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Международный университет информационных технологий

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Программирование на Java

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Java Programming Language

Lecture 3.

Using Loop Constructs

9 - 1

Topics

•Create a while loop

•Develop a for loop

•Nest a for loop and a while loop

•Use an array in a for loop

•Code and nest a do/while loop

•Compare loop constructs

9 - 2

Loops

Loops are frequently used in programs to repeat blocks of statements until an expression is false.

There are three main types of loops:

•while loop: Repeats while an expression is true

•do/while loop: Executes once and then continues to repeat while true

•for loop: Repeats a set number of times

9 - 3

Creating while Loops

Syntax:

while (boolean_expression) {

code_block;

} // end of while construct // program continues here

If the boolean expression is true, this code block executes.

If the boolean expression is false, program continues here.

9 - 4

while Loop in Elevator

public void setFloor() {

//Normally you would pass the desiredFloor as an argument to the

//setFloor method. However, because you have not learned how to

//do this yet, desiredFloor is set to a specific number (5)

// below.			If the boolean
			expression returns
int desiredFloor = 5;			true, execute the
			while loop.
while (	currentFloor != desiredFloor	){	while loop.
while (	currentFloor != desiredFloor	){
if (currentFloor < desiredFloor) {
goUp();
}
else {
goDown();
}
}
}

9 - 5

Types of Variables

public class Elevator {

public boolean doorOpen=false; public int currentFloor = 1; public final int TOP_FLOOR = 10; public final int BOTTOM_FLOOR = 1;

... < lines of code omitted > ...

public void setFloor() {
int desiredFloor = 5;	Local variable

while ( currentFloor != desiredFloor ){ if (currentFloor < desiredFloor) {

goUp(); } else {

goDown();

}

} // end of while loop } // end of method

}// end of class

Instance variables (fields)

Scope of desiredFloor

9 - 6

while Loop: Example 1

Example:

float square = 4; // number to find sq root of float squareRoot = square; // first guess

while (squareRoot * squareRoot - square > 0.001) { // How accurate? squareRoot = (squareRoot + square/squareRoot)/2; System.out.println("Next try will be " + squareRoot);

}

System.out.println("Square root of " + square + " is " + squareRoot);

Result:

Next try will be 2.5

Next try will be 2.05

Next try will be 2.0006099

Next try will be 2.0

The square root of 4.0 is 2.0

9 - 7

while Loop: Example 2

Example:

int initialSum = 500;
int interest = 7;			// per cent		Check if money has
int years = 0;					doubled yet.
int currentSum = initialSum * 100; // Convert to pennies
while (		currentSum <= 100000		) {		If not doubled,
						add another
	currentSum += currentSum * interest/100;					add another
	currentSum += currentSum * interest/100;
	years++;					year’s interest.
	years++;
	System.out.println("Year " + years + ": " + currentSum/100);
}

Result:

... < some results not shown > ...

Year 9: 919

Year 10: 983

Year 11: 1052

The while loop iterates 11 times before the boolean test evaluates to true.

9 - 8

while Loop with Counter

Example:		Declare and
System.out.println("	/*");	initialize a
System.out.println("	/*");	counter variable.
		counter variable.
int counter = 0;
while ( counter < 4 ) {		Check to see if counter
System.out.println("	*");	has exceeded 4.
System.out.println("	*");
counter ++;
}
System.out.println("	*/");	Print an asterisk and
		increment the counter.
Output:
/*
*
*
*
*
*/

9 - 9