More loops

Horstmann Ch 7 continued.

The do-loop

continue-condition?

loop-body statements

next statement

false

true

WHILE-LOOP

continue-condition?

loop-body statements

next statement

false

true

DO-LOOP

Using a do-loopUse this form when you want the loop body to be executed at least once

int data;

int sum = 0;

String dataString;

do

{ dataString=JOptionPane.showInputDialog(null,”enter number:”);

data = Integer.parseInt(dataString);

sum += data;

} while (data != 0);

JOptionPane.showMessageDialog(null,”Numbers sum to ”+sum);

Things to know about do-loops

• The loop body will always be executed one

• The first execution is done before the guard is checked

• The do-loop is nothing more than a small variant of the while-loop

• It is often useful for checking user input

When to use the do-loop?int sum;

int data;

String dataString;

dataString=JOptionPane.showInputDialog(null,”enter number:”);

data = Integer.parseInt(dataString);

while(data != 0)

{

dataString=JOptionPane.showInputMessage(null,”enter number:”);

data = Integer.parseInt(dataString);

sum = sum + data; //could have used: sum += data;

}

JOptionPane.showMessageDialog(null,”Numbers sum to ”+sum);

A better wayint data;

int sum = 0;

String dataString;

do

{

dataString=JOptionPane.showInputDialog(null,”enter number:”);

data = Integer.parseInt(dataString);

sum += data;

} while (data != 0);

JOptionPane.showMessageDialog(null,”Numbers sum to ”+sum);

We don’t have to get the first piece of data, add it to the sum variable, and then start the loop.

The for-loop

• The for-loop is another variant of the while loop

• Every for-loop can be written as a while loop

• Not every while-loop can be written as a for-loop

• Use a for-loop when you know exactly how many times the loop body should be executed

The for loop: Three things in one line

for ( <initialization>; <guard>; <adjustment>)

{

...

}

1. Initialization: (declare and) set a counter, to 0. This is done before everything else.

2. Guard: loop ends when this is false. This is tested before each pass through the loop.

3. Adjustment: e.g. increment a counter. This is done at the end of each pass through the loop.

Examplesfor (int i=0; i<10; i++)

{

JOptionPane.showMessageDialog(null, “ ”+i);

}

int i;

for (i=10; i>0; i--)

{

JOptionPane.showMessageDialog(null, “ ”+i);

}

From while to forint i = startValue;

while (i < endValue)

{

.....

i++;

}

for (int i=startValue; i<endValue; i++)

{

...

}

Flow of control in a for-loop

initialization

test guard

loop body

next statement

adjust

true

false

Rules and guidelines for “for”

• Counters can be declared and initialized in one go

• Never (never) change the value of the counter inside the loop body

• I mean it. Never do that!• If the loop body is one statement only, you

can omit the braces—but please don’t!• Indent the code within the loop body..

Examples// Compute sum = 0.01 + 0.02 + … + 1;

...

double sum = 0;

// Keep adding 0.01 to sum

for (double i=0.01; i <= 1.0 ; i = i+0.01)

{

sum += i;

}

JOptionPane.showMessageDialog("The sum is " + sum);

Common error

• The 3 elements of the for-loop header are sparated by semi-colons, not commas!

• Do this:• for (int i=0; i<10; i++)

• Not this:• for (int i=0, i<10, i++)

• Keep your loops as simple as possible

Nesting for-loops

• Inside the loop body of a for-loop, we can put another for-loop

• Each time through the 1st for-loop, we execute the 2nd loop until its guard is false

• Handy for printing tables like this: 1 1 1 1 1 1 2 2 2 2 2 2 3 3 3 3 3 3 4 4 4 4 4 4

Simple examplefor (int i=0; i<5; i++)

{

for (int j=0; j<3; j++)

{

System.out.print(i+“ ”);

}

System.out.println();

}

Nested Loops

• Create triangle pattern

• Loop through rows

[][][][][][][][][][]

for (int i = 1; i <= n; i++){   // make triangle row}

for (int i = 1; i <= n; i++){   

// make triangle row

}

for (int i = 1; i <= n; i++){    for (int j = 1; j <= i; j++) {   r = r + "[]"; } r = r + "\n"; }

Nested Loops

• Make triangle row is another loop

• Put one loop inside the other → Nested loops

for (int j = 1; j <= i; j++) {   r = r + "[]";}r = r + "\n"; \\ “\n” means end of line

/** This class describes triangle objects that can bedisplayed with printed shapes */

public class Triangle{

private int width;

/** Constructs a triangle. @param aWidth number of [] in the last row of triangle. */

public Triangle(int aWidth){ width = aWidth;}

/** Computes a string representing the triangle. @return a string consisting of [] and newline characters */

public String toString(){ String r = ""; for (int i = 1; i <= width; i++){ for (int j = 1; j <= i; j++) { r = r + "[]"; } r = r + "\n"; } return r; }}

File TriangleTester.java

/** This program tests the Triangle class.*/public class TriangleTester{ public static void main(String[] args){ Triangle small = new Triangle(3); System.out.println(small.toString());

Triangle large = new Triangle(15); System.out.println(large.toString()); }}

break-ing out of a loop

• Inside a loop body, the statement break; causes execution to leave the loop immediately.

• This is usually unnecessary, and shows that not enough though has gone into the development of a guard.

Example of break

int sum = 0;

int item = 0;

while (item < 5)

{

item ++;

sum += item;

if (sum >= 6) break;

}

System.out.println("The sum is " + sum);

continue

• This keyword also stops interation of the loop body

• But...the program then starts the next iteration of the loop (testing the loop guard first)

• This is also usually unnecessary, and can be replaced by an if-statement.

Example of continue

int sum = 0;

int item = 0;

while (item < 5)

{

item++;

if (item == 2) continue;

sum += item;

}

System.out.println("The sum is " + sum);

Object-oriented approach to averages/** Computes the average of a set of data values. */public class DataSet{

private double sum; private double maximum; private int count;

/** Constructs an empty data set. */ public DataSet(){ sum = 0; count = 0; maximum = 0; }

/** Adds a data value to the data set @param x a data value */ public void add(double x){ sum = sum + x; if (count == 0 || maximum < x) { maximum = x; } count++; }

(continued)

public double getAverage(){ if (count == 0) { return 0; } else { return sum / count; } }

public double getMaximum(){ return maximum; }} // close the DataSet class

Public class DataSestTester{ public static void main(String[] args){ DataSet myData = new DataSet(); String input; do { input = JOptionPane.showInputDialog(null, “number?”); myData.add(Double.parseDouble(input) ); } while(input.length() > 0); JOptionPane.showMessageDialog(null,“the average of those numbers is ”+ myData.getAverage() ); System.exit(0);}}