chapter 5 loops, printf statements, nested control statements and gui input & output section 1 -...

Chapter 5Loops, Printf Statements, Nested Control

Statements and GUI Input & Output

Section 1 - While Loops

Section 2 - Printf Statements

Section 3 - For Loops

Section 4 - Floating Point Precision & Loop Errors

Section 5 - Other Math Class Methods

Section 6 - GUI Input & Output Dialog Boxes and Parsing Strings to int and double

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Chapter 5 Section 1

While Loops

2

5.1 What are Control Statements?We call ….

if statements

if-else statements

extended if statements

while loops

and for loops

control statements

because they control the order of execution in a Java program.

3

5.1 General Form of a while loopThe while loop executes statements inside its body repeatedly for as long as its boolean condition remains true.

Every time the statements inside a while loop run, we say one iteration, one pass, or one repetition has occurred.

while (condition) No semicolon goes here!{

statement;

statement;

}

No curly braces required for just one line of code in the loop. 4

5.1 A while loop Stops with a False ConditionWhen the boolean condition of a while loop becomes false, the while loop will stop. There should be some code in the

loop to make it stop, otherwise you will have an infinite loop (never-ending loop).

while (condition) No semicolon goes here!{

statement;

statement;

}

No code is executed if the boolean condition is initially false.The loop is skipped!

5

5.1 A Count Up while Loop ExampleA count-up while loop can be written to sum the integers from 1 to 3:

int sum = 0;

int cntr = 1;

while (cntr <= 3)

{

sum += cntr;

cntr++;

}

System.out.println(sum);

Notice that here cntr, the loop control variable, is initialized to 1 before the loop and then it is incremented by 1 each time the body of the loop is executed. When cntr becomes 4, then the while loop condition will evaluate to false and the loop will immediately end.

Since the loop control variable cntr increases by 1 every time the body of the loop executes, we call it a count-up loop.

6

5.1 Tracing a Count Up while LoopIt is important to be able to trace the order of execution of a while

loop to be able to fully understand how it works:

int sum = 0; 1int cntr = 1; 2while (cntr <= 3) 3 6 9 12

{sum += cntr; 4 7 10

cntr++; 5 8 11

}System.out.println(sum); 13

Note: you are not required to used cntr in the mathematics or work of the loop as in the line sum += cntr; We just happen to be using it here as an easy way to help sum the numbers from 1 to 3.

7

The blue numbers indicate the order the lines of code are executed in.

5.1 Skeleton of a Count Up while LoopIf you initially think about the problem of summing the numbers

from 1 to 100, you should conclude that a loop is needed that will run 100 times. So the place to start is just to think about the basic parts of the loop that you need. Don’t worry about the mathematics or the work to be done inside the loop. Pull the necessary components together that will give you a loop that will run 100 times. Here is where you should start:

int cntr = 1; while (cntr <= 100){

cntr++; }

declare and initialize the loop control variable

design a loop condition that will make the loop run the correct number of times

increment the loop control variable at the bottom of the loop so the loop will eventually stop.

8

5.1 Adding other Lines to a while LoopNext, declare and initialize any additional variables needed for the

mathematics that will make the loop calculate the correct value. These should be added before the loop! Then, add the appropriate lines of code inside the loop to make the calculation.

int sum = 0;

int cntr = 1;

while (cntr <= 100)

{

sum += cntr;

cntr++;

}

System.out.println(“The sum is: ” + sum);

declare and initialize other variables

add the lines of code to the loop necessary to make the calculation.

9

The value 5050 is printed.

5.1 Varying while Loops Conditions

What if the boolean expression of the original code was changed from:

cntr <= 100 to cntr < 100

int sum = 0, cntr = 1;while(cntr < 100){

sum += cntr;

cntr++;

}

Then the loop runs only 99 times instead of 100 and the value stored in sum is 4950 instead of 5050.

10

5.1 Count-Controlled while Loops

The variable cntr is the counter variable or loop control variable (lcv) and

is declared and initialized before the while loop.

This while loop repeats 100 times, because when the loop is encountered

the value of cntr is 1 and it is incremented by 1 every time the body of

the loop runs. The loop will run the final time when cntr is 100 and then

when it becomes 101 inside the loop, then the loop condition evaluates

to false and the body of the loop won’t execute anymore.

Consider the following code that sums the integers from 1 to 100:

int sum = 0;int cntr = 1;while(cntr <= 100){

sum += cntr;

cntr++;

}

The sum is calculated as the number 5050.

In a count-controlled loop, the loop control variable is incremented by one each time the loop runs.

5.1 Summing from 51 to 100If we wish to sum the numbers from 51 to 100, then we only

need to make a slight variation in our code by changing the value that we initialize cntr to … 51.

int sum = 0;

int cntr = 51;

while (cntr <= 100)

{

sum += cntr;

cntr++;

}


12

5.1 Summing from 51 to 150If we wish to sum the numbers from 51 to 150 instead of 100,

then we also need to change the upper limit value of the loop.

int sum = 0;

int cntr = 51;

while (cntr <= 150)

{

sum += cntr;

cntr++;

}


13

5.1 Count Down while Loops

Countdown Loop code:

int sum = 0;int cntr = 100;while(cntr >= 1){

sum += cntr;

cntr--;

}

Note the main line of code in the loop is the same . Only the loop header and the initialization value of the lcv have been changed and the use of the decrement operator instead of the increment operator. Compare the two codes to see the differences.

