CSC 1214: Object-Oriented Programming

Transcription

1 CSC 1214: Object-Oriented Programming J. Kizito Makerere University www: materials: e-learning environment: office: block A, level 3, department of computer science alt. office: institute of open, distance, and elearning, room 2 Control Flow Statements, Arrays, and Iterators Kizito (Makerere University) CSC 1214 February, / 17

2 Overview 1 Control Flow Introduction Conditional Statements Looping Statements Branching Arrays Iterators Kizito (Makerere University) CSC 1214 February, / 17

3 Introduction Control Flow Statements Statements and Blocks An expression followed by a semicolon becomes a x = 0; i++; System.out.print(...); The semicolon is a terminator Braces { and are used to group s into a compound or block There is no semicolon after the right brace that ends a block Kizito (Makerere University) CSC 1214 February, / 17

4 Introduction Control Flow Statements Overview Conditional s: a conditional lets us choose which will be executed next 1 The if 2 The if-else 3 The else-if 4 The switch Iteration/looping s: a looping lets us execute a several times 1 The while loop 2 The do-while loop 3 The for loop Branching 1 The break 2 The continue Kizito (Makerere University) CSC 1214 February, / 17

5 Conditional Statements Control Flow Statements If[-Else] Syntax Examples if (expression) 1 [else 2] 1 if(a < b) a = 0; else b = 0; 2 3 if(a % 2 == 0) System.out.print("even"); if (bytesavailable > 0) { ProcessData(); bytesavailable -= n; else waitformoredata(); Kizito (Makerere University) CSC 1214 February, / 17

6 Conditional Statements Control Flow Statements If[-Else] Syntax Examples if (expression) 1 [else 2] 1 if(a < b) a = 0; else b = 0; 2 3 if(a % 2 == 0) System.out.print("even"); if (bytesavailable > 0) { ProcessData(); bytesavailable -= n; else waitformoredata(); Kizito (Makerere University) CSC 1214 February, / 17

7 Conditional Statements Control Flow Statements Else-If Syntax Example if (expression) else if (expression) else if (expression) //... [else ] if(month == 12 month == 1 month == 2) season = "Winter"; else if(month == 3 month == 4 month == 5) season = "Spring"; else if(month == 6 month == 7 month == 8) season = "Summer"; else if(month == 9 month == 10 month == 11) season = "Autumn"; else season = "Bogus Month"; Kizito (Makerere University) CSC 1214 February, / 17

8 Conditional Statements Control Flow Statements Else-If Syntax Example if (expression) else if (expression) else if (expression) //... [else ] if(month == 12 month == 1 month == 2) season = "Winter"; else if(month == 3 month == 4 month == 5) season = "Spring"; else if(month == 6 month == 7 month == 8) season = "Summer"; else if(month == 9 month == 10 month == 11) season = "Autumn"; else season = "Bogus Month"; Kizito (Makerere University) CSC 1214 February, / 17

9 Conditional Statements Control Flow Statements Switch Syntax Example switch (expression) { case value1: s break; case value2: s break; //... default: s for(int i=0; i<6; i++) switch(i) { case 0: System.out.println("i is zero."); break; case 1: System.out.println("i is one."); break; case 2: System.out.println("i is two."); break; case 3: System.out.println("i is three."); break; default: System.out.println("i is greater than 3."); Kizito (Makerere University) CSC 1214 February, / 17

10 Conditional Statements Control Flow Statements Switch Syntax Example switch (expression) { case value1: s break; case value2: s break; //... default: s for(int i=0; i<6; i++) switch(i) { case 0: System.out.println("i is zero."); break; case 1: System.out.println("i is one."); break; case 2: System.out.println("i is two."); break; case 3: System.out.println("i is three."); break; default: System.out.println("i is greater than 3."); Kizito (Makerere University) CSC 1214 February, / 17

11 Looping Statements Loops While and For Syntax Examples while (expression) for (expr1; expr2; expr3) Conventionally, expr1 initialization, expr2 condition, and expr3 iteration 1 2 int n = 10; while(n > 0) { n--; for(n=10; n>0; n--) Kizito (Makerere University) CSC 1214 February, / 17

12 Looping Statements Loops While and For Syntax Examples while (expression) for (expr1; expr2; expr3) Conventionally, expr1 initialization, expr2 condition, and expr3 iteration 1 2 int n = 10; while(n > 0) { n--; for(n=10; n>0; n--) Kizito (Makerere University) CSC 1214 February, / 17

13 Looping Statements Loops While and For Syntax Examples while (expression) for (expr1; expr2; expr3) Conventionally, expr1 initialization, expr2 condition, and expr3 iteration 1 2 int n = 10; while(n > 0) { n--; for(n=10; n>0; n--) Kizito (Makerere University) CSC 1214 February, / 17

14 Looping Statements Loops While and For Syntax Examples while (expression) for (expr1; expr2; expr3) Conventionally, expr1 initialization, expr2 condition, and expr3 iteration 1 2 int n = 10; while(n > 0) { n--; for(n=10; n>0; n--) Kizito (Makerere University) CSC 1214 February, / 17

15 Looping Statements Control Flow Statements Loops do-while Syntax Example do while (expression); int n = 10; do { n--; while(n > 0); // more efficiently... do { while(--n > 0); Kizito (Makerere University) CSC 1214 February, / 17

16 Looping Statements Control Flow Statements Loops do-while Syntax Example do while (expression); int n = 10; do { n--; while(n > 0); // more efficiently... do { while(--n > 0); Kizito (Makerere University) CSC 1214 February, / 17

17 Looping Statements Control Flow Statements Loops do-while Syntax Example do while (expression); int n = 10; do { n--; while(n > 0); // more efficiently... do { while(--n > 0); Kizito (Makerere University) CSC 1214 February, / 17

18 Branching Control Flow Statements Break, Continue, and Return int i, sum; int a[] = { 31, -28, 31, 30, -31, 30, 31, 31, 0, 31, 30, 31 ; startloop: // label for (i = 0, sum = 0; i < n; i++) { if (a[i] == 0) // 0 terminates loop break; else if (a[i] == -1) break startloop; // a form of Goto else if (a[i] == -2) return; else if (a[i]<0) /* skip negative elements */ continue; sum += a[i]; // do elements greater than 0 System.out.println("sum: " + sum); // do more if didn t return Kizito (Makerere University) CSC 1214 February, / 17

19 Arrays Arrays An array is a group of like-type variables that are referred to by a common name Arrays can be of any type Arrays may have one or more dimensions A specific element in an array is accessed by its index Kizito (Makerere University) CSC 1214 February, / 17

20 Arrays Arrays One-Dimensional Arrays General form type var-name[]; var-name = new type[size]; Examples 1 2 int month days[]; month days = new int[12]; int month days[] = new int[12]; Indexing the array month days[1] = 28; System.out.println(month days[3]); Kizito (Makerere University) CSC 1214 February, / 17

21 Arrays One-Dimensional Arrays Example (1) class Array { public static void main(string args[]) { int days[] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 ; System.out.println("April has " + days[3] + " days."); Kizito (Makerere University) CSC 1214 February, / 17

22 Arrays Multidimensional Arrays Examples two dimensional int twod1[][] = new int[4][5]; double m[][] = { {0*0, 1*0, 2*0, 3*0, {0*1, 1*1, 2*1, 3*1, {0*2, 1*2, 2*2, 3*2, {0*3, 1*3, 2*3, 3*3 ; char[][] twod2 = new char[3][4]; Kizito (Makerere University) CSC 1214 February, / 17

23 Arrays Two-Dimensional Arrays Example (2) class TwoDArray { public static void main(string args[]) { int i, j, k = 0, twod[][] = new int[4][5]; for(i = 0; i < 4; i++) for (j = 0; j < 5; j++) twod[i][j] = k++; for(i = 0; i < 4; i++) { for(j = 0; j < 5; j++) System.out.print(twoD[i][j] + " "); System.out.println(); Output: Kizito (Makerere University) CSC 1214 February, / 17

24 Arrays Two-Dimensional Arrays Example (2) class TwoDArray { public static void main(string args[]) { int i, j, k = 0, twod[][] = new int[4][5]; for(i = 0; i < 4; i++) for (j = 0; j < 5; j++) twod[i][j] = k++; for(i = 0; i < 4; i++) { for(j = 0; j < 5; j++) System.out.print(twoD[i][j] + " "); System.out.println(); Output: Kizito (Makerere University) CSC 1214 February, / 17

25 Iterators Iterators Collection classes provide an iterator() method that returns an iterator to the start of the collection Collection classes and interfaces include: - Classes AbstractCollection AbstractList AbstractSequentialList LinkedList ArrayList AbstractSet HashSet TreeSet Interfaces Collection List ListIterator Iterator Set SortedSet Text: java.util Part 1: The Collections Framework. Chapter 15 of The Complete Reference Java TM 2, by Patrick Naughton & Herbert Schildt, Fifth Edition We await discussion on classes, and later on, interfaces... Kizito (Makerere University) CSC 1214 February, / 17

26 Iterators Iterators Example (3) import java.util.*; class IteratorDemo { public static void main(string args[]) { // create an array list ArrayList al = new ArrayList(); al.add("c"); // add elements to the array list al.add("a"); al.add("e"); al.add("b"); al.add("d"); al.add("f"); // use iterator to display contents of al System.out.print("Original contents of al: "); Iterator itr = al.iterator(); while(itr.hasnext()) { Object element = itr.next(); System.out.print(element + " "); System.out.println(); // modify objects being iterated ListIterator litr = al.listiterator(); while(litr.hasnext()) { Object element = litr.next(); litr.set(element + "+"); System.out.print("Modified contents of al: "); itr = al.iterator(); while(itr.hasnext()) { Object element = itr.next(); System.out.print(element + " "); System.out.println(); // now, display the list backwards System.out.print("Modified list backwards: "); while(litr.hasprevious()) { Object element = litr.previous(); System.out.print(element + " "); System.out.println(); The output of this program is shown below: Original contents of al: C A E B D F Modified contents of al: C+ A+ E+ B+ D+ F+ Modified list backwards: F+ D+ B+ E+ A+ C+ Kizito (Makerere University) CSC 1214 February, / 17