很简洁的用法： index < rating return true or false
for (index, button) in ratingButtons.enumerate() {
// If the index of a button is less than the rating, that button should be selected.
button.selected = index < rating
}
大大简化了语句，可读性很强。对比老写法：
button.selected = false
if index < rating {
button.selected = true
}
or button.selected = (index < rating)? true:false
This code iterates through the button array to set the state of each button according to whether its index in the array is less than the rating. If it is, index < rating evaluates to true, which sets the button’s state to selected and makes it display the filled-in star image. Otherwise, the button is unselected and shows the empty star image.
Swift 另类判断语句用法 button.selected = index < rating
原文：http://www.cnblogs.com/iyoubo/p/4615048.html