3. The while Statement
Syntax for the while Statement
while ( <boolean expression> )
<statement>
while ( number <= 100 ) {
sum = sum + number;
number = number + 1;
}
Statement
(loop body)
Boolean Expression
4. Control Flow of while
int sum = 0, number = 1
number <= 100 ?
false
sum = sum + number;
number = number + 1;
true
5. int age;
Scanner scanner = new Scanner(System.in);
System.out.print("Your Age (between 0 and 130): ");
age = scanner.nextInt( );
while (age < 0 || age > 130) {
System.out.println(
"An invalid age was entered. Please try again.");
System.out.print("Your Age (between 0 and 130): ");
age = scanner.nextInt( );
}
Example: Testing Input Data
Priming Read
For Integer input
6. Caution
Don’t use floating-point values for
equality checking in a loop control. Since
floating-point values are approximations,
using them could result in imprecise
counter values and inaccurate results.
This example uses int value for data. If a
floating-point type value is used for
data, (data != 0) may be true even though
data is 0.
Make sure the loop body contains a statement that will
eventually cause the loop to terminate.
Make sure the loop repeats exactly the correct number of
times.
If you want to execute the loop body N times, then
initialize the counter to 0 and use the test condition
counter < N or initialize the counter to 1 and use the test
7. Loop Pitfall - 1
Infinite Loops
Both loops will not
terminate because the
boolean expressions will
never become false.int count = 1;
while ( count != 10 ) {
count = count + 2;
}
2
int product = 0;
while ( product < 500000 ) {
product = product * 5;
}
1
9. The do-while Statement
do {
sum += number;
number++;
} while ( sum <= 1000000 );
do
<statement>
while ( <boolean expression> ) ;
Statement
(loop body)
Boolean Expression
10. Control Flow of do-while
int sum = 0, number = 1
sum += number;
number++;
sum <= 1000000 ?
true
false
11. The for Statement
for ( i = 0 ; i < 20 ; i++ ) {
number = scanner.nextInt();
sum += number;
}
for ( <initialization>; <boolean expression>; <increment> )
<statement>
Initialization
Boolean
Expression
Increment
Statement
(loop body)
12. The for Statement
int i, sum = 0, number;
for (i = 0; i < 20; i++) {
number = scanner.nextInt( );
sum += number;
}
These statements are
executed for 20 times
( i = 0, 1, 2, … , 19).
13. Control Flow of for
i = 0;
false
number = . . . ;
sum += number;
true
i ++;
i < 20 ?
14. More for Loop Examples
for (int i = 0; i < 100; i += 5)1
i = 0, 5, 10, … , 95
for (int j = 2; j < 40; j *= 2)2
j = 2, 4, 8, 16, 32
for (int k = 100; k > 0; k--) )3
k = 100, 99, 98, 97, ..., 1
15. Which Loop to Use?
The three forms of loop statements, while, do, and for, are
expressively equivalent; that is, you can write a loop in any
of these three forms.
I recommend that you use the one that is most intuitive
and comfortable for you. In general, a for loop may be used
if the number of repetitions is known, as, for example,
when you need to print a message 100 times. A while loop
may be used if the number of repetitions is not known, as
in the case of reading the numbers until the input is 0. A
do-while loop can be used to replace a while loop if the
loop body has to be executed before testing the
continuation condition.
16. Caution
Adding a semicolon at the end of the for clause
before the loop body is a common mistake, as shown
below:
for (int i=0; i<10; i++);
{
System.out.println("i is " + i);
}
Similarly, the following loop is also wrong:
int i=0;
while (i<10);
{
System.out.println("i is " + i);
i++;
}
In the case of the do loop, the following semicolon is
needed to end the loop.
int i=0;
do {
System.out.println("i is " + i);
i++;
} while (i<10);
Wrong
Correct
17. Loop-and-a-Half Repetition Control
• Loop-and-a-half repetition control can be
used to test a loop’s terminating condition in
the middle of the loop body.
• It is implemented by using reserved words
while, if, and break.
18. Example: Loop-and-a-Half Control
String name;
Scanner scanner = new Scanner(System.in);
while (true){
System.out.print("Your name“);
name = scanner.next( );
if (name.length() > 0) break;
System.out.println("Invalid Entry." +
"You must enter at least one character.");
}
19. Example: Loop-and-a-Half Control
String name;
Scanner scanner = new Scanner(System.in);
while (true){
System.out.print("Your name“);
name = scanner.next( );
if (name.length() > 0) break;
System.out.println("Invalid Entry." +
"You must enter at least one character.");
}
24. Introducing Methods, cont.
•parameter profile refers to the
type, order, and number of the
parameters of a method.
•method signature is the
combination of the method name and
the parameter profiles.
•The parameters defined in the
method header are known as formal
parameters.
•When a method is invoked, its
26. Calling Methods, cont.
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
pass i
pass j
27. Calling Methods, cont.
The main method
i:
j:
k:
The max method
num1:
num2:
result:
pass 5
5
2
5
5
2
5
pass 2
parameters
