一個普通的 while 循環由 while 關鍵字開始，后跟一個開口的圓括號 (，其后是條件表達式和一個閉口的圓括號 ) 。然后跟一個循環體表達式： > 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; ~~~ 條件表達式必須為 Bool 類型。 > The condition expression has to be of type Bool. 對應每次迭代，條件表達式都會執行。如果它執行結果為 false ，循環則停止，否則執行循環體。 > 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(); } } } ~~~ 這類的while循環不保證一定執行循環體表達式：如果條件從開始就不匹配，它不會執行。這和 do-while循環（第5.15節）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).