Chapter 3 Part 2 More about Strings - PowerPoint Presentation

Slide1

Chapter 3 Part 2

More about Strings andConditional StatementsLoops (for and while)

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Repetition Statements

Repetition statements

allow us to execute a statement multiple times

Often they are referred to as

loops

Like conditional statements, they are controlled by boolean expressions

The text covers two kinds of repetition statements:

the

while loop

the

for loop

The programmer should choose the right kind of loop for the situationSlide3

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The while Statement

The

while statement

has the following syntax:

while (

condition

)

statement;

while

is areserved word

If the

condition

is true, the statement is executed.Then the condition is evaluated again.

The

statement

is executed repeatedly until

the

condition

becomes false.Slide4

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Logic of a while Loop

statement

true

condition

evaluated

falseSlide5

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The while Statement

Note that if the condition of a

while

statement is false initially, the statement is never executed

Therefore, the body of a

while

loop will execute zero or more times

See

Counter.java (page 147)See Average.java (page 148)A sentinel

value indicates the end of the inputThe variable

sum maintains a running sumSee WinPercentage.java (page 151)A loop is used to validate the input, making the program more robustSlide6

while (value != 0) // sentinel value of 0 to terminate loop

{ count++; sum += value; System.out.println

("The sum so far is " + sum);

System.out.print

("Enter an integer (0 to quit): ");

value =

scan.nextInt

();

} System.out.println (); System.out.println ("Number of values entered: " + count); average = (double)sum / count; DecimalFormat fmt

= new DecimalFormat ("0.###");

System.out.println ("The average is " + fmt.format(average)); }}6Slide7

while (won < 0 || won > NUM_GAMES)

{ System.out.print ("Invalid input. Please reenter: "); won = scan.nextInt

();

}

ratio = (double)won / NUM_GAMES;

NumberFormat

fmt

= NumberFormat.getPercentInstance(); System.out.println (); System.out.println ("Winning percentage: " + fmt.format(ratio)); }}

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Infinite Loops

The body of a

while

loop eventually must make the condition false

If not, it is an

infinite loop

, which will execute until the user interrupts the program

This is a common logical error

You should always double check to ensure that your loops will terminate normallySee Forever.java (page 152) Slide9

public class Forever

{ //----------------------------------------------------------------- // Prints ever-decreasing integers in an INFINITE LOOP! //----------------------------------------------------------------- public static void main (String[]

args

)

{

int

count = 1;

while (count <= 25)

{ System.out.println (count); count = count - 1; } System.out.println ("Done"); // this statement is never reached }}

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Nested Loops

Similar to nested

if

statements, loops can be nested as well

That is, the body of a loop can contain another loop

Each time through the outer loop, the inner loop goes through its full set of iterations

See

PalindromeTester.java

(page 155)Slide11

while (

another.equalsIgnoreCase("y")) // allows y or Y { System.out.println ("Enter a potential palindrome:");

str

=

scan.nextLine

();

left = 0;

right =

str.length

() - 1; while (str.charAt(left) == str.charAt(right) && left < right) { left++; right--; }

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System.out.println(); if (left < right) System.out.println ("That string is NOT a palindrome.");

else

System.out.println

("That string IS a palindrome.");

System.out.println

();

System.out.print ("Test another palindrome (y/n)? "); another = scan.nextLine(); } }}12Slide13

Conditional Operator

Conditional operators are a shortcut way to express an if-else situation. Format:

result = (condition)? value1 : value2;

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“if” part

“else” partSlide14

Conditional Operator

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Example: (

orig

if-else

)

i

f (x > 0)

count = count + 1;

else count = count – 1;

Example:

(cond. operator)count =(x > 0)? (count +

1) : (count – 1);Slide15

The Do-While Loop

The do-while loop works like the while loop, BUT in a different order. The do portion is done first, before the while condition is checked.Format:

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do

{

… statements for what to do

}

w

hile (condition); Slide16

Example of Do-While loop

int count = 0; int sum = 0;

do

{

sum = sum + count;

count ++;

} while (count < 10);16

Notice the while condition is not checked until the do loop is executed at least once.Slide17

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The for Statement

The

for statement

has the following syntax:

for (

initialization

;

condition

; increment )

statement

;Reserved

word

The

initializationis executed oncebefore the loop begins

The

statement

is

executed until the

condition

becomes false

The

increment

portion is executed at the end of each iteration

The

condition

-

statement

-

increment

cycle is executed repeatedlySlide18

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The for StatementA

for

loop is functionally equivalent to the following

while

loop structure:

initialization

;

while (

condition )

{

statement; increment;}Slide19

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Logic of a for loop

statement

true

condition

evaluated

false

increment

initializationSlide20

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The for Statement

Like a

while

loop, the condition of a

for

statement is tested prior to executing the loop body

Therefore, the body of a

for

loop will execute zero or more timesIt is well suited for executing a loop a specific number of times that can be determined in advanceSee Counter2.java (page 161)See Multiples.java (page 163)See

Stars.java (page 165)Slide21

public class Counter2

{ //----------------------------------------------------------------- // Prints integer values from 1 to a specific limit. //----------------------------------------------------------------- public static void main (String[]

args

)

