Java Programming: Loops

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22001144 -- 22001155

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Motivations
Suppose that you need to print a string (e.g.,
"Welcome to Java!") a hundred times. It would be
tedious to have to write the following statement a
hundred times:
System.out.println("Welcome to Java!");
So, how do you solve this problem?

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Opening Problem
…
…
…
Problem:
100
times

4
Introducing while Loops
int count = 0;
while (count < 100) {
System.out.println("Welcome to Java");
count++;
}

5
Objectives
To write programs for executing statements repeatedly using a while loop.
To develop a program for Guess Number and Subtraction Quiz Loop.
To follow the loop design strategy to develop loops.
To develop a program for Subtraction Quiz Loop.
To control a loop with a sentinel value.
To obtain large input from a file using input redirection rather than typing from
the keyboard .
To write loops using do-while statements.
To write loops using for statements.
To discover the similarities and differences of three types of loop statements.
To write nested loops.
To learn the techniques for minimizing numerical errors.
To learn loops from a variety of examples (GCD, FutureTuition,
MonteCarloSimulation).
To implement program control with break and continue.
(GUI) To control a loop with a confirmation dialog .

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while Loop Flow Chart
Initial action ( start the loop)
while (loop-continuation-condition) {
// loop-body; ( End of the loop)
Statement(s);
iteration (increment or decrement)
}
int count = 0;
while (count 100) {
System.out.println(Welcome to Java!);
count++;
}
Loop
Continuation
Condition?
true
Statement(s)
(loop body)
false
count = 0;
(count 100)?
true
false
count++;
(A) (B)

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Trace while Loop
int count = 0;
while (count 2) {
count++;
}
Initialize count

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Trace while Loop, cont.
int count = 0;
while (count 2) {
count++;
}
(count 2) is true

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int count = 0;
while (count 2) {
count++;
}
Print Welcome to Java

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int count = 0;
while (count 2) {
count++;
}
Increase count by 1
count is 1 now

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int count = 0;
while (count 2) {
count++;
}
(count 2) is still true since
count is 1

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int count = 0;
while (count 2) {
count++;
}

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int count = 0;
while (count 2) {
count++;
}
Increase count by 1
count is 2 now

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int count = 0;
while (count 2) {
count++;
}
(count 2) is false since count is
2 now

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Trace while Loop
int count = 0;
while (count 2) {
count++;
}
The loop exits. Execute the next
statement after the loop.

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Caution
Don’t use floating-point values for equality checking in a loop control.
Since floating-point values are approximations for some values, using
them could result in imprecise counter values and inaccurate results.
Consider the following code for computing 1 + 0.9 + 0.8 + ... + 0.1:
double item = 1; double sum = 0;
while (item != 0) { // No guarantee item will be 0
sum += item;
item -= 0.1;
}
System.out.println(sum);
Variable item starts with 1 and is reduced by 0.1 every time the loop body
is executed. The loop should terminate when item becomes 0. However,
there is no guarantee that item will be exactly 0, because the floating-point
arithmetic is approximated. This loop seems OK on the surface, but
it is actually an infinite loop.

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do-while Loop
do {
// Loop body;
Statement(s);
Statement(s)
(loop body)
Loop
Continuation
Condition?
true
false
} while (loop-continuation-condition);

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for Loops
for (initial-action; loop-continuation-
condition;
action-after-each-iteration) {
// loop body;
Statement(s);
}
int i;
for (i = 0; i 100; i++) {
System.out.println(
Welcome to Java!);
}
Initial-Action
Loop
Continuation
Condition?
true
Statement(s)
(loop body)
false
Action-After-Each-Iteration
(A)
i = 0
(i 100)?
true
System.out.println(
Welcome to Java);
false
i++
(B)

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Trace for Loop
int i;
for (i = 0; i 2; i++) {
System.out.println(
Welcome to Java!);
}
Declare i

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Trace for Loop, cont.
int i;
for (i = 0; i 2; i++) {
System.out.println(
Welcome to Java!);
}
Execute initializer
i is now 0