The following code uses a variation of the previous code to sum the integers from 100 down to 1. We call this a count down loop:

Original code:int sum = 0;int cntr = 1;while(cntr <= 100){

sum += cntr;cntr++;

}

14

5.1 Reverse Summing with a Count Down Loop

Here is a while loop that REVERSE sums the numbers from 51 to 100 by adding them like this 100 + 99 + .... + 52 + 51.

sum = 0;

cntr = 100;

while(cntr >= 51)

{

sum += cntr;

cntr--;

}

System.out.println("The Reverse sum of 100 to 51 is: " + sum);

15

5.1 A Count-Down that Decrements by 5You can make a loop control variable decrease in value as well as increase. It can decrease by 1 or by any other value you indicate. Here number is the loop control variable.

int number = 25;

while (number >= 10)

{

System.out.print("The square root of " + number);

System.out.println(” is " + Math.sqrt(number));

number -= 5;

}

Output in console window:The square root of 25 is 5.0The square root of 20 is 4.47213595499958The square root of 15 is 3.872983346207417The square root of 10 is 3.1622776601683795 16

5.1 A Count-Down While Loop VariationWhat if the boolean expression was changed from:

number >= 10 to number > 10

int number = 25;

while (number > 10)

{

System.out.print("The square root of " + number);

System.out.println(” is " + Math.sqrt(number));

number -= 5;

}

The loop runs one less times and the Output is:The square root of 25 is 5.0The square root of 20 is 4.47213595499958The square root of 15 is 3.872983346207417

17

5.1 Task-Controlled While LoopsA loop can execute until some task is accomplished. This code seeks to find the value of number once sum is greater than 1,000,000.

int sum = 0;

int number = 0;

while ( sum <= 1000000)

{

number++;

sum

+= number;

}

System.out.print(“The first value of number for which”);

System.out.println(“ sum is over 1,000,000 is: " + number);In this example, the loop control variable is sum but it is not incremented by one or any other constant value each time the loop runs. 18

5.1 Interesting While Loop ExampleCode Example: Generate a random integer between -5 and 5 inclusive and

store it in x. If x is positive find the square root of all values between 1 and x and print them out, otherwise do nothing.

int x = (int) (Math.random() * 11 ) - 5;

while (x > 0)

{

double root = Math.sqrt(x);

System.out.println(“The square root of x is ” + root);

x--;

}

Notice that if the value stored in x is between -5 and 0 inclusive, the loop will NOT run at all.

Also notice that when x is positive, the loop will run and then x will be decremented by 1 and eventually x will become 0 and when it does then 0 > 0 evaluates to false and the loop stops. 19

5.1 User-Controlled While Loop ExampleThe following code sums all the integers between two integers

(inclusive) entered from the keyboard:

Scanner reader = new Scanner (System.in);

System.out.print(“Enter a starting value: ”);

int startingValue = reader.nextInt();

System.out.print(“Enter an ending value greater than starting value: ”);

int endingValue = reader.nextInt();

int sum = 0;

int cntr = startingValue;

while (cntr <= endingValue)

{

sum += cntr ;

cntr++;

}

System.out.println(sum); 20

5.1 Generate 10 Random IntegersThe following code prints 10 random integers in the range of 25 to 75 inclusive:

int cntr = 1;

while (cntr <= 10)

{

int x = (int) (Math.random() * 51) + 25;

System.out.println("The random integer is " + x);

cntr++;

}

Note: here we want to print each random integer, so our

System.out.println statement is inside the loop.

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5.1 Entering Values from the Keyboard in a Loop

The following loop runs 5 times and asks the user to enter a

vocab word each time the loop runs. The vocab word is then

printed out.

int cntr = 1;

while (cntr <= 5)

{

System.out.print("Enter a vocab word and press return: ");

String word = reader.nextLine();

System.out.println();

System.out.println(”The vocab word is: " + word);


cntr++;

}

22

5.1 Branching Statements Inside While LoopsBranching statements may be nested or placed inside loops. Almost

every programming problem can be solved with the combination of loops and branching statements. The following code prints whether each integer between 1 and 10 is even or odd.

int cntr = 1;

while (cntr <= 10)

{

if (cntr % 2 == 0)

System.out.println(cntr + “ is even”);

else

System.out.println(cntr + “ is odd”);

cntr++;

} 23

5.1 Branching Statements Inside LoopsNotice that we could also use …

int cntr = 1;

while (cntr <= 10)

{

if (cntr % 2 == 0)

System.out.println(cntr + “ is even”);

else if (cntr % 2 == 1)

System.out.println(cntr + “ is odd”);

cntr++;

}24

5.1 Branching Statements Inside Loops

Again, many programming problems are solved with the

combination of loops and branching statements.

You will solve a lot of problems with loops and branching

statements in the programs that you will write in this

chapter.

