The break and continue statements

Introduction There are 2 special statements that can affect the execution of loop statements (such as a while-statement) The special statements are: We will study their meaning and how to use these special statements inside the while-statement break continue

The break statement Syntax: Effect: break ; When the break statement is executed inside a loop- statement, the loop-statement is terminated immediately The execution of the program will continue with the statement following the loop-statement

The break statement (cont.) Schematically:

Programming example using the break statement: find the GCD Problem description: Write a Java program that reads in 2 numbers x and y... and prints the largest common divisor of both x and y

Programming example using the break statement: find the GCD (cont.) A concrete example: Input: x = 24 and y = 16 Output: 8

Programming example using the break statement: find the GCD (cont.) What would you do to solve this problem ? Suppose: x = 24 and y = 16 The lesser of the values is 16 Therefore, all divisors are ≤ 16

Programming example using the break statement: find the GCD (cont.) Check if 16 and 24 are divisible by 16: no Check if 16 and 24 are divisible by 15: no... Check if 16 and 24 are divisible by 10: no Check if 16 and 24 are divisible by 9: no Check if 16 and 24 are divisible by 8: YES Print 8 and STOP

Programming example using the break statement: find the GCD (cont.) Rough algorithm: input x, y; min = min(x, y); // this is the range of the brute force search for every value a = {min, min-1, min-2,..., 1} do { if (x and y are divisible by a) { print a; exit the while loop !!! }

Programming example using the break statement: find the GCD (cont.) Algorithm (structured diagram):

Programming example using the break statement: find the GCD (cont.) Java program: import java.util.Scanner; public class GCD01 { public static void main(String[] args) { Scanner in = new Scanner(System.in); int x, y, a, min = 0; x = in.nextInt(); // Read in number y = in.nextInt(); // Read in number if ( x < y ) min = x; else min = y;

Programming example using the break statement: find the GCD (cont.) a = min; while ( a >= 1 ) // Run a = min(x,y), min(x,y)-1,..., 1 { if ( x % a == 0 && y % a == 0 ) { // a is a divisor of x and y System.out.println(a); // Print a (because it's a common divisor) break; // Exit while loop !!! (Only need the largest) } else { a--; // Move to the next number !! }

Programming example using the break statement: find the GCD (cont.) Example Program: (Demo above code) –Prog file: GCD01.java How to run the program: Right click on link and save in a scratch directory To compile: javac GCD01.java To run: java GCD01

The continue statement Syntax: continue;

The continue statement (cont.) Effect: When the continue statement is executed inside a loop- statement, the program will skip over the remainder of the loop-body to the end of the loop Note: What happens next when the program reaches the end of a loop depends on the type of loop statement !!!

The continue statement (cont.) Effect of a continue statement in a while-loop: As given previously: In the case of a while-loop, when the program reaches end of the loop, the program will jump back to the testing of the loop-continuation-condition the program will skip over the remainder of the loop-body to the end of the loop

The continue statement (cont.) Schematically:

Programming example using the continue statement: find all divisors of a number Problem description: Write a Java program that reads in an integer n... and prints all its divisors

Programming example using the continue statement: find all divisors of a number (cont.) Previously discussed solution: We try every number a = 1, 2,..., n For each number a, we check if n % a == 0.

Programming example using the continue statement: find all divisors of a number (cont.) We can re-write the same algorithm differently using a continue statement as follows:

Programming example using the continue statement: find all divisors of a number (cont.) Notice that the if-condition has been changed to x % a != 0, meaning: a is not a divisor of x When a is not a divisor of x, (the then-part), we increment a (to try next number) and jump to the end of the while- loop using the continue statement. When x % a != 0 is false, the program will print a and increment a (to try next number)

Programming example using the continue statement: find all divisors of a number (cont.) Java program: public class Continue01 { public static void main(String[] args) { Scanner in = new Scanner(System.in); int n, a; n = in.nextInt(); // Read in number a = 1;

Programming example using the continue statement: find all divisors of a number (cont.) while ( a <= n ) // Run a = 1, 2,..., n { if ( n % a != 0 ) { // a is NOT a divisor of n a++; continue; // Jump to end of while loop } /* We reach here ONLY when "n % a != 0" is FALSE I.e.: a is a divisor of x */ System.out.println(a); // Print a (because it's a divisor) a++; // Make sure we more to the next number !! // or else: infinite loop !!! }

Programming example using the continue statement: find all divisors of a number (cont.) Example Program: (Demo above code) –Prog file: Continue01.java How to run the program: Right click on link and save in a scratch directory To compile: javac Continue01.java To run: java Continue01

Programming advice Good programming practice: A computer program will be easier to understand if it is transparent. One way to improve transparency is a consistent flow of control Meaning, the program always take the same path of execution The break and the continue commands will alter the flow of control Therefore, they make a computer program less transparent It is a general recommendation to avoid using break and continue statements when you can write the algorithm easily without them.