do-while 循环由 do 关键字开始，后跟循环体表达式。其后跟while 关键字，然后是一个 开口的圆括号 (，再跟条件表达式和一个闭口的圆括号 ) ：
A do-while loop starts with the do keyword followed by the loop body expression. After that follows the while keyword, an opening parenthesis (, the condition expression and a closing parenthesis ):
do expression while(condition); 条件表达式必须为Bool类型。
The condition expression has to be of type Bool.
如语法所暗示的，循环体表达式至少会执行一次，这和 while循环（第5.14节）不同。
As the syntax suggests, the loop body expression is always evaluated at least once, unlike while (5.14) loops.