Download Java Programming, Second edition

IT259 Foundation of
Programming Using Java
Unit 7 Seminar :
(Chapter 6 )
Instructor : Vladimir Gubanov, PhD
Email : [email protected]
A reminder:
1.
2.
3.
Our Seminars :
they will be each Thursday , from 9
PM to 10 PM EST
My Office Hours :
Mondays, 7PM to 8PM
Saturdays, 9AM to 10 AM
My email :
[email protected]
Java Programming,
Fifth Edition
Chapter Six:
Looping
Objectives
•
Use a while loop
•
Use shortcut arithmetic operators
Use a for loop
How and when to use a do...while loop
•
•
•
•
Nested loops
Improving loop performance
Java Programming, Fifth
Edition
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Repetition Structure
(the last control structure in Java,
what are the other two ?)
1- 5
© 2001 by The McGraw-Hill Companies, Inc. All rights reserved.
Java Loop Structures
•
•
•
Repeating a series of instructions
Each repetition is called an iteration
Three types of loops
– while
• Loop-controlling Boolean expression is the first
statement
– for
• A concise format in which to execute loops
– do...while
• Loop-controlling Boolean expression is the last
statement
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Learning About the Loop Structure
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while Loop
•
while loop
– Executes body of statements continually
• As long as Boolean expression that controls
entry into loop continues to be true
– Consists of keyword while
• Followed by Boolean expression within
parentheses
• Followed by body of loop; can be single
statement or block of statements surrounded
by curly braces
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Using a while Loop To Create
a Definite Loop
What will happen if the second line in the
loop body will be removed ?
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Using a while Loop To Create a Definite
Loop
•
•
•
•
Definite loop
– Performs task a predetermined number of times
Write a definite loop
– Initialize loop control variable
• Variable whose value determines whether loop
execution continues
– While loop control variable does not pass limiting value
• Program continues to execute body of while loop
Write body of loop
• Must include statement that alters loop control
variable
Infinite loop ? When will10it happen ?
Java Programming, Fifth
Edition
Using a while Loop
NNNN
How many iterations will be executed ?
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Using a while Loop To Create
a Definite Loop
•
Prevent while loop from executing infinitely
– Named loop control variable initialized to
starting value
– Loop control variable tested in while statement
– If test expression true
• Body of while statement takes action
– Alters value of loop control variable
• Test of while statement must eventually
evaluate to false
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Using a while Loop To Create
a Definite Loop
Is there any error here ?
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NNN
Is there any error here ?
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Using a while Loop To Create
a Definite Loop
•
•
•
Increment variable
– Alter value of loop control variable by adding 1
(or any other reasonable value)
Decrement variable
– Subtract 1 ( or any other reasonable value)
from loop control variable
Clearest and best method
– Start loop control variable at 0 or 1
– Stop when loop control variable reaches limit
– Increment by 1 each time through loop
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Using a while Loop To Create
a Definite Loop
How many times will this loop execute ? What will be
the value of loopCount when the loop breaks ?
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How may iterations will
this loop perform ?
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Java Programming, Fifth
Edition
Using Shortcut Arithmetic Operators
•
•
Accumulating
– Repeatedly increasing value by some
amount
Java provides shortcuts for incrementing
and accumulating:
+= add and assign
-= subtract and assign
*= multiply and assign
/= divide and assign
%= remainder and assign
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Using Shortcut Arithmetic Operators
•
Prefix and postfix increment operators
++someValue, someValue++
– Use only with variables
– Unary operators
• Use with one value
– Increase variable’s value by 1
• No difference between operators (unless
other operations in same expression)
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Using Shortcut Arithmetic Operators
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Using Shortcut Arithmetic Operators
•
•
Prefix and postfix increment operators
– Prefix ++
• Result calculated and stored
• Then variable used
– Postfix ++
• Variable used
• Then result calculated and stored
Prefix and postfix decrement operators
--someValue
someValue-– Similar logic to increment operators
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Using a for Loop
•
for Loop
– Used when definite number of loop
iterations is required
– One convenient statement
• Assign starting value for loop control
variable
• Test condition that controls loop entry
• Alter loop control variable
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for Loop Flow Chart
23
Using a for Loop
What will be the results of processing of each of the
loops above ?
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Using a for Loop
•
Other uses for three sections of for loop
– Initialization of more than one variable
• Place commas between separate statements
– Performance of more than one test using AND or
OR operators
– Decrementation or performance of some other task
– Altering more than one value
– Can leave one or more portions of for loop
empty
• Two semicolons still required as placeholders
• Use same loop control variable in all three parts of
for statement
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Edition
for Loop Example
For loop is executed as
shown in the numbered
steps
System.out.println(I + “\t”);
System.out.println(“”n” + “Finished”);
Why don’t you have curly braces here ? Where to place those
if needed ?
