The while loop
allows programs to repeat a statement or series of statements, over and over, as
long as a certain test condition is true. The format is shown in the box
to the right.
The test condition must be
enclosed in parentheses. The block of code is known as the body of the loop and is enclosed in braces and indented for
readability. (The braces are not required if the body is composed of only ONE statement.) Semi-colons follow
the
statements within the block only.
When a program encounters a while loop, the test condition is evaluated first. If it is TRUE, the program executes the body of the loop. The program then returns to
the test condition and reevaluates. If still TRUE,
the body executes again. This cycle of testing and execution continues
until the test condition evaluates to FALSE. If you want the loop
to eventually terminate, something within the body of the loop must affect the test
condition. Otherwise, a disastrous INFINITE
LOOP is the result! In Eclipse, you can cancel (stop) an infinite loop by clicking on the Terminate button on the right side of the Console window.
The while loop is an entry-condition loop. If the test
condition is FALSE to begin
with, the program never executes the body of the loop.
Similar to the "if" statement, any variable declared in the block for a while loop has block scope. They will not be recognized by coding outside of the block
The
following program fragment prints the characters of a string (one per line) and counts the total number of characters of the string:
String name="Corky";
int i = 0; //begin with the first cell of string
while (i < name.length( )) // loop test condition
{
//print each cell on a new line
System.out.println(name.charAt(i));
i++;
//increment counter by 1
}
System.out.println("There are "+ i + " characters.");
/*This last statement prints when the test condition is
false and the loop terminates - finishes.*/ |