{

final

int

LIMIT = 5;

for (

int count=1; count <= LIMIT; count++) System.out.println (count); System.out.println ("Done"); }}21Slide22

final

int PER_LINE = 5; int value, limit, mult, count = 0;

Scanner scan = new Scanner (

System.in

);

System.out.print

("Enter a positive value: ");

value =

scan.nextInt(); System.out.print ("Enter an upper limit: "); limit = scan.nextInt(); System.out.println (); System.out.println ("The multiples of " + value + " between " + value + " and " + limit + " (inclusive) are:");

for (

mult = value; mult <= limit; mult += value) { System.out.print (mult + "\t"); // Print a specific number of values per line of output

count++; if (count % PER_LINE == 0) System.out.println();

} }

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public class Stars

{ //----------------------------------------------------------------- // Prints a triangle shape using asterisk (star) characters. //----------------------------------------------------------------- public static void main (String[]

args

)

{

final

int

MAX_ROWS = 10;

for (

int row = 1; row <= MAX_ROWS; row++) { for (int star = 1; star <= row; star++) System.out.print ("*"); System.out.println(); }

}}

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The for StatementEach expression in the header of a for loop is optional

If the

initialization

is left out, no initialization is performed

If the

condition

is left out, it is always considered to be true, and therefore creates an infinite loop

If the

increment is left out, no increment operation is performedBoth semi-colons are always required in the for loop headerSlide25

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Choosing a Loop Structure

When you can’t determine how many times you want to execute the loop body, use a

while

statement

If you can determine how many times you want to execute the loop body, use a

for

statementSlide26

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Program Development

We now have several additional statements and operators at our disposal

Following proper development steps is important

Suppose you were given some initial requirements:

accept a series of test scores

compute the average test score

determine the highest and lowest test scores

display the average, highest, and lowest test scoresSlide27

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Program Development

Requirements Analysis – clarify and flesh out specific requirements

How much data will there be?

How should data be accepted?

Is there a specific output format required?

After conferring with the client, we determine:

the program must process an arbitrary number of test scores

the program should accept input interactively

the average should be presented to two decimal placesThe process of requirements analysis may take a long timeSlide28

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Program Development

Design – determine a possible general solution

Input strategy? (Sentinel value?)

Calculations needed?

An initial algorithm might be expressed in pseudocode

Multiple versions of the solution might be needed to refine it

Alternatives to the solution should be carefully consideredSlide29

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Program Development

Implementation – translate the design into source code

Make sure to follow coding and style guidelines

Implementation should be integrated with compiling and testing your solution

This process mirrors a more complex development model we'll eventually need to develop more complex software

The result is a final implementation

See

ExamGrades.java

(page 170)Slide30

max = min = grade;

// Read and process the rest of the grades while (grade >= 0) { count++; sum += grade;

if (grade > max)

max = grade;

else

if (grade < min)

min = grade;

System.out.print

("Enter the next grade (-1 to quit): "); grade = scan.nextInt (); }30Slide31

// Produce the final results

if (count == 0) System.out.println ("No valid grades were entered."); else {

DecimalFormat

fmt

= new

DecimalFormat

("0.##");

average = (double)sum / count; System.out.println(); System.out.println ("Total number of students: " + count); System.out.println ("Average grade: " + fmt.format(average)); System.out.println ("Highest grade: " + max);

System.out.println ("Lowest grade: " + min);

} }31Slide32

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Program Development

Testing – attempt to find errors that may exist in your programmed solution

Compare your code to the design and resolve any discrepancies

Determine test cases that will stress the limits and boundaries of your solution

Carefully retest after finding and fixing an errorSlide33

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More Drawing Techniques

Conditionals and loops can greatly enhance our ability to control graphics

See

Bullseye.java

(page 173)

See

Boxes.java

(page 175)

See BarHeights.java (page 177) Slide34

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SummaryChapter 3 has focused on:

program development stages

the flow of control through a method

decision-making statements

expressions for making complex decisions

repetition statements

drawing with conditionals and loopsSlide35

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Assignment 3

COMPLETE ALL ASSIGNMENTS IN THE ORDER LISTED

Lab 2 -- Grades

Lab 3 Phase 1

Counting and Looping – Love CS

Powers of 2 – Powers

A guessing Game – Guess

Chapter 3 Exercises continued

numbers 9- 13Lab 3 Phase 2Factorials – Factorial (Ex. Credit)More Guessing – GuessDoChapter 3 Exercises continuednumbers 14 - 22 Slide36

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Assignment 4

COMPLETE ALL ASSIGNEMENTS IN THE ORDER LISTED

Lab 4 Phase 1

Counting Characters –

CharCount

Chapter 3 Programming Projects (pp. 187 – 188)

3.1, 3.4, 3.5, 3.7, 3.8, 3.12 a, b, 3.13

Lab 4 Phase 2Finding Maximum & Minimum Values – MaxMinChapter 3 Exercises Continuednumbers 23 – 27Chapter 3 Exercises from textbook pp. 1893.15 – 3.17 (Ex. Credit)Lab 5

A Rainbow Applet – Rainbow (Ex. Credit)