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do…while Statements
•
Posttest : condition
continue looping or not is
tested after the loop body
is executed
•
General form:
do
{
statement;
}
while( conditional
expression);
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How and When to Use a do...while
Loop
•
do...while loop
– Posttest loop
– Checks value of loop control variable
• At bottom of loop
• After one repetition has occurred
– Performs task at least one time
– Never required to use this type of loop
– Use curly brackets to block statement
• Even with single statement
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a do...while Loop
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How and When to Use a
do...while Loop
How many times will this loop execute ?
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Another do…while Example
int counter = 10;
do
// No semicolon on this line
{
System.out.writeln(counter + "\t" + math.pow(counter, 2));
counter--;
}
while (counter > 6);
What values of
counter variable will
this loop print ?
Change this loop in order it
would print 7 numbers.
Type your answer.
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Nested Loops
•
Inner and outer loops
– Inner loop must be entirely contained in
outer loop
– Loops can never overlap
•
To print mailing labels for customers :
for(customer = 1; customer <= 20;
++customer)
for(label = 1; label <= 3; ++label)
printLabelMethod();
How many mailing labels will be printed ?
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Nested Loops (continued)
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Improving Loop Performance
•
•
•
Make sure loop does not include
unnecessary operations or
statements
Consider order of evaluation for
short-circuit operators
Make comparisons to 0
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Avoiding
Unnecessary Operations
• Do not use unnecessary operations or statements:
– Within loop’s tested expression
– Within loop body
•
Avoid
while (x < a + b)
// loop body
•
Instead use
int sum = a + b;
while(x < sum)
// loop body
Why is it better ?
Java Programming, Fifth
Edition
Considering the Order of Evaluation
for Short-Circuit Operators
•
Short-circuit evaluation
– Each part of an AND or an OR
expression is evaluated only as much as
necessary to determine the value of
expression
•
Important to consider number of
evaluations that take place
– When loop might execute many times
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Comparing to Zero
•
Making comparison to 0
– Faster than making comparison to any
other value
•
Improve loop performance
– Compare loop control variable to 0
•
Do not code Do-nothing loop
– Performs no actions other than looping
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Comparing to Zero
Code and run this loop ( in the textbook ) : compare
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their
relative
speed
Edition
Employing Loop Fusion
•
Loop fusion
– Technique of combining two loops into
one
– Will not work in every situation
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Summary
•
Loop structure allows repeated
execution of block of statements
– Infinite loop
– Definite loop
– Nest loop
•
•
Must change loop control variable
within looping structure
Use while loop to:
– Execute statements while some
condition
is true
Java Programming,
Fifth
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Edition
Summary (continued)
•
Execute while loop
– Initialize loop control variable, test in while
statement, and alter loop control variable
•
Prefix ++ and postfix ++
– Increase variable’s value by 1
– Variable used
• Result calculated and stored
•
Unary operators
– Use with one value
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Summary (continued)
•
Binary operators
– Operate on two values
•
Shortcut operators +=, -=, *=, and /=
– Perform operations and assign result in one
step
•
for loop
– Initializes, tests, and increments in one
statement
•
do...while loop
– Tests Boolean expression after one
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repetition
Edition
Unconditional Transfer of
Control
•
Break
– Used with Switch statement
– Place in the body of a loop to provide immediate
exit
•
Continue
– When reached, a new iteration of the nearest
enclosing while, do…while, for, or foreach
statement is started
•
Other jump statements
– goto, throw, and return
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Quiz
• True or False: Within a looping structure, a floating-point expression is
evaluated.
• Answer: False
• A(n) ____ loop is one in which the loop-controlling Boolean
expression is the first statement in the loop.
• Answer: while
• To write a definite loop, you initialize a(n) ____, a variable whose
value determines whether loop execution continues.
• Answer: loop control variable
• A loop controlled by the user is a type of ____ loop because you don’t
know how many times it will eventually loop.
• Answer: indefinite
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Quiz (cont.)
• The statement count ____1; is identical in meaning to count = count + 1.
• Answer: +=
• The prefix and postfix increment operators are ____ operators because
you use them with one value.
• Answer: unary
• You begin a for loop with the keyword ____ followed by a set of
parentheses.
• Answer: for
• A while loop is a(n) ____ loop—one in which the loop control variable is
tested before the loop body executes.
• Answer: pretest
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Quiz (cont.)
• The do...while loop is a(n) ____ loop—one in which the loop control
variable is tested after the loop body executes.
• Answer: posttest
• When loops are ____, each pair contains an inner loop and an outer
loop.
• Answer: nested
• True or False: Whether you decide to use a while, for, or do...while
loop in an application, you can improve loop performance by making
sure the loop does not include unnecessary operations or statements.
• Answer: True
• Under what circumstances would you use a for loop rather than a while
loop?
46
Quiz (cont.)
• Why do you think the Java language provides three different types of
loops if all loops can be written using the while statement?
• True or False: Given the for statements form, for (initialization; test;
update), the initialization is performed with each iteration through the
loop.
• Given the following statement, what would be the output produced?
for (int cnt = 0; cnt < 3; cnt++)
System. out.println(cnt) ;
• Used with a loop, how does a continue statement differ from a break
statement?
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• dialog box
This is the end of our Unit 7
Seminar
•Any questions ?
48