25

5.1 A While Loop May Never Run

// Why does the following loop never run?

int a = 100;

int b = 50;

while (a < b)

{

System.out.println("This will print if the loop runs!");

a += b;

}

System.out.println("The value of a is: " + a);

System.out.println("The value of b is: " + b);

System.out.println("The loop never runs!!! ");

26

5.1 While Loops That Never Ends … Infinite Loops

A while loop that never ends is called an infinite loop.

Here is an example of a loop that starts but never ends. Can you tell why?

int a = 0;

int b = 101;

while (a != b)

{

a += 2;

}

System.out.println(“The value of a is ” + a);

27

5.1 While Loops That Never Ends … Infinite Loops

A while loop that never ends is called an infinite loop.

Here is an example of a loop that starts but never ends. Can you tell why?

int a = 0;

int b = 101;

while (a != b) // use of != causes the infinite loop

{

a += 2;

}

System.out.println(“The value of a is ” + a);

Changing != to <= causes the loop to run

Changing != to >= causes the loop to be skipped 28

5.1 Order & Structure of a While Loop

1) Initialize the loop control variable and other variables.

2) Test the loop control variable in the condition to try to enter the while loop.

3) If successful entry, execute the body of the loop and then perform calculations and change the lcv or other variables.

4) When the condition becomes false, the loop stops without executing the body.

initialize loop control variable // initialize

while (condition) // test the loop control variable in the condition

{ // execute the body of loop

perform calculations and

change the lcv or other variables involved in the condition

}

29

5.1 Using a while (true) loop with Break Since true is a valid boolean value, a while(true) loop can be used.

A while (true) loop continues to run until some condition makes it stop. This can be done by embedding a break statement inside an if statement inside the loop, so that when the if condition becomes true, the break statement will be executed and the loop will immediately stop.

while (true)

{

System.out.print(“Enter a number or -1 to quit: ”);

int x = reader.nextInt();

if (x == -1)

break;

…. // lines of code that use x if it is not -1

}

-1 is called the sentinel because it makes the loop stop

30

5.1 A while (true) loop Example Here is a while-true loop example that …

while(true){

System.out.print("Enter the person's age or -1 to quit: ");age = reader.nextInt();

if (age == -1)break;

reader.nextLine(); // consume the new line character

System.out.print("Enter the person's name: ");name = reader.nextLine();

if(age < 0 || age > 120)System.out.println("There is NO WAY you are alive " +

name);else{

System.out.println("The name of the person is: " + name);System.out.println("The age of the person is: " + age);

}} // end of while(true) loop 31

5.1 Flow Chart for The while Statement

32

boolean

condition

Statements

true

false

Statements after loop

Notice the return arrow after the last statement in the loop. It makes contact above the condition diamond.

All the statements inside the loop go here.

Chapter 5 Section 2

Formatting Console Output

with printf statements

33

5.2 Introduction to printf statementsprint and println statements don’t allow us to easily format output to

the screen or a file in an organized manner. However, a printf statement does. Just like the ln in println is an abbreviation for line. The f in printf is an abbreviation for format.

Many times when we use loops to output a lot of data to the screen, we want it to be more readable. We can use printf statements to make output more readable by placing data in columns.

printf statements allow us to left justify output or right justify output. You can use both in the same statement to do things like start printing at the left margin but align other output in columns. This makes the output more readable to the user of a program.

Freezing = 3 2Boiling = 2 1 2Fiery = 5 1 3 4Nice = 8 7Super Hot = 6 9 2 4 1

The words are left justified.

The integers are right justified.

34

5.2 The Form of a printf Statement

A printf statement always has the following form:

System.out.printf ("format string", dataValue1, dataValue2, ....);

Notice the format string and all data values are separated by commas NOT + signs.

The format string is inside double quotes and there must be a format specifier in it for each data value listed in the remainder of the statement. You can have any number of data values, but you must have one format specifier for every data value. The data values are NOT within double quotes unless they are String values.

Format specifiers always begin with the % symbol.35

5.2 printf Statement Examples

Here are three examples of printf statements.

int fahrenheit = 212;

double percentage = 97.524791;

String phrase = “Java is Cool!”;

System.out.printf ("%7d”, fahrenheit);

System.out.printf ("%10.4f”, percentage );

System.out.printf ("%20s”, phrase);

We will explain what is in the parentheses of each printf statement above shortly.

36

5.2 The printf Format SpecifiersThere are three primary format specifiers that we will use:

the letter d for decimal (base-10) int values as in %7d

the letter f for decimal floating point values as in %10.4f

the letter s for string values as in %20s

%n is a format specifier that tells Java to display a new-line character, but you can also use the escape sequence \n in its place. When Java sees %n, it knows not to apply it to any data value after the format string.

If a dash - follows the % symbol of a format specifier, then the data value will be left justified. If there is no dash then the data value is automatically right justified. The - is called a format flag. You’ll see examples coming up.

37

decimal doesn’t indicate decimal point

5.2 The Format Specifier for intprintf statements can right justify integer output. Consider

this code:

int fahrenheit = 212;System.out.printf("%7d”, fahrenheit);

Once java sees the format specifier %7d then it retrieves the

int data value stored in the variable fahrenheit (212) after

the comma. Java figures out that it needs 3 spaces to print

212 and then it calculates that it needs 7-3 spaces or 4

spaces before 212 since it is right justifying the output. So

it skips 4 spaces from the left and then begins printing 212.

