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Java iteration statements
● Iteration statements are statements which appear in the
source code only once , but it execute many times.
● Such kind of statements are called loops.
● Almost all the programming languages support looping
instructions.
a = b 5 ; if( a>=c 5 ) while( a>= 5 ) b = a / 10 ; a = a + 4 ; { c = a + b); else b = 9 + a; a = a – 7 ; a --; } // while
Iteration statements - Loops
Java has three kinds of iteration statements ( לולאות/ הוראות חזרה ) :
WHILE loop
FOR loop
DO... WHILE loop
Iteration (repetition) statements causes Java to execute one or more statements as long as a condition exist. Each repetition statement request :
- a control variable /loop counter ( משתנה הלולאה/ מונה הלולאה )
- the initial value of control variable
- the increment ( decrement ) by which the control variable is modified each time through the loop
- the loop continuation condition that determines if looping should continue.
Java Loop structures
Do-while structure While structure
While loop statement
while (loop-continuation-condition) { // loop-body; Statement(s); } int count = 0 ; while (count < 100 ) { System.out.println("Welcome to Java!"); count++; } // while Loop Continuation Condition? true Statement(s) (loop body) false (count < 100)? true System.out.println("Welcome to Java!"); count++; false (A) (B) count = 0;
While loop example 1
- Input :
- Two integers – num 1 and num 2
- Output :
- How many times num 1 contains num 2 This is the result of the integer division num 1 /num 2
- Note :
- Do not use the division operator ( / )!
Solution
public class Ex 1 While
static Scanner reader = new Scanner(System.in);
public static void main(String[ ] args)
int res= 0 ; // help variable
System.out.println(“enter two integers : “);
int num 1 =reader.nextInt();
int num 2 =reader.nextInt();
while ( (res+ 1 ) * num 2 <= num 1 )
res ++;
System.out.println(“num 1 contains num 2 : “+res+” times”);
} // main
} // class
Every iteration res is incremented by 1 Declaration during input
Infinite loops
● It is the programmer’s responsibility to ensure that the
condition of a loop will eventually become false.
If it doesn’t, the loop body will execute forever.
● This situation, called an infinite loop ( לולאה אין סופית ).
Example 1 : Example 2 : int count= 1 ; double num=2.0; while (count != 50 ) while ( num !=0.0) count+ = 2 ; num = num - 0.1; This loop will never terminate because count will never equal 50. This loop will never terminate because num will never have a value exactly equal to 0.
FOR loop statement
- Equivalent to while… Any for loop can be converted to while loop and vice versa.
- If we want to perform something for a predefined number of times, better use for.
- If we just wait for something to happen (not after a certain number or iterations), better use while.
- The for loop has three expressions that are contained within parentheses and separated with a semicolon.
FOR loop – order of execution
initialization conditional iteration For loop body false true Three kinds of for loop header expressions ● Before the loop begins the first part of the header, called initialization , is executed. ● The second part of the header is the boolean condition , which is evaluated before the loop body. If true ,the body is executed. ● The iteration part is executed after each iteration of the loop. Initialization is executed only once
FOR loops example 1
This program section reads an integer and computes its
factorial ( n!).
int fact= 1 ; // factorial System.out_._ println("enter an integer : "); int n=reader.nextInt(); // input variable for (int i= 1 ; i<=n ;i++) fact *= i; System.out.println( “ factorial is :" + fact); n i i<=n fact output 3 1 3 1 T 1 3 2 T 2 3 3 T 6 3 4 F 6 Before loop execution Trace table ( טבלת מעקב ) A trace table is a technique used to test programs.
input: 3
FOR loops example 2
int prime = 1 ; / / help variable System.out_._ println( “ enter an integer : “ ); int x = reader.nextInt(); / / input variable if (x> 3 ) { for( int i = 2 ; i <x && prime == 1 ; i++ ) if( x%i == 0 ) prime = 0 ; switch (prime) { case 1 : System.out.println( x +" is prime number “ ); break; case 0 : System.out.println (x +" is not prime number “ ); break; } // switch } / / if (x> 3 ) else System.out.println (x +" is prime number "); This program section checks if the input integer is the prime number
Do …while loop statement
do { statement(s) } while (expression) ;
- Similar to while loops
- Except the condition is evaluated after the loop body.
The condition is written at the end of the loop to indicate
that it is not evaluated until the loop body is executed.
- The loop body is always executed at least once , even if
the expression is never true.
Note that the loop begins with the reserved word do Note that the loop ends with semicolon
Do …while loop example 1
// program statement before the do…while loop
System.out.print( "Please, enter a positive number: “ );
do
int num = reader.nextInt();
if (num <= 0 )
System.out.println( “Input error! Try again “ );
} while ( num <= 0 );
// program statements after the do…while loop
Waiting for legal input
Do …while loop example 2
int reversNum = 0 ; // reversed number
System.out.println("enter an integer : ");
int num=reader.nextInt( ); // input variable
do
int lastDigit = num % 10 ;
reversNum = (reversNum* 10 )+ lastDigit;
num = num / 10 ;
} while (num > 0 );
System.out.println(" That number reversed is "+reversNum);
This program section reads an integer and reverses its digit mathematically.
