2. Objective
On completion of this lecture you would be able to
know
• The various iteration statements in Java
2
3. Recap
In the previous class we have discussed
• Various selection statements available in Java
3
4. Iterative statements in Java
• Java language provides three loop or iterative
statements
• They are
• while statement
• do statement
• for statement
4
5. The while Statement
int sum = 0, number = 1;
while ( number <= 100 ) {
sum = sum + number;
These statements are
These statements are
executed as long as number
executed as long as number
is less than or equal to 100.
is less than or equal to 100.
number = number + 1;
}
5
6. Syntax for the while Statement
while ( <boolean expression> ) {
<statement>
}
Boolean Expression
Boolean Expression
while ( number <= 100 ){
Statement
Statement sum = sum + number;
(loop body)
(loop body) number = number + 1;
}
6
7. Control Flow of while
int sum = 0, number = 11
int sum = 0, number =
true
number <= 100 ??
number <= 100
false
sum = sum + number;
sum = sum + number;
number = number + 1;
number = number + 1;
7
8. while Loop Pitfall - 1
1
1 int product = 0;
while ( product < 500000 ) {
product = product * 5;
} Infinite Loops
Infinite Loops
Both loops will not
Both loops will not
terminate because the
terminate because the
boolean expressions will
boolean expressions will
2
2 int count = 1; never become false.
never become false.
while ( count != 10 ) {
count = count + 2;
}
8
9. while Loop Pitfall - 2
1
1 float count = 0.0f;
while ( count != 1.0f ) {
count = count + 0.3333333f;
} Using Real Numbers
Using Real Numbers
Loop 22terminates, but Loop 11
Loop terminates, but Loop
does not because only an
does not because only an
approximation of aareal
approximation of real
number can be stored in aa
2
2 float count = 0.0f; number can be stored in
computer memory.
computer memory.
while ( count <= 1.0f ) {
count = count + 0.3333333f;
}
9
10. while Loop Pitfall - 3
• Goal: Execute the loop body 10 times.
1 count = 1;
1 2 count = 1;
2
while (count < 10) { while (count <= 10) {
... ...
count++; count++;
} }
3 count = 0;
3 4 count = 0;
4
while (count <= 10) { while (count < 10) {
... ...
count++; count++;
} }
1 and 3 exhibit off-by-one error.
1 3
10
11. The do-while Statement
int sum = 0, number = 1;
do {
These statements are
These statements are
sum += number; executed as long as sum is
executed as long as sum is
less than or equal to
less than or equal to
number++; 1,000,000.
1,000,000.
} while ( sum <= 1000000 );
11
12. Syntax for the do-while Statement
do {
<statement>
} while (<boolean expression>);
do {
sum += number; Statement
Statement
number++; (loop body)
(loop body)
} while (sum <= 1000000);
Boolean Expression
Boolean Expression
12
13. Control Flow of do-while
int sum = 0, number = 11
int sum = 0, number =
sum += number;
sum += number;
number++;
number++;
true
sum <= 1000000 ??
sum <= 1000000
false
13
14. Pre-test vs. Post-test loops
• Use a pre-test loop for something that may be
done zero times
• Use a post-test for something that is always done
at least once
14
15. Checklist for Repetition Control
1. Watch out for the off-by-one error (OBOE).
2. Make sure the loop body contains a statement that
will eventually cause the loop to terminate.
3. Make sure the loop repeats exactly the correct
number of times.
15
16. Syntax of the for Statement
for ( <initialization>; <boolean expression>; <update> )
<statement>
Boolean
Boolean
Initialization
Initialization Update
Update
Expression
Expression
for ( i=0 ; i < 20 ; i++ ){
number = inputBox.getInteger(); Statement
Statement
sum += number; (loop body)
(loop body)
}
16
17. Control Flow of for
ii= 0;
= 0;
ii< 20 ??
< 20
false
true
number = inputBox.getInteger( );
number = inputBox.getInteger( );
sum += number;
sum += number;
ii++;
++;
17
18. The for Statement
int i, sum = 0, number;
for (i = 0; i < 20; i++) {
number = inputBox.getInteger();
sum += number;
} These statements are
These statements are
executed for 20 times
executed for 20 times
((i i= 0, 1, 2, … , ,19).