It prints 212 in the 5th, 6th, and 7th spaces of the field

width of 7 spaces. If we use an underscore character ‘_’ to

represent a blank space, then the output would look like this:

_ _ _ _ 212

The printf statement right justifies the value stored in the variable fahrenheit in a field width of 7 spaces.

38

5.2 The Format Specifier for int

We can also add other string information inside the quotation marks of the format string that we also want printed out prior to 212:

int fahrenheit = 212;System.out.printf(“Temperature = %7d”, fahrenheit);

Java will first print Temperature = at the first of a line of

output and then when it sees the format specifier %7d it

retrieves the int data value stored in the variable

fahrenheit (212). Again, Java figures out that it needs 3

spaces to print 212 and then skips 4 spaces from the left of

the last thing printed and then begins printing 212. The

output would look like the following with underscores again

representing blank spaces:

Temperature = _ _ _ _ 212 39


We can also add other string information inside the format string that we want printed out after 212:

int fahrenheit = 212;System.out.printf(“Temperature = %7d Fahrenheit”, fahrenheit);

Java will first print Temperature = and then when it sees the

format specifier %7d then it retrieves the int data value

stored in the variable fahrenheit (212) and prints it right

justified in the field width of 7. It then prints the word

Fahrenheit after 212. The output would look like the

following:

Temperature = _ _ _ _ 212 Fahrenheit

Make sure you realize that the %7d applies to fahrenheit the variable NOT Fahrenheit the word.

40


You might be wondering … “Why would we want to use %7d?”

The reason is a program may need to output temperature

values of different sizes with all of the ones digits

lined up, all of the tens digits lined up, all of the

hundreds digits lined up, etc. In other words, we

would want output to look like this with the numbers

right justified:

Temperature = _ _ _ _ _ 3 2 FahrenheitTemperature = _ _ _ _ 2 1 2 FahrenheitTemperature = _ _ _ 5 1 3 4 FahrenheitTemperature = _ _ _ _ _ 8 7 FahrenheitTemperature = _ _ 6 9 2 4 1 Fahrenheit

41


You may be wondering what would Java do if the temperature was over

9,999,999, then Java would have to take an extra space to print all of the

digits of the temperature (because it won’t chop the number off) and then

the formatting would be off on that one line as seen below.

Temperature = _ _ _ _ _ 3 2 FahrenheitTemperature = _ _ _ _ 2 1 2 FahrenheitTemperature = _ _ _ 5 1 3 4 FahrenheitTemperature = _ _ _ _ _ 8 7 FahrenheitTemperature = 3 7 5 6 9 2 4 1 Fahrenheit

So it is important to choose a field width that is larger than the biggest number you expect to have in output.

42

5.2 Right Justified Output for int

Here are random integers between 1 and 2000 that are right justified using a prinf statement with a field width of 4. Notice the ones digits, tens digits, hundreds digits, and thousands digits are lined up.

1102 295 1493 536 3 424 47 1983 1995 740

43

5.2 Left Justification for intYou can use a printf statement to left justify output also.

Consider the code:

int fahrenheit = 212;System.out.printf ( "%-7d”, fahrenheit );

Once java sees the format specifier %-7d then it retrieves the

data value stored in the variable fahrenheit (212). Java

figures out that it needs 3 spaces to print 212 and then

begins printing 212 at the left margin. It then advances 4

spaces to get ready to print the next thing. It prints 212

in the 1st, 2nd, and 3rd spaces of the field of 7 spaces.

The output would look like the following:

2 1 2 _ _ _ _

The printf statement uses the - format flag to left justify the value stored in the variable fahrenheit in a field width of 7 spaces.

44

5.2 The Format Specifier for doubleprintf statements can be set to round what is printed out.

They don’t actually round what is in the variable, but they round what is seen on the screen. In this example, we will print right justified a floating point value in a field width of 10 with a precision of 4.

double percentage = 99.3456789;System.out.printf (“ %10.4f ”, percentage );

Once java sees the format specifier %10.4f then it retrieves the data value stored in the variable percentage (99.3456789). Java figures out that it needs 4 spaces to print the part of the number to the right of the decimal point. It uses the number in the 5th decimal place (7) to round the number to .3457. It then uses the remaining spaces: 10 - 4 = 6 to print the decimal point and the part of the number to the left of the decimal point. Note how the number is rounded when it is printed.

The output would look like the following:_ _ _ 9 9 . 3 4 5 7 45

5.2 Left Justification for doubleWe can use a printf statement to round but use left

justification instead. In this example, we will print left justified a floating point value in a field width of 10 with a precision of 4.

double percentage = 99.3456789;

System.out.printf (“ %-10.4f ”, percentage );

Note how the number is rounded and left justified when it is printed. Any other output will start after the 3 blank spaces.


9 9 . 3 4 5 7 _ _ _

46

5.2 The Format Specifier for Stringprintf statements can be used to display string information

left or right justified. In this example, we will print right justified a string value in a field width of 20.

String phrase = “Java is Cool!”;

System.out.printf (“ %20s ”, phrase);

Once java sees the format specifier %20s then it retrieves

the data value stored in the variable phrase (“Java is

Cool!”). Java figures out that it needs 13 spaces to

print phrase, including any spaces between the words. It

uses the remaining spaces: 20 - 13 = 7 to print 7 spaces

before phrase. The output would look like the following:

_ _ _ _ _ _ _ J a v a _ i s _ C o o l !

