The last loop statement: the do-statement
- The do-statement is different from the while-statement and the for-statement in that:
- Due to the different ordering in the test/execute steps, the do-statement is rarely used in computer algorithms
  Because in most computer algorithms that have been developed, the algorithm
  - first test the condition and
  - execute the statements in the body only when the condition is satisfied

Syntax and meaning of the do-statement

The general syntax of the do-statement is as follows:

Explanation:

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

Following the keyword do, you can write (only) one statement

This is the body of the do-statement
The body will be execute once before the LOOP-CONTINUATION-CONDITION is tested.
Furthermore, the body will be executed as long as the loop-continuation-condition is true

The keyword while and a conditional clause ( LOOP-CONTINUATION-CONDITION ) follow the body

Note:

The execution of the do-statement can be explained more clearly with a flow chart (discussed in the next topic).

As usual, the body of a do-statement will usually be a statement block.
A typical do-statement will look like this:

Flow chart of a do-statement

Execution of the do-statement:

The computer executes the statements in the body of the do-statement once (without testing the loop-continuation-condition)
Then it tests the loop-continuation-condition
If loop-continuation-condition is true, the do-statement is repeated (from the beginning)
Otherwise, the do-statement is terminated and the execution continues with the statement that follows the do-statement.

Example:

a = 1; do { System.out.println(a); a++; } while ( a <= 4 ) System.out.println("Done");

Output: 1 2 3 4 Done

Flow chart of this program:

Structure diagram of a do-statement

Structure diagram representing a do-statement:

How to "read" the diagram:

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

Example:

a = 1; do { System.out.println(a); a++; } while ( a <= 4 ) System.out.println("Done");

Structure diagram of this program:

Difference in behavior between do-statement and while-statement

The difference between a do-statement and a while-statement is the first iteration:

The do-statement will always execute the body once, even if the loop-continuation-condition is false in the first iteration
In contrast, if the loop-continuation-condition is false in the first iteration, the while-statement will terminate without executing the body.

Example:

While-statement	Do-statement
a = 10; while ( a <= 4 ) { System.out.println(a); a++; } System.out.println(a); // prints 10	a = 10; do { System.out.println(a); a++; } while ( a <= 4 ) ; System.out.println(a); // Prints 11
The while-statement exits immediately The variable `a` contains the value 10 (i.e., *unchanged) after the while-statement*	The *body* of the do-statement is executed once The variable `a` contains the value 11 (changed by `a++;`) after the do-statement

Re-writing a do-statement using a while-statement

The do-statement is not necessary in a programming language
It is possible (but clumpsy) to re-write a do-statement using a while-statement

How to re-write a do-statement using a while-statement:

Explanation:

Because the body of the do-statement is executed before testing the loop-continuation-condition, we place a copy of the body before the while-loop
This will cause the body to be executed once before testing the loop-continuation-condition
The while-statement will executed the body as long as the loop-continuation-condition is true
Therefore, we can replaced the do-statement using a while-statement
However, we have duplicated the statements in body (which is very clumpsy)

Break and continue statement used in a do-statement

The effect of the break and the continue statement used inside a do-statement is the same as inside a while-statement:

The break statement will cause the do-statement to terminate immediately:
The execution will continue with the statement following the do-statement.
The continue statement will cause the do-statement to jump to the end of the body
The execution will continue with testing the loop-continuation-condition.
If the condition is true, the do-statement is repeated, and otherwise (if the condition is false), the do-statement is terminated.