Control Statements

1. Control Statements Loops

2. Types of Control Structures

3. Why is Repetition Needed? • There are many situations in which the same statements need to be executed several times. • Example: • Formulas used to find average grades for students in a class. • Loop Group of instructions that a computer executes repeatedly while some condition remains true

4. Repetition • Java has three repetition, or looping, structures that let you repeat statements over and over again until certain conditions are met: • while • for • do…while

5. counter-controlled loop • The loop shown below in pseudo code is called a counter-controlled while loop because its repetition is managed by a loop control variable whose value represents a count. Set loop control variable to an initial value While loop control variable < final value ... //Do something multiple times Increase loop control variable by 1 (or step size).

6. Syntax: while (expression) statement Statements must change value of expression to false. A loop that continues to execute endlessly is called an infinite loop (expression is always true). The while Looping (Repetition) Structure

7. Example 1This slide shows a program fragment that computes and displays the gross pay for seven employees. The loop body is the compound statements (those between { and }) count_emp = 1; while (count_emp <= 7) count_emp = 0; while (count_emp < 7) { System.out.print("Hours:“); hours = input.nextInt(); System.out.print("Rate:“); rate = input.nextInt(); pay = hours * rate; System.out.print(“Pay is:”+ pay); count_emp = count_emp + 1; //or count_emp++; } System.out.print("\n All employees processed\n“);

8. Syntax of the while Statement: • Initialization. i.e. count_emp = 0; • Testing(condition). i.e. count_emp < 7 • Updating i.e. count_emp = count_emp + 1; • The above steps must be followed for every while loop. • If updating is skipped, it produce an infinite loop

9. Example 2: Computing Sum • If we want to compute , we need to do 1+2+3+...+100 int sum =0, i = 1; while (i <= 100) { sum = sum + i; //or sum+=i; i = i + 1; //or i++; ++i; i+=1; } System.out.print(“Sum is :“+sum );

10. The for Looping (Repetition) Structure • Specialized form of while loop. • Its primary purpose is to simplify the writing of counter-controlled loops. For this reason, the forloop is typically called a counted or indexed for loop. . • Syntax: for (initial statement; loop condition; update statement) statement Rewrite the previous examples using for loop

11. The for Looping (Repetition) Structure • The following for loop outputs the word Hello and a star (on separate lines) five times: for (i = 1; i <= 5; i++) { System.out.println("Hello"); System.out.println("*"); } 2. The following for loop outputs the word Hello five times and the star only once: for (i = 1; i <= 5; i++) System.out.println("Hello"); System.out.println("*");

12. The for Looping (Repetition) Structure • Does not execute if loop condition is initially false. • Update expression changes value of loop control variable, eventually making it false. • If loop condition is always true, result is an infinite loop. • Infinite loop can be specified by omitting all three control statements.

13. Conditional Loops • In many programming situations, we will not be able to determine the exact number of loop repetitions before loop execution begins. • Sentinel-Controlled loops • Flag-Controlled loop

14. Sentinel-Controlled while Loop • Used when exact number of entry pieces is unknown, but last entry (special/sentinel value) is known. • General form: Input the first data item into variable; while (variable != sentinel) { . . input a data item into variable; . . }

15. Example :Program to compute the product of entered numbers import java.util.*;publicclass SentinelControlledWhileLoop{static Scanner console = new Scanner(System.in);staticfinalint SENTINEL = -999;publicstaticvoid main (String[] args){int number, prod= 0; SSystem.out.println("Enter positive integers ending with " + SENTINEL); number = console.nextInt();// Initialization while (number != SENTINEL) //testing { prod= prod * number; System.out.println("Enter positive integers ending with " + SENTINEL); number = console.nextInt(); //updating } System.out.println(“the product is “ + prod);}} Read input before loop

16. Flag-Controlled while Loop • Boolean value used to control loop. • General form: boolean found = false; while (!found) { . . if (expression) found = true; . . . }

17. bool found = false; int num =0;   while(!found) {         num++;         if (num == 3) { found = true; System.out.println("value of flag is:“ +found ); }          } }

18. The do…while Loop (Repetition) Structure • Syntax: do statement while (expression); • Statements are executed first and then expression is evaluated. • Statements are executed at least once and then continued if expression is true.

19. do…while Loop (Post-Test Loop)

20. do…while Loop (Post-Test Loop) Example : i = 0 ; do { System.out.print(i + “ “ ) ; i = i + 5 ; }while ( i <= 30 ) ; output : 0 5 10 15 20 25 30

21. break Statements • Used to • exit early from a loop. (while, for, and do...while) • skip remainder of switch structure. • Can be placed within if statement of a loop. • If condition is met, loop is exited immediately. • After the break statement executes, the program continues to execute with the first statement after the structure

22. break Statements Example : int count ; for ( count = 1 ; count <= 10 ; count ++ ) { if ( count == 5) break ; System.out.print(count + “ ” ); } Output 1 2 3 4

23. continue Statements • Used in while, for, and do...while structures. • When executed in a loop, the remaining statements in the loop are skipped; proceeds with the next iteration of the loop. • When executed in a while/do…while structure, expression is evaluated immediately after continue statement. • In a for structure, the update statement is executed after the continue statement; the loop condition then executes.

24. continue Statements Example : int count ; for ( count = 1; count <= 10 ; count ++ ) { if ( count == 5) continue; System.out.print(count + “ ” )}; } Output 1 2 3 4 6 7 8 9 10

25. Nested Control Structures • Provides new power, subtlety, and complexity. • if, if…else, and switch structures can be placed within while loops. • for loops can be found within other for loops. • Nested loops consist of an outer loop with one or more inner loops. • Each time the outer loop is repeated, the inner loops are reentered • Their loop control expressions are reevaluated • All required iterations are performed again

26. Example: Sum of Scores of 3 sectionscalculate the total scores in each section for 3 sections. • Each section's scores are terminated by the sentinel -99.

27. int sec, score, sum; System.out.print ("sum of scores for each section\n“); for (sec= 1 ; sec <= 3 ; ++sec) { sum = 0; for (score=input.nextInt(); score != -99; score=input.nextInt()) {sum += score; } System.out.print("Section: " +sec+ " sum of scores :” +sum); }

28. Nested Control Structures for (inti = 1; i <= 5; i++) { for (int j = 1; j <= i; j++) System.out.print("*"); System.out.println(); } Output: * ** *** **** *****

29. Self read