The `while`-statement

The loop statements in Java

What is a loop-statement:

A loop-statement is a statement that repeatedly executes statements contained in its body as long as a loop conditional is satisfied.

I.e.: the loop-statement is executed until the loop condition is not valid

Examples:

/* --------------------------------------------------------- Execute a statement for as long as some condition is true --------------------------------------------------------- */ as long as ( condition is true ) execute this statement; /* --------------------------------------------------------- Execute a statement for each of the value 1, 2, ..., 10 --------------------------------------------------------- */ repeat ( for i = 1, 2, 3, .... 10 ) execute this statement;

Purpose of loop-statements:

A loop-statement can accomplish a complex task incrementally using smaller steps
The body of a loop-statement achieves only a small step towards completing a complex task
By executing the body repeatedly, the algorithm can perform the task incrementally using smaller (simpler) steps

Different loop-statements in programming languages
- Most programming languages provides 3 types of loop-statements:
- The main loop-statement is the while-statement
- The for-statement and the do-while-statement can be re-written as a while-statement (but the result can be very verbose)
- We will first study the while-statement

Syntax and meaning of the while-statement

Syntax of the while-statement:

/* ---------------------------------- Meaning of the while-statement: ---------------------------------- */ as long as ( loop-continuation-condition is true ) execute ONE-statement;

Explanation:

The keyword while announces (to the Java compiler) that we started an while-statement

A conditional clause ( LOOP-CONTINUATION-CONDITION ) follows the keyword while

The LOOP-CONTINUATION-CONDITION is a Boolean expression (exactly the same as in the condition clause of an if-statement)
This is the condition of the while-statement

Following the loop-continuation-condition clause, you can write (only) one statement

Common practice in loop-statements
- Fact:
- Common practice in while-loops:
- A typical while-loop looks like this:

Example while-statement: print the numbers 1 through 10 (board)

Consider the following Java program:

public class While01 { public static void main(String[] args) { int a; a = 1; while ( a <= 10 ) // While-statement { System.out.println(a); // Print a a++; // Increment a } System.out.println("Done"); System.out.println("Exit: a = " + a); } }

Execution of this Java program:

Program begins:
The program reserve memory space for the variable a
Then executes the assignment statement:
Stores the value 1 into the variable a
Then executes the while-statement.
First, the loop-continuation-condition is tested:
The result is true and the while-body is executed

Execution of the while-statement's body:

System.out.println(a);
a++;
When the execution of the while-body is completed:
the while-statement resumes from the start again !!!

The execution the while-statement continues.
First, the loop-continuation-condition is tested again:
The result is true and the while-body is executed

Execution of the while-statement's body:

System.out.println(a);
a++;
When the execution of the while-body is completed:
the while-statement resumes from the start again !!!

And so on...
The while statement will continue executing until a = 11 because then the loop-continuation-condition a ≤ 10 becomes false (which will cause the while statement to terminate).
This while-statement will loop 10 times !
Result:
The number 1, 2, ..., 10 will be printed (and the variable a will have the value 11).

Example Program: (Demo above code)
- Prog file: click here
How to run the program:

Output:

1 2 3 4 5 6 7 8 9 10 Done Exit: a = 11

Flow chart of a while-statement

While-statement:
Flow chart representing a while-statement:

How to "read" the flow chart:

When the loop-cont-condition is true, the execution takes the downward branch:
When the loop-cont-condition is false, the execution takes the side branch

Example:

a = 1; while ( a <= 10 ) { System.out.println(a); a++; } System.out.println("Done");

Flow chart of this program:

Execution:

Structure diagram of a while-statement

While-statement:

Structure diagram representing a while-statement:

How to "read" the diagram:

The outer rectangle containing the LOOP-CONTINUATION-CONDITION represents the while-statement:
The while-statement is one (single) statement)
The while-statement will only complete execution when the LOOP-CONTINUATION-CONDITION becomes false
The inner rectangle (shaded with a green color) is the while-loop body
The statements in the inner rectangle are executed when the LOOP-CONTINUATION-CONDITION is true

Example:

a = 1; while ( a <= 10 ) { System.out.println(a); a++; } System.out.println("Done");

Structure diagram of this program:

The while-statement will keep on executing as long as a ≤ 10
As long as a ≤ 10, the while-loop executes these statements:


Program "loops" as long as `a ≤ 10`	Program exits loop when `a > 10`

The while-statement

The `while`-statement