47

5.2 Left Justification for StringsIn this example, we will print left justified a string value

in a field width of 20.

String phrase = “Java is Cool!”;System.out.printf (“ %-20s ”, phrase);

Once java sees the format specifier %-20s then it retrieves

the data value stored in the variable phrase(“Java is

Cool!”). Java figures out that it needs 13 spaces to

print phrase. It uses the remaining spaces: 20 - 13 = 7

to print 7 spaces after phrase. Any other output has to

begin after those 7 spaces.


J a v a _ i s _ C o o l ! _ _ _ _ _ _ _

48

5.2 The New Line Format SpecifierNone of the previous examples start a new line after

printing the data values. To start a new line after printing the data value, the new line format specifier %n is added in the format string. All of the lines below will print the data values and start a new line.

System.out.printf(“%7d%n”, fahrenheit);

System.out.printf(“%-7d%n”, fahrenheit);

System.out.printf(“%10.4f%n”, percentage );

System.out.printf(“%-10.4f%n”, percentage );

System.out.printf(“%20s%n”, phrase);

System.out.printf(“%-20s%n”, phrase);

Note: when Java sees the %n format specifier, it doesn’t look for a data value to the right of the comma like it does with other format specifiers.

Notice there doesn’t need to be a space between any two format specifiers. If you put one, it will be added to output.

49

5.2 The New Line Format Specifier

You can choose to use \n in the place of %n. So the code on the previous slide could be changed to:

System.out.printf(“%7d\n”, fahrenheit);

System.out.printf(“%-7d\n”, fahrenheit);

System.out.printf(“%10.4f\n”, percentage );

System.out.printf(“%-10.4f\n”, percentage );

System.out.printf(“%20s\n”, phrase);

System.out.printf("%-20s\n”, phrase);

50

5.2 Multiple Format Specifiers in One Line

All of the previous printf statements have only one data value that is formatted whether a new line is created or not.

Numerous format specifiers and data values can be included in one printf statement. All of the format specifiers must appear in the format string between the double quotes.

System.out.printf("%7d%10.4f%20s%n”, fahrenheit, percentage, phrase);

The output all on one line would look like this:

____212___99.3457_______Java is Cool!

Note there are commas to separate each of the variables that represent data values. 51

5.2 Multiple Format Specifiers in One LineSometimes we want to have numerous format specifiers and

data values in one printf statement, but we want to also print out additional string information. Here is an example of how you might do that:

…. loop that calculates the square and cube of a set of numbers and then uses the following printf statement … (i is the lcv)

System.out.printf("The square of %5.1f is %6.1f and the cube is %7.1f%n",

i, square, cube);

The output for each line of output would look something like this:

The square of 1.0 is 1.0 and the cube is 1.0The square of 2.0 is 4.0 and the cube is 8.0The square of 3.0 is 9.0 and the cube is 27.0The square of 4.0 is 16.0 and the cube is 64.0The square of 5.0 is 25.0 and the cube is 125.0 52


When printf statements get too long, you can break them up into separate printf statements. Compare these two different ways of printing the exact same thing:

System.out.printf("The square of %5.1f is %6.1f and the cube is %7.1f%n", i, square,

cube);

and

System.out.printf(“The square of %5.1f is %6.1f”, i, square);System.out.printf(“ and the cube is %7.1f%n", cube);

both produce …

The square of 1.0 is 1.0 and the cube is 1.0The square of 2.0 is 4.0 and the cube is 8.0The square of 3.0 is 9.0 and the cube is 27.0The square of 4.0 is 16.0 and the cube is 64.0The square of 5.0 is 25.0 and the cube is 125.0 53


Sometimes we can print the same thing using separate printf statements where we want to left or right justify the string information but we don’t want the info in the format string.

Here is the original printf statement:

System.out.printf("The square of %5.1f is %6.1f and the cube is %7.1f%n", i, square, cube);

Here is one that splits the original into two printf statements

that formats the strings using format specifiers, but it left

justifies both “The square of” and “and the cube is” treating

them as data values.

System.out.printf(“%-14s%5.1f is %6.1f”, “The square of”, i, square);

System.out.printf(“%-15s%7.1f%n”, “and the cube is”, i, cube);54

5.2 The Comma Format Flag for Numbers

The comma format flag makes large numbers even more readable. Here is an example:

Let’s assume we have calculated the 8th perfect number and we want to print it with commas. We could use the line of code:

System.out.printf ("The 8th Perfect Number is %,30d", num);

This gives the output:

The 8th Perfect Number is 2,305,843,008,139,952,128

It is worthwhile to note that d will not only format int values but also long values and this value is in the long range.

55

note the comma

Chapter 5 Section 3

For Loops

56

5.3 Definition of a For LoopIn Java you can execute a segment of code over and over again using

a “for” loop. The key difference between a for and a while loop is you can place the declaration and initialization of the lcv (loop control variable), the test condition, and the lcv update statement all in the for loop header.

A for loop looks like this:

for (declare & initialize lcv; test condition; update lcv) {

<lines of code inside the loop body>}

No code is executed if the test condition is initially false.

