一个普通的 while 循环由 while 关键字开始，后跟一个开口的圆括号 (，其后是条件表达式和一个闭口的圆括号 ) 。然后跟一个循环体表达式： > A normal while loop starts with the while keyword, followed by an opening parenthesis (, the condition expression and a closing paranthesis ). After that follows the loop body expression: ~~~ while(condition) expression; ~~~ 条件表达式必须为 Bool 类型。 > The condition expression has to be of type Bool. 对应每次迭代，条件表达式都会执行。如果它执行结果为 false ，循环则停止，否则执行循环体。 > Upon each iteration, the condition expression is evaluated. If it evaluates to false, the loop stops, otherwise it evaluates the loop body expression. ~~~ class Main { static public function main() { var f = 0.0; while (f < 0.5) { trace(f); f = Math.random(); } } } ~~~ 这类的while循环不保证一定执行循环体表达式：如果条件从开始就不匹配，它不会执行。这和 do-while循环（第5.15节）不同。 > This kind of while-loop is not guaranteed to evaluate the loop body expression at all: If the condition does not hold from the start, it is never evaluated. This is different for do-while loops (5.15).