exercise Clss Exercise 3 Clss Exercise Wht is the output of the following? int x = 9; Integer intoject = new Integer(x); System.out.println("The vlue stored in intoject = " + intoject.intvlue()); (5 == 4) && ( < ) (5 == 5) ( < ) (5 == 4) && ( < ) (5 == 5) ( < ) if (( >= ) && ( >= c)) { lrgest = ; else if ( >= c) { lrgest = ; else { lrgest = ; // end if. Arrys Arry Nme Index Vlue Arrys coursemks[]= 95; 3 An rry is single nme for collection of dt vlues, ll of the sme dt type. System.out.println(courseMrks.length); Length The numer of elements in rry It determines the mount of memory llocted for the rry elements It is estlished in the declrtion nd cnnot e chnged unless the rry is redeclred. Index (suscript) The position of n element. It must e n int, or n expression tht evlutes to n int It uses zero-numering indexes go from to length- Vlue of the indexed vrile (n element of the rry) It cn e ccessed nd modified in constnt time using its index. The type of ll its components is the sme. 4 95

2 5. Arrys Declring & Creting Arrys The three steps for creting n rry re Declring n Arry int coursemrks[]; Syntx: String[] rgs; <type><nme>[]; //or <type>[] <nme>; No memory spce llocted No length specified The rry hs not een initilized Creting New Arry coursemrks = new int[]; Syntx: <type>[] <nme> = new <type>[length]; Memory spce llocted Initilizing elements Syntx: <nme> = new <type>{ vlues ; All elements utomticlly initilized to defult vlues if we don t specify the initil vlues coursemrks = new int[]{6,73,55,97; e.g. for int rrys 6. Arrys Creting Arrys (con t) Comine declrtion nd cretion The first two steps of creting n rry cn e comined s one: int[] coursemrks = new int[]; Comine declrtion & initiliztion Arry elements cn e initilized in the declrtion sttement y putting commseprted list in rces The length of n rry is utomticlly determined when the vlues re explicitly initilized in the declrtion doule[] reding = {5., 3., 9.65; The memory spce of n rry cn e llocted dynmiclly during the run-time of the progrm. The minimum size is. But it mens we cn t store nything in it. If the size is -, it will led to run-time error Note: The Jv compiler will NOT indicte tht something is wrong. int[] coursemrks = new int[-];. Arrys Using Arrys Arry processing is esily done in loop A for loop is commonly used to initilize rry elements nd print out ll vlues Initiliztion int[] reding = new int[5]; for (int i=; i<reding.length; i++) { reding[i] = i; Note: The loop counter/rry index goes from to length- It counts through length=5 itertions using the zero-numering of the rry index Printing for (int i=; i<reding.length; i++) { System.out.println(reding[i]); Using n index lrger thn length- cuses run time (not compiler) error An ArryIndexOutOfBoundsException is thrown Outside the vlid rnge System.out.println(reding[5]);. Arrys Arguments for the method min The heding for the min method shows prmeter tht is n rry of Strings: pulic sttic void min(string[] rgs) {... When you run progrm from the commnd line, ll words fter the clss nme will e pssed to the min method in the rgs rry. >jv TestArry Hello World The following min method in the clss TestArry will print out the first two rguments it receives: pulic sttic void min(string[] rgs) { for (int i=; i<rgs.length; i++) System.out.println(rgs[i]); >jv TestArry Hello World Hello World 7 8

3 9. Arrys Primitive & Reference Types Rememer tht Jv distinguishes etween primitive types nd reference types Primitive Types The primitive types re: yte, short, int, long, flot, doule, chr, oolen. A vrile of primitive type is directly ssocited with memory loction storing the vlue of the vrile. int x = 4; x 4 Reference Types: Any dt type tht is not primitive type is reference type A vrile of reference type is ssocited with memory loction tht stores the ddress of the memory loction storing the vlue. If vrile is not initilized explicitly, the vrile is given the specil vlue null. int[] = {,,; Jv rrys re reference types:. Arrys Assignment Primitive type: The vrile y contins copy of the vlue held in the vrile x. There re two independent copies of the 3-it integer 4. Reference type: int x = 4; int y = x; x 4 int[] = {,,; int[] = ; y 4 The vrile holds copy of the reference held in the vrile. There is still only one copy of the rry.. Arrys Equlity Primitive type: if (x == y) System.out.println("Equl"); else System.out.println("Not Equl"); The condition is true if the stored vlues for vriles vr nd vr re identicl Reference type: if ( == ) System.out.println("Equl"); else System.out.println("Not Equl"); x 4 y 4 The condition is true if oth vr nd vr store the sme ddress. Arrys Testing for Equlity The following exmple cretes two rrys of ints. Testing for Equlity int i; int[] = new int[3]; int[] = new int[3]; for(i=; i <.length; i++) [i] = i; for(i=; i <.length; i++) [i] = i; if ( == ) System.out.println("Equl"); else System.out.println("Not Equl"); Vriles nd re oth 3-element rrys of ints The output of this exmple is Not Equl ecuse the ddresses of the rrys re not equl.