= 0, 1, 2, … 19).
18
19. Indefinite vs. Definite loops
• For loops and while loops are exchangeable
• But use a for loop when the number of iterations
are definite
• Use a while or do-while when the number of
iterations depends on statements in the loop body
19
20. Summary
In this class we have discussed
• Various iterative statements available in Java
20
21. Quiz
1.Which of the following loop is deterministic
a) while
b) do
c) for
d) all of the Above
21
22. Quiz
2.If altering statement is missed in a loop, then it
becomes
a) fast loop
b) deterministic loop
c) non deterministic loop
d) infinite loop
22
24. Assignment
• Write a Java program to find factorial of a given
no.
• Write a Java program to find Fibonacci series.
• Write a Java program to find whether the given no
is prime or not.
24
25. Frequently asked questions
• List the various iterative statements available in
Java
• Explain the working of various iterative
statements
• Compare while with do statement
• Compare while, do with for loop
25
26. swings
Struts
jdbc
hibernate
home
java previous question papers
OCT/NOV-2012 QUESTION PAPER
April / May 2012 c-09
October/ November-2011 c-09
April/ May 2011 c-09
April/ May 2011 c-05
26
Editor's Notes
Chapter 7 - There are basically two types of terminating the loop: 1. count-controlled and 2. sentinel-controlled. Count-controlled loops terminate the execution of the loop after the loop body is executed for a fixed number of times. Sentinel-controlled loops terminate the execution of the loop after one of the designated values called a sentinel is encountered.
Chapter 7 -
Chapter 7 -
Chapter 7 - Note: In theory, this while statement is an infinite loop, but in programming languages other than Java, this loop will eventually terminate because of an overflow error. An overflow error will occur if you attempt to assign a value larger than the maximum value the variable can hold. When an overflow error occurs, the execution of the program is terminated in almost all programming languages. With Java, however, an overflow will not cause the program termination. When an overflow occurs in Java, a value that represents infinity (IEEE 754 infinity, to be precise) is assigned to a variable and no abnormal termination of a program will happen. Also, in Java an overflow occurs only with float and double variables; no overflow will happen with int variables. When you try to assign a value larger than the maximum possible integer an int variable can hold, the value “wraps around” and becomes a negative value. Whether the loop terminates or not because of an overflow error, the logic of the loop is still an infinite loop, and we must watch out for it. When you write a loop, you must make sure that the boolean expression of the loop will eventually become false.
Chapter 7 - Although 1/3 + 1/3 + 1/3 == 1 is mathematically true, the expression 1.0/3.0 + 1.0/3.0 + 1.0/3.0 in computer language may or may not get evaluated to 1.0 depending on how precise the approximation is. In general, avoid using real numbers as counter variables because of this imprecision.
Chapter 7 - Yes, you can write the desired loop as count = 1; while (count != 10 ) { ... count++; } but this condition for stopping the count-controlled loop is dangerous. We already mentioned about the potential trap of an infinite loop.
Chapter 7 - The following is the routine presented earlier that inputs a person’s age using the do–while statement. do { age = inputBox.getInteger("Your Age (between 0 and 130):"); if (age < 0 || age > 130) { messageBox.show("An invalid age was entered. " + "Please try again."); } } while (age < 0 || age > 130); This code is not as good as the version using the while statement it includes an if statement inside its loop body and the if test is repeating the same boolean expression of the do–while. Since the loop body is executed repeatedly, it is important not to include any extraneous statements. Moreover, duplicating the testing conditions tends to make the loop statement harder to understand. For this example, we can avoid the extra test inside the loop body and implement the control flow a little more clearly by using a while statement. In general, the while statement is more frequently used than the do–while statement.
Chapter 7 -
Chapter 7 -
Chapter 7 -
Chapter 7 -
Chapter 7 - The <initialization> component also can include a declaration of the control variable. We can do something like this: for (int i = 0; i < 10; i++) instead of int i; for (i = 0; i < 10; i++)
