If a user must express a choice that requires a numeric value within specific boundaries, give her a control that intrinsically communicates those limits and prevents her from entering a value outside of the boundaries.

• 如果程序需要一个必须保持在指定界限内的数值，呈现给用户的控件应该表明那些限制。 并且阻止用户输入界限以外的值，滑块控件就是这样。