4 . Arrys Pss y Vlue Primitive type: Similr difference in ehvior etween primitive types nd reference types occurs when rguments re pssed to methods pulic sttic void increment(int x) { x = x + ; System.out.println(x); pulic sttic void testbyvlue() { int z = 4; increment(z); System.out.println(z); testbyvlue z:4 increment x:4 -> 5 Since x is primitive type, the method hs its own privte copy of this vlue.. Arrys Pss y Reference Reference type: pulic sttic void pssbyreference(int[] ) { []=; pulic sttic void testbyreference() { int[] = {,,; pssbyreference(); System.out.println([]); testbyreference -> The method pssed copy of the reference held in vrile to the vrile. Now oth the vrile nd the method prmeter hold references to the sme oject. The method cn use its reference to chnge the contents of the oject. Therefore, the output is. Chnged to 3 4. Arrys Pss y Reference (con t) Reference type: A tricky exmple: pulic sttic void pssbyreference(int[] ) { = new int[3]; []= -; Crete new rry: pulic sttic void testbyreference() { int[] = {,,; pssbyreference(); System.out.println([]); ->- The ddress of the rry is pssed into the other method. However, the method crete new rry nd overwrite the existing locl rry. The vlues of the vrile re unchnged.. Arrys Copying Arrys Note: ssigning one rry to nother only chnges the references. No ctul copying is performed. To perform copying, use the System.rrycopy void rrycopy(oject src, int srcpos, Oject dest, int destpos, int length) Copies n rry from the specified source rry, eginning t the specified position, to the specified position of the destintion rry. int[] src = {,,3; int[] copy = new int[3]; System.rrycopy(src,, copy,, ); for (int i=; i<copy.length; i++) System.out.println(copy[i]); Note: It is useful when copying prts of n rry The copy rry must e first creted. src 3 copy if (copy == src) //checking reference System.out.println("Equl"); else System.out.println("not Equl"); 5 6

5 . Arrys Cloning Arrys To copy the entire rry, use the clone method Oject clone() Cretes nd returns copy of this oject int[] src = {,,3; int[] copy = (int []) src.clone(); The type csting is needed ecuse clone lwys returns n oject. It converts vlue of type oject to vlue of type int rry. The type csting will fil if the vlue is of the wrong underlying type src 3 copy 3 if (copy == src) //checking reference System.out.println("Equl"); else System.out.println("not Equl");. Arrys Exercise Wht is the output of the following method? pulic sttic void rryex() { int[] mrks = { 4, 7,, 8 ; int[] mrks = chngemrk(mrks); if ( mrks == mrks) { System.out.println("Sme Reference"); for (int i =; i< mrks.length; i++) System.out.print(mrks[i] + " "); System.out.println(); pulic sttic int[] chngemrk(int[] ) { for (int i =; i<.length; i++) [i] = [i] + i; return ; 7 8. Arrys of Ojects Arry of Ojects Aprt from rrys of primitive types we cn hve rrys of reference types (ojects) p Creting n rry of Point Syntx: Crete the rry <clss>[] <nme> = new <clss>[length]; Point[] p = new Point[3]; Note: Memory spce re llocted for the rry Crete the oject element <nme>[row] = new <clss>(); p[] = new Point(, ); p[] = new Point(, 3); Note: In n rry of reference types not the ojects re stored in the rry, ut references to the ojects. The ojects themselves hve to e creted nd memory spce hs to e llocted for them seprtely. The element re initilized with the null reference. x: y: x: y:3 NULL. Arrys of Ojects Using Arry of Ojects Accessing To ccess ll components of n rry of reference types we cn use the sme code tht we hve used for primitive types A for loop is used to print out vlues. for (int i=; i<; i++) System.out.println(p[i].toString()); You must crete the oject element efore ccessing it. Otherwise, NullPointerException is thrown. for (int i=; i<p.length; i++) System.out.println("(" + p[i].x + ","+ p[i].y + ")"); (,) (,3) Exception in thred "min" jv.lng.nullpointerexception... 9

