The break and continue statements

1
The break and continue statements
2
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

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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

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The break statement (cont.)

• Schematically

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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

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Programming example using the break statement
find the GCD (cont.)

• A concrete example

• Input x 24 and y 16       
• Output 8

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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

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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

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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 !!!

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Programming example using the break statement
find the GCD (cont.)

• Algorithm (structured diagram)

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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 lt y ) min x else min
y
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Programming example using the break statement
find the GCD (cont.)

• a min
• while ( a gt 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 !!

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Programming example using the break statement
find the GCD (cont.)

• Example Program (Demo above code)
• Prog file http//mathcs.emory.edu/cheung/Courses
  /170/Syllabus/07/Progs/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

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The continue statement

• Syntax

continue
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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 !!!

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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-condi
  tion

• the program will skip over the remainder of the
  loop-body to the end of the loop

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The continue statement (cont.)

• Schematically

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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

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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.

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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

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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)

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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
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Programming example using the continue statement
find all divisors of a number (cont.)

• while ( a lt 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 !!!

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Programming example using the continue statement
find all divisors of a number (cont.)

• Example Program (Demo above code)
• Prog file http//mathcs.emory.edu/cheung/Courses
  /170/Syllabus/07/Progs/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

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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.