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int i;
for (i = 0; i 2; i++) {
System.out.println( Welcome to Java!);
}
(i 2) is true
since i is 0

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int i;
for (i = 0; i 2; i++) {
}

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int i;
for (i = 0; i 2; i++) {
}
Execute adjustment statement
i now is 1

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int i;
for (i = 0; i 2; i++) {
}
(i 2) is still true
since i is 1

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int i;
for (i = 0; i 2; i++) {
}

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int i;
for (i = 0; i 2; i++) {
}
Execute adjustment statement
i now is 2

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int i;
for (i = 0; i 2; i++) {
}
(i 2) is false
since i is 2

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int i;
for (i = 0; i 2; i++) {
}
Exit the loop. Execute the next
statement after the loop

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Note
The initial-action in a for loop can be a list of zero or more
comma-separated expressions. The action-after-each-iteration
in a for loop can be a list of zero or more comma-separated
statements. Therefore, the following two for
loops are correct. They are rarely used in practice,
however.
for (int i = 1; i 100; System.out.println(i++));
for (int i = 0, j = 0; (i + j 10); i++, j++) {
// Do something
}

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Note
If the loop-continuation-condition in a for loop is omitted,
it is implicitly true. Thus the statement given below in (a),
which is an iinnffiinniittee lloooopp, is correct. Nevertheless, it is
better to use the equivalent loop in (b) to avoid confusion:
for ( ; ; ) {
// Do something
}
(a)
Equivalent while (true) {
// Do something
}
(b)

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Caution
Adding a semicolon at the end of the for clause before
the loop body is a common mistake, as shown below:
Logic
Error
for (int i=0; i10; i++);
{
System.out.println(i is + i);
}

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Caution, cont.
Similarly, the following loop is also wrong:
int i=0;
while (i 10);
Logic Error
{
i++;
}
In the case of the do loop, the following semicolon is
needed to end the loop.
int i=0;
do {
i++;
} while (i10);
Correct

for ( ; loop-continuation-condition; ) // Loop body
}
initial-action;
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Which Loop to Use?
The three forms of loop statements, while, do-while, and for, are
expressively equivalent; that is, you can write a loop in any of these
three forms. For example, a while loop in (a) in the following figure
can always be converted into the following for loop in (b):
A for loop in (a) in the following figure can generally be converted into the
following while loop in (b) except in certain special cases
for (initial-action;
loop-continuation-condition;
action-after-each-iteration) {
// Loop body;
}
(a)
Equivalent
// Loop body;
action-after-each-iteration;
(b)
}
// Loop body
}
(a)
Equivalent
(b)

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Recommendations
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.

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Nested Loops
Problem: Write a program that uses nested for
loops to print a multiplication table.
Note: look practical part

Problem: Predicating the Future Tuition
Problem: Suppose that the tuition for a university is $10,000 this year
and tuition increases 7% every year. In how many years will the
tuition be doubled?
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Note: look practical part

Problem: Predicating the Future Tuition
double tuition = 10000; int year = 1 // Year 1
tuition = tuition * 1.07; year++; // Year 2
...
37

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Using break and continue
Examples for using the break and continue
keywords:

Problem: Displaying Prime Numbers
Problem: Write a program that displays the first 50 prime numbers in
five lines, each of which contains 10 numbers. An integer greater
than 1 is prime if its only positive divisor is 1 or itself. For example,
2, 3, 5, and 7 are prime numbers, but 4, 6, 8, and 9 are not.
Solution: The problem can be broken into the following tasks:
•For number = 2, 3, 4, 5, 6, ..., test whether the number is prime.
•Determine whether a given number is prime.
•Count the prime numbers.
•Print each prime number, and print 10 numbers per line.
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(GUI) Controlling a Loop with a
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Confirmation Dialog
A sentinel-controlled loop can be implemented using a confirmation
dialog. The answers Yes or No to continue or terminate the loop. The
template of the loop may look as follows:
int option = 0;
while (option == JOptionPane.YES_OPTION) {
System.out.println(continue loop);
option = JOptionPane.showConfirmDialog(null, Continue?);
}

Java Programming: Loops

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