Note the curly braces { } that encompass all of the statements inside the for loop body. 57

5.3 Skeleton of a for Loop

for (int cntr = 1; cntr <= 3; cntr++){

}

int cntr = 1;

while (cntr <= 3)

{

cntr++;

}

Here we compare the skeleton of a for loop with the skeleton of a while loop. Each of these loops is designed to iterate three times.

Observe the placement of the lcv (loop control variable) cntr in the two different loops.

58

5.3 A Simple for Loop Example

int sum = 0;

for (int cntr = 1; cntr <= 3; cntr++)

{

sum += cntr;

}


int sum = 0;

int cntr = 1;

while (cntr <= 3)

{

sum += cntr;

cntr++;

}


Any while loop can be written as a for loop and vice-versa. Here is the code to sum the integers from 1 to 3. Compare it to the while loop version of the code.

Again, observe the placement of the lcv (loop control variable) cntr in the two different loops. The next slide will show you the order of execution of the statements in the for loop. 59

5.3 The for Loop Order of ExecutionIt is important to be able to trace the order of execution of a for

loop. This will help you fully understand what is going on.

The order of execution is indicated with the blue numbers.

int sum = 0;

for (int cntr = 1; cntr <= 3; cntr++) this is the for loop header{

sum += cntr;

}System.out.println(sum);

Note: the loop control variable cntr is updated at the bottom of the loop, after the statements in the body have been executed.

1

2 3 6 9 12

4 7 10

5 8 11

13

Having cntr++ in the for loop header is like having cntr++ here

60

5.3 A Second While & For Loop ComparisonCompare the parts of the for loop and while loop versions of the

code that sums all of the integers from 1 to 100:

int sum = 0;for (int cntr = 1; cntr <= 100; cntr++){

sum += cntr;}System.out.println(sum);

int sum = 0;int cntr = 1;while (cntr <= 100){

sum += cntr;cntr++;

}System.out.println(sum);

For loops provide an efficient way to write quickly the skeleton of a loop and help you avoid loop errors. For loops are actually more readable, because you can see on one line if all of the necessary parts are present.

61

5.3 Setting Up A Count Up for LoopIf you initially think about the problem of summing the numbers

from 1 to 100, you should conclude that a loop is needed that will run 100 times. So the place to start is just to think about the basic parts of the loop that you need. Don’t worry about the mathematics or the work to be done. Pull the necessary components together that will give you a loop that will run 100 times. Here is where you should start:

for (int cntr = 1; cntr <= 100; cntr++){

}

declare and initialize the loop control variable

design a loop condition that will make the loop run the correct number of times

increment the loop control variable so the lcv will change and the loop will eventually stop.

62

5.3 Adding Other Lines to a for LoopNext, declare and initialize any additional variables needed for

the mathematics that will make the loop calculate the correct value. These should be added before the loop! Then, add the appropriate lines of code inside the loop to make the calculation.


sum += cntr;}System.out.println(“The sum is: ” + sum);

add the lines of code to the loop necessary to make the calculation.

63

declare and initialize other variables

The value 5050 is printed.

5.3 Summing from 51 to 100

If we wish to sum the numbers from 51 to 100, then we only need to make a slight variation in our code by changing the value that we initialize cntr to … 51.



64

5.3 Summing from 51 to 100If we wish to sum the numbers from 51 to 150 instead of 100,

then we also need to change the upper limit value of the loop.



65

5.3 Summing Random IntegersWrite a for loop that will print 100 random integers in the

range from 1 to 300 inclusive. The loop should print each random integer as it is generated inside the loop, but print only 5 random integers per line.

System.out.println("Here are 100 random ints 5 per line.");

for(int cntr = 1; cntr <= 100; cntr++)

{

int x = (int) (Math.random() * 300) + 1;

System.out.printf("%7d", x);

if (cntr % 5 == 0)


} 66

5.3 Reverse Summing from 100 to 51

If we wish to reverse sum the numbers from 100 down to 51, then we only need to make a slight variations in our code ….

int sum = 0;

for (int cntr = 100; cntr >= 51; cntr--)

{

sum += cntr;

}


67

5.3 A Count-Down for Loop

A count-down for loop can be used similar to a while loop:

// Display the square roots of 25, 20, 15, and 10

for ( int number = 25; number >= 10; number -= 5)

{System.out.println(“The square root of ” + number + “ is ” + Math.sqrt(number));

}

Notice as before with a while loop the test condition uses >= instead of <=. You could use > instead of < also.

68

5.3 Count-Controlled Input with a for LoopThis code sums a list of numbers entered from the keyboard. You will find that i is used a lot as a loop control variable. This is by tradition.

double number;

double sum2 = 0;

System.out.print(“How long is the list? ”);

int count = reader.nextInt();

for (int i = 1; i <= count; i++)

{

System.out.print(“Enter a positive number: ”);

number = reader.nextDouble();

sum2 += number;

}

Note: the letter i has long been used as a lcv. This is a tradition that has continued over the years with programmers.

69

5.3 String Input in a for Loop

Write a for loop that will run 5 times and allow the user to enter a vocabulary word from the keyboard each time the loop runs. The word will be printed after it is received from the keyboard along with how many characters are in the word.

