为什么我的Swift循环失败，出现错误“Can not form range with end<启动"？ [英] Why is my Swift loop failing with error "Can't form range with end < start"?

问题描述

我有一个for循环来检查一个数是否是一个数的因子，然后检查这个因子是否为素数，然后把它加到数组中。根据原来的号码，我会得到一个错误说：

致命错误：无法形成范围结束< / p>开始

几乎每次都发生这种情况，但是对于一些数字来说，它可以正常工作。我找到的唯一数字是9,15和25。

这里是代码：

  let num = 16 //或任何Int 
 var primes = [Int]（）
 
 for i in 2 ...（num / 2）{
 
 if（（num％i）== 0）{
 var isPrimeFactor = true 
 
 for l in 2 ... i-1 {（
 if（（i％l）== 0）{
 isPrimeFactor = false; 
} // end if 
} // end for 
 
 if（isPrimeFactor == true）{
 primes.append（i）
} / / end if 
 
 // end if 
 $ b} // end for 
  

$ b $如果你需要一个动态值范围的循环，我建议使用 stride（to：by：）而不是 ..< 或 ...

基本上 ..< 或 ... 会在 start_index时崩溃> end_index 。

例如：

 让我们在10 ... k {print（i = \（i））} 
为i中的10，k = 5 
。< k {print（i = \\（i））} 
  

如何解决：

  // swift 2.3 
 let k = 5 
 for 10.stride（to：k + 1，by：1）{print i = \（i））} 
 for 10.stride（to：k，by：1）{print（i = \（i））} 
 
 // swift 3.0 
 let k = 5 
 for my stride（from：10，through：k，by：1）{print（i = \（i））} 
为我的步伐（从10到k：1）{print（i = \（i））} 
  以上的2个块不会打印出任何东西，但其中一个会在执行时崩溃。
 
  
 
 
参考： 
 
 
 
 
  http://michael-brown.net/2016/using-stride-to-convert-c-style -for-loops-to-swift-2.2 /  
 
 
 < a href =http://swiftdoc.org/v2.1/protocol/Strideable/ =noreferrer> http://swiftdoc.org/v2.1/protocol/Strideable/  
 
 
 
I have a for loop that checks if a number is a factor of a number, then checks if that factor is prime, and then it adds it to an array.  Depending on the original number, I will get an error saying

  
fatal error: Can't form range with end < start
This happens almost every time, but for some numbers it works fine.  The only numbers I have found to work with it are 9, 15, and 25.

Here is the code:
let num = 16 // or any Int
var primes = [Int]()

for i in 2...(num/2) {

    if ((num % i) == 0) {
        var isPrimeFactor = true

        for l in 2...i-1 {
            if ((i%l) == 0) {
                isPrimeFactor = false;
            }//end if
        }//end for

        if (isPrimeFactor == true) {
            primes.append(i)
        }//end if

    }//end if

}//end for

解决方案
If you need a loop with dynamic value-range, I suggest that using stride(to:by:) instead of ..< or ...

Basically ..< or ... will be crashed if start_index > end_index.

Ex:
let k = 5
for i in 10...k { print("i=\(i)") }
for i in 10..<k { print("i=\(i)") }
How to fix:
// swift 2.3
let k = 5
for i in 10.stride(to: k+1, by: 1) { print("i=\(i)") }
for i in 10.stride(to: k, by: 1) { print("i=\(i)") }

// swift 3.0
let k = 5
for i in stride(from: 10, through: k, by: 1) { print("i=\(i)") }
for i in stride(from: 10, to: k, by: 1) { print("i=\(i)") }
Note: 2 blocks above won't print out anything, but one of them will be crashed when execute.

Reference: 


http://michael-brown.net/2016/using-stride-to-convert-c-style-for-loops-to-swift-2.2/
http://swiftdoc.org/v2.1/protocol/Strideable/


                        
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