Loops - while
A while loop is a control structure for repeating a block of code as long as the condition given is true.
The definition has 3 parts:
- keyword while
- a condition enclosed in parentheses
- 1 or more statements enclosed in curly brackets
This definition is very similar to the if statement. The key difference is that with if, the code block is run 0 or exactly 1 time. With while, the code block is run 0 or more times.
Integer i = 1;
while (i < 10){
printLine(i.toString());
i++;
}
In this example, the counter i must be less than 10. The loop will be repeated a total of 9 times, printing the numbers 1 through 9 in ascending order.
Condition
The condition determines if the code inside the block will be run or not. It is tested at the beginning of each pass through the loop. If it evaluates to true, the enclosed statements are run. If it evaluates to false, the loop will terminate and we go to the 1st statement after the loop.
You can use both conditional and logical operators and also statements that evaluate to true or false.
Warning
It is crucial that you update the counter! If not, your loop will run forever (or until it crashes the browser).
Usage
You should use a while loop when you don't know the number of iterations ahead of time.
It is also very useful when the counter updates don't follow a fixed pattern such as ++. For example, when you receive a group sign-up, the new number of attendees must reflect the size of the group.