6 . Arrys of Ojects Assignment - Arrys of Ojects Reference type: Point[] p = new Point[3]; p[] = new Point(, ); p[] = new Point(, 3); p[] = new Point(3, 4); Point[] copy = p; The vrile copy holds copy of the reference held in the vrile p. There is still only one copy of the rry. Thus chnging the oject pointed to y one of the vriles will lso cuse the contents of the other vrile to chnge (since the sme oject in memory is eing ltered). p copy x: -> y: x: y:3 x:3 Y:4. Arrys of Ojects Copying Arrys of Ojects An rry cn e copied to reproduce n exct copies (or prt) of the originl y using the System.rrycopy method However, if the rry elements re reference vriles, oth source nd destintion now refer to the sme oject (pointing to the sme physicl oject in memory) Point[] p = new Point[3]; p[] = new Point(, ); p[] = new Point(, 3); p[] = new Point(3, 4); Point[] copy = new Point[]; System.rrycopy(p,, copy,, ); Thus chnging the oject pointed to y one of the vriles will lso cuse the contents of the other vrile to chnge (since the sme oject in memory is eing ltered). p x: -> y: x: y:3 x:3 Y:4 copy copy[].x = ; for (int i=; i<copy.length; i++) System.out.println(p[i].toString()); copy[].x = ; for (int i=; i<copy.length; i++) System.out.println(p[i].toString());. Arrys of Ojects Cloning Arrys of Ojects Shllow Clone An rry cn e cloned to reproduce n exct copies of the originl It lloctes spce to hold the sme size s the originl It mkes copies of the originl nd hs the sme prolem s efore (pointing to the sme physicl oject in memory) Type csting is needed. Arrys of Ojects Deep Clone A deep copy is copy tht contins the complete dt of the originl oject, llowing it to e used independently of the originl oject Algorithm: Clone the rry Wlk through ech element in the new rry Clone the element in the rry to produce new copy Point[] p = new Point[]; p[] = new Point(, ); p[] = new Point(, 3); Point[] copy = (Point[])p.clone(); copy[].x = ; System.out.println(p[].toString()); p x: -> y: x: y:3 copy Point[] q = new Point[]; q[] = new Point(, ); q[] = new Point(, 3); Point[] dest = (Point[])q.clone(); for (int i = ; i < q.length; i++) dest[i] = (Point)q[i].clone( ); dest[].x = ; System.out.println(q[].toString()); q x: y: x: y:3 dest x: -> y: x: y:3 3 4

7 . Arrys of Ojects Exercise Wht is the output of the following method? pulic sttic void rryex() { Point[] p = new Point[]; p[] = new Point(, ); p[] = new Point(, ); Point[] p = chnge(p); if ( p == p) { System.out.println("Sme Reference"); for (int i =; i< p.length; i++) System.out.print(p[i] + " "); for (int i =; i< p.length; i++) System.out.print(p[i] + " "); pulic sttic Point[] chnge(point[] ) { for (int i =; i<.length; i++) ((Point) [i]).trnslte(, ); return ;.3 Multidimensionl Arrys Multidimensionl Arrys Multidimensionl Arrys Arrys with more thn one index numer of dimensions = numer of indexes Multidimensionl rrys in Jv re implemented s rrys of rrys Exmples: int[][][] threed = new int [][3][4]; int[][][][] fourd = new int [][3][768][4]; Two-dimensionl rrys A two-dimensionl rry cn e thought of s tle of vlues, with rows nd columns A two-dimensionl rry element is referenced using two index vlues Exmple: int[][] x = new int[5][7]; 5 rows 7 columns: Multidimensionl Arrys -Dimensionl Arrys Declrtion & Initiliztion -D rrys Syntx for -D rrys is similr to -D rrys <type>[][] <nme> = new <type>[rows][columns]; Exmple : int[][] x = new int[5][7]; The rry should hve five rows nd seven columns The rel picture: Exmple : int[][] x = new int [4][]; The rry hs 4 rows. -D Arry Initilizer You cn lso initilize t declrtion with D rry literl Use nested comm seprted lists, enclosed in { s int[][] y = { {,,3, {4,5,6; Multidimensionl Arrys Creting Multidimensionl Arrys We cn use existing rrys for initiliztion 3 does not store copies of nd in Arry3[] nd 3[] respectively Insted it stores reference to nd int[] = {,,4; int[] = {,,4; int[][] 3 = {, ; If we chnge the vlue of [], we lso chnge the vlue of 3[][] [] = ; System.out.println(3[][]); To crete copies of n existing -dimensionl rrys, we cn use the clone method int[][] 4={(int[]).clone(), (int[]).clone();