String word = “”; // initialized to empty string

for (int i = 1; i <= 5; i++)

{

System.out.print(“Enter a vocab word: ”);

word = reader.nextLine();

System.out.print("The number of characters in " + word + " is ");

System.out.println( word.length() );

}

70

5.3 A for Loop May Never Run

Why won’t the for loop seen here run?

int b = 50;

int a;

for(a = 100; a < b; a++)

{

System.out.println("This will print if the loop runs!");

a += b;

}

System.out.println("The value of a is: " + a);

System.out.println("The value of b is: " + b);

System.out.println("The for loop never runs!!! ");71

5.3 For Loops That Never Ends … Infinite Loops

A for loop that never ends is also called an infinite loop.

Here is an example of a for loop that starts but never ends. Can you tell why?

int d = 50;

for(int c = 1; c != 300; c += 2)

{

d += 100;

}

System.out.println("The value of d is " + d);

72

5.3 Nan Stands for Not A Number

BY THE WAY …

"NaN" stands for "not a number". "Nan" is produced if a

floating point operation has some input parameters that

cause the operation to produce some undefined result.

For example, 0.0 divided by 0.0 is arithmetically

undefined. Taking the square root of a negative number

is also undefined.

73

Chapter 5 Section 4

Floating Point Precision

Loop Errors

5.4 Effects of Floating-Point Precision Normally, we don’t use double variables as loop control variables, we

use int variables. However, if you need to for some reason, you need to understand that not all double values can be represented accurately in Java in a program.

If we divide 1.0 / 3, we know the answer is a repeating decimal of 0.3

However, this value cannot be accurately represented by the computer. At some point, Java must terminate it. It is being converted to binary so the division can take place and then back to base-10. In running the following code:

double num = 1.0 / 3;

num will hold the value 0.3333333333333333

See the next slide to see how this could cause a problem.

75

5.4 Approximate Floating-Point Precision Numbers that are declared as double have about 18 decimal digits of precision.

This is good but is not perfect and can lead to unexpected errors.

Here is an example:

for (double x = 0.0; x != 1.0; x+= 0.1){

System.out.println(x + " ");}

This code goes into an infinite loop because of the use of the

line x != 1.0; The line x <= 1.0 should be used.

Here is what happens: the base-10 versions of 0.0 and 0.1 are converted to binary and every time 0.1 is added to x, the binary equivalent of 0.1 is added to x. As x grows and it is printed, it is converted back to base 10 where there is a loss of accuracy. So …

76

5.4 Approximate Floating-Point Precision … what is printed out is not 0.0, 0.1, 0.2, 0.3, 0.4, etc. but rather

0.0 0.1 0.2 0.30000000000000004 0.4 0.5 0.6 0.7 0.7999999999999999 0.8999999999999999 0.9999999999999999 1.0999999999999999… unending output indicating an infinite loop

77

5.4 Debugging Errors in Loops

If an error is suspected, check these four things:

• make sure all variables including the loop control variable

are initialized correctly before entering the loop

• that the terminating condition stops the loop when the loop

control variable or other test variables have reached the

intended limit

• that the statements in the body of the loop are correct

• that the update statement is positioned correctly and that it

modifies the loop control variable or other test variables so

that they eventually pass the limits tested in the terminating

condition so the loop will stop78

Chapter 5 Section 5

Other Math Class Methods

79

5.5 Other Math Class MethodsThe Math class has numerous other methods that are static and can

be called in the same manner that Math.abs(), Math.pow(), Math.sqrt(), and Math.random() are called by prefixing the method by the name of the class. Here are some other methods we will use:

public static double log10 (double a)

public static double sin (double a)

public static double cos (double a)

public static double tan (double a)

public static double toRadians (double angleDegrees)

80

5.5 The Math Class Log10 Method

If you want to find the base 10 logarithm of a number you can use the method Math.log10(). The method signature is as follows:

public static double log10 (double a)

If the parameter a is equal to 10n for integer n, then the result is n.

Examples: Math.log10 (10) returns 1

Math.log10 (100) returns 2



81

5.5 The Math Class Log MethodIf you want to find the base e logarithm (natural logarithm) of a number

you can use the method Math.log(). The Math class has the constant E and it can be used by writing Math.E where E is “the double value that is closer than any other to e, the base of the natural logarithms.” You may know that the constant is a non-terminating decimal like PI that is represented by the number 2.71828182845904523536028747135266249775724709369995... (from WikiPedia)

The method signature is as follows:

public static double log (double a)

Returns the natural logarithm (base e) of a double value.

Even though we won’t use it, it is worth knowing that Java has the constant E and the method Math.log().

82

5.5 The Math Class toRadians MethodBefore some trigonometric calculations, like sine, cosine, and tangent,

can be made in Java, angle measures must be converted to radians. This can be done using the toRadians() method.

public static double toRadians (double angleDegrees)

The API says that this method converts an angle measured in degrees to an approximately equivalent angle measured in radians. Example:

double angle = 30.0;

double radians = Math.toRadians(angle);

83

5.5 The Math Class Sin Method

If you want to find the sine of an angle, then you can convert an angle first to radians and then call the sin() method. The method signature is as follows:

public static double sin (double a)

This method returns the trigonometric sine of an angle, where the parameter a is in radians.

