当迭代次数未知时, 将While和Repeat while循环用作for-in循环的替代方法。 while循环执行一组语句, 直到出现错误条件为止。当你不知道迭代次数时, 通常使用此循环。
Swift中有两种类型的循环:
- while循环
- 重复While循环
Swift的While循环
Swift while循环在每次传递开始时评估其条件。
句法:
while (TestExpression) {
// statements
}
在这里, TestExpression是一个布尔表达式。如果是真的
- 在while循环内的语句被执行。
- 并且, 再次对TestExpression求值。
这个过程一直进行到TestExpression的值为假。当TestExpression获得错误条件时, while循环终止。
While循环流程图
例:
var currentLevel:Int = 0, finalLevel:Int = 6
let gameCompleted = true
while (currentLevel <= finalLevel) {
//play game
if gameCompleted {
print("You have successfully completed level \(currentLevel)")
currentLevel += 1
}
}
//outside of while loop
print("Terminated! You are out of the game ")
输出
You have successfully completed level 0
You have successfully completed level 1
You have successfully completed level 2
You have successfully completed level 3
You have successfully completed level 4
You have successfully completed level 5
You have successfully completed level 6
Terminated! You are out of the game
在上面的程序中, 执行while循环, 直到条件被评估为false为止, 并在获得false条件后立即终止。