8 9.3 Multidimensionl Arrys Length Vriles Syntx: rrynme.length returns returns the numer of rows in the rry rrynme[rowtoget].length Note: returns the numer of columns in row rowtoget int[][] x = new int[5][7]; System.out.println(x.length); System.out.println(x[].length); 5 7 If x is rectngulr rry, ll rows hve the sme length System.out.println(x[].length); System.out.println(x[].length); System.out.println(x[].length); Multidimensionl Arrys Using D Arrys Accessing elements To ccess elements, we use pretty much the sme technique s D rry Initiliztion & Printing Nested loops re used to initilize rry elements nd print out vlues Algorithm: System.out.println(myArr[rowToGet][colToGet]); Strting from the first row ( th ) to the lst row (n-) outer loop For ech row, strting from the first column ( th ) to the lst column (m-) inner loop Print out the vlue Use y.length to nvigte the lst row int[][] y = { {,,3, {4,5,6; for (int row = ; row < y.length; row++) { for (int col = ; col < y[row].length; col++) System.out.print(y[row][col] + " "); System.out.println(); Use y[row].length to nvigte the lst column of ech row.3 Multidimensionl Arrys Cloning of Multi-D Arrys.4 Rgged Arrys Rgged Arrys It is shllow clone only Rememer tht multi-dimensionl rrys in Jv re rrys of rrys. Cloning only genertes copy of the root rry int[][] 5 = {{,,4, {,,4; int[][] 6 = (int[][])5.clone(); Note: 5[] nd 5[] from n -dimensionl rry tht store the memory loction m nd m of two -dimensionl rrys. If we clone 5, then we only crete new -dimensionl rry tht stores m nd m. Rgged rrys hve rows of unequl length Ech row hs different numer of columns, or entries Declrtion & Initiliztion rgged rry Syntx: <type>[][] <nme> = new <type>[rows][]; <nme>[] = new <type>[colcountforrow];... int[][] ; = new int[3][]; [] = new int[]; [] = new int[3]; [] = new int[]; Arry Initilizer int[][] = {{3,, {3,,, {;

9 .4 Rgged Arrys Using Rgged Arrys Length Vriles: rrynme.length returns returns the numer of rows in the rry rrynme[rowtoget].length returns the numer of columns in row rowtoget Exmple:.length = 3 int[][] y = {{3,, {3,,, {; [].length returns [].length returns 3 [].length returns [3].length <- ERROR Initiliztion & Printing Nested loops re used to initilize rry elements nd print out vlues Note: You must use y[row].length to get the numer of columns in ech row. The numer my not e the sme for ll rows. for (int row = ; row < y.length; row++) { for (int col = ; col < y[row].length; col++) System.out.print(y[row][col] + " "); System.out.println();.4 Rgged Arrys Exmple: Pscl's Tringle Pscl s Tringle using rgged rry The first row hs element The second row hs elements The third row hs 3 elements... The n row hs (n+) elements Solution: Crete n-row rry Fill the first row, crete column nd fill y For the second to the lst row(n), crete (n+) columns Clculte the vlue For the first or lst column, vlue = Else, vlue[row][col] = vlue of the previous row nd previous col + vlue of the previous row ut the sme col Code:.4 Rgged Arrys Exmple: Pscl's Tringle pulic sttic void pscltringle() { finl int NUM_ROWS = 5; int tringle[][] = new int[num_rows][]; tringle[] = new int[]; tringle[][] = ; for (int row = ; row < NUM_ROWS; row++) { tringle[row] = new int[row+]; tringle[row][] = ; tringle[row][row] = ; for (int col = ; col < row; col++) tringle[row][col] = tringle[row-][col-] + tringle[row-][col]; Summry Advntges & Disdvntges Advntges Esy to specify (declrtion, lloction of memory spce, initiliztion cn ll e done in one line of code) Direct ccess to ny element vi the index Fst Access Disdvntges A potentil disdvntge of rrys is the need to know the size t time of lloction Creful design is required to mke sure tht the rry will e lrge enough to hold the lrgest possile group of dt 35 36

10 Exercise 3 Wht will hppen len = grid[].length; for(int row=; row < len; row++) { for(int col=; col < len; col++) { System.out.print(grid[row][col] + " "); System.out.println(); int[][] grid = { { 3, 5, 6, 9,{, 6, 9, 4 ; int len; How to fix it? len = grid.length; for(int row=; row < len; row++) { for(int col=; col < len; col++) { System.out.print(grid[row][col] + " "); System.out.println(); 37