Sample code use:



double sine = Math.sin(radians);

84

5.5 The Math Class Cos Method

If you want to find the cosine of an angle, then you can convert an angle first to radians and then call the cos() method. The method signature is as follows:

public static double cos (double a)

This method returns the trigonometric cosine of an angle, where the parameter a is in radians.

Sample code use:



double cosine = Math.cos(radians);

85

5.5 The Math Class Tan Method

If you want to find the tangent of an angle, then you can convert an angle first to radians and then call the tan() method. The method signature is as follows:

public static double tan(double a)

This method returns the trigonometric tangent of an angle, where the parameter a is in radians.

Sample code use:



double tangent = Math.tan(radians);

86

Chapter 5 Section 6

GUI Input & Output Dialog Boxes

87

5.6 The JOptionPane ClassThe JOptionPane class allows us to use dialog boxes to receive input and to display output.

For input, we us …JOptionPane.showInputDialog();

and for output we use …JOptionPane.showMessageDialog();

88

5.6 Receiving Input Using showInputDialogA convenient way to accept input from a user, even in a console program, is to

use an input dialog box that prompts the user to enter a value by calling the method showInputDialog of the JOptionPane class:

String inputStr = JOptionPane.showInputDialog(null, "Enter the radius", "0");

Notice the method requires 3 parameters. Null is a necessary first parameter if this line does not appear in an applet program. The prompt “Enter the radius” is the second parameter and a default value (here zero) is the third parameter and appears in the text entry field. The user can use the default value or enter something else like 10 and then click OK. 89

Notice there is also a Cancel button.

5.6 Canceling an Input Dialog BoxWhen using an input dialog box, a programmer should build in

protection in case the user decides not to enter a value and clicks Cancel instead. If the code for an input dialog box is in a method, we can add the following code to terminate the input process and the method:

String inputStr = JOptionPane.showInputDialog(null, "Enter the radius", "0");if (inputStr == null)

return;

This works because if the user clicks cancel the value null is returned instead of some number in string form.

Maybe you remember that when a String variable is declared but not initialized or given a value, it then references the value null.

This is why we can use if (inputStr == null) to see if cancel was clicked. 90

5.6 Parsing Numeric InputAll values from an input dialog box are returned as String values. So if the expected input is a number, you must parse it using either …

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g

e

r

.

p

a

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s

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I

n

t

(

)

o

r

D

o

u

b

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e

.

p

a

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D

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)

These methods convert the input String value to an int or double number so the input can be processed as a number.

parseInt() and parseDouble() are static methods of the Integer and Double classes, which we have mentioned in passing. Therefore, they must be called using the name of the class, similar to Math.pow and

Math.sqrt.

returns an int

returns a double

91

5.6 Canceling an Input Dialog Box

So if we assume the user will enter a floating-point value for the radius, then the code needs

to be the following:

String inputStr = JOptionPane.showInputDialog(null, "Enter the radius", "0");if (inputStr == null)

return;

double radius = Double.parseDouble(inputStr); So we get the

number as a String,

and then parse it to

a double, and store

it in a double

variable.

92

5.6 Output Using showMessageDialogA convenient way to display output to a user, in a console or an

applet program, is to use an output dialog box by calling the method showMessageDialog of the JOptionPane class:

JOptionPane.showMessageDialog(null, "The area is " + area, "Output Window", JOptionPane.INFORMATION_MESSAGE );

required

output message

title bar label

message icon

Four parameters separated by commas

INFORMATION_MESSAGE Icon

93

5.6 Output Using showMessageDialogWe can display an output dialog box with an Error icon rather than an

Information icon if we want to. To do this the last parameter changes:

JOptionPane.showMessageDialog(null, "Error: Radius must be >= 0", "Output Window", JOptionPane.ERROR_MESSAGE);

required

output message

title bar label

message icon

Four parameters separated by commas

ERROR_MESSAGE Icon

94

5.6 CircleAreaDemo Program CodeHere is some of the code of the CircleAreaDemo program:

String inputStr = JOptionPane.showInputDialog(null, "Enter the radius", "0");

if (inputStr == null) return;

double radius = Double.parseDouble(inputStr);

if (radius < 0) JOptionPane.showMessageDialog(null,

"Error: Radius must be >= 0","Output Window",JOptionPane.ERROR_MESSAGE);

else{ double area = Math.PI * Math.pow(radius, 2); JOptionPane.showMessageDialog(null,

"The area is " + area,"Output Window",JOptionPane.INFORMATION_MESSAGE);

}

95

Chapter 5 Review• When testing a loop, try running varying test data to make sure the

loop processes all values correctly. Make sure you test the initial

value of the loop and the loop limit value just before the loop stops

to make sure they process everything correctly so you don’t have

an OBO (Off-By-One error).

• If a loop produces errors, use debugging output statements to verify

the values of the control variable on each pass through the loop.

• Branching statements, such as an if, if-else, and

extended if statements, can be nested within loops to provide a

powerful mechanism for solving problems.

• A break statement can be used with an if statement to terminate

a while or for loop early.

96

chapter 5 loops, printf statements, nested control statements and gui input & output section 1 -...

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