close

 

let rid = "23456"

let index = rid.index(rid.endIndex, offsetBy: -6, limitedBy: rid.startIndex)

let positionSerialNumber = rid.substring(from: index ?? rid.startIndex)

 

(超出索引可變nil，不會crash)

 

(可取出倒數6位字串)

 

參考資料：

https://developer.apple.com/reference/swift/indexable/1779407-index

http://stackoverflow.com/questions/25113672/how-do-you-get-the-last-character-of-a-string-without-using-array-on-swift

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