Quick reference to Java Loop Control

Question

Need of Loops

A computer program is a set of statements , which are usually executed sequentially. However, in certain situations it is necessary to repeat certain steps to meet a specified condition.

For example, if the user wants to write a program that calculates and display the sum of the first 10 numbers 1,2,3,...,10.

On way to calculate the same is as follows:

1+2=3

3+3+=6

6+4=10

10+5=15

……

and so on.

This Technique is suitable for relatively small calculations. However, if the program requires adding the first 200 numbers, it would b tedious to add up all the numbers from 1 to 200 using the above mentioned technique. In such situations, iterations or loops come to our rescue.

Definition and Types

A loop comprises a statement or a block of statements that are executed repeatedly untill a particular condition evaluates to true or false.

Loop enable programmers to develop concise programs, which otherwise would require thousands of program statements.

The loop statements supported by java programming language are:

• While

• Do-while

• For

While Statement

The while statement in java is used to execute a statement or a block of statements while a particular condition is true. The condition is checked before the statement are executed. The condition for the while statement can be any expression which returns a boolean value.

Syntax:

while (expression)

{

//one or more statements

}

Rules

 The following points should be notes when using the while statement.

• The values of the variables used in the expression must be set at some point before the while loop is reached. This process is called the initialization of variables and has to be performed once before the execution of the loop.

• The body of the loop has an expression that changes the value of the variable that is a part of the loop’s expression. A variable is said to be incremented if its value increases in the body of the loop and is said to be decremented if its value decreases.

Do-while Statement

The do-while statement check the condition at the end of the loop rather than at the beginning to ensure that the loop is executed at least once. The condition of the do-while statement usually comprises of an expression that evaluates the boolean value.

Syntax:

do

{

Statement(s);

} while (expression);

For statement

The for statement is similar to the while statements in its function. The statements within the body of the loop are executed as long as the condition is true. Here too, the condition is checked before the statements are executed.

Syntax:

for(initialization; condition; Increment/decrement)

{

//one or more statements

}

Nested Loop

The placing of a loop in the body of another loop is called nesting. For example, a while statement can be enclosed within a do-while statement and a for statement can be enclosed within a while statement. When you nest two loops, the outer loop controls the number of times the inner loop is executed. For each iteration of the outer loop the inner loop will execute all its iterations. There can be any number of combinations between the three loops. While all types of loop statements may be nested, the most commonly nested loops are formed by for statements.

Syntax

for (initialization; condition; Increment/decrement)

{

for (initialization; condition; Increment/decrement)

{

//statements;

}

}

Here is a difference between while/for and do-while loops:

While/for	Do-while
Loop is pre-tested. The condition is checked before the statements within the loop are executed	Loop is post-tested. The condition is checked after the statements within the loop are executed.
The loop does not get executed if the condition is not satisfied at the beginning.	The loop gets executed at least once even if the condition is not satisfied at the beginning.

For more watch this tutorial video: