Swift String.firstIndex(of: "\n") returns nil even though string has \n

Question

I'm confused by what's happening here. This should be simple. I'm checking for the first index of a newline character, but it's returning nil. I've inspected the string in the debugger and it has \r\n at the end of the string.

rfidstring = {String} "900083000020758\r\n"

if rfidstring.firstIndex(of: "\n") != nil {
    print("rfid string has \\n")
}

Am I making some kind of silly mistake and just totally missing it here? When I call contains("\n") that works correctly and returns true. So it knows there is a \n in the string when I'm calling contains() but doesn't think there is one when I call firstIndex(of:)?

// but ...
rfidstring.contains("\n") // returns true

I'm receiving data from an inputstream as I read from an RFID reader, and the reader sends one rfid per line. So to make sure that I've read the entire rfid (sometimes it'll send it in two blocks of data) I need to read to the end of the line. That's why I'm trying to find the newline.

UPDATE: This seems weird to me:

rfidstring.firstIndex(of: "\r\n") // returns valid index: 15[utf8]

rfidstring.firstIndex(of: "\r") // returns nil
rfidstring.firstIndex(of: "\n") // returns nil

Are you sure [`firstIndex`](https://developer.apple.com/documentation/swift/string/firstindex(of:)) is the method you want? — tadman, May 19 '23 at 19:44
@tadman I believe so. It's always been very common when doing these things to use index(of:) but that has been deprecated and replaced with firstIndex(of:). Why wouldn't I want to use firstIndex(of:)? — Kenny Wyland, May 19 '23 at 19:51
@LeoDabus What? I've always thought that Carriage Return / CR / \r was one character and Line Feed / LF / \n was one character. — Kenny Wyland, May 19 '23 at 19:59
what you actually need is probably `firstIndex(where: \.isNewline)` — Leo Dabus, May 19 '23 at 20:02
@LeoDabus Hm. I'm really not happy that `"\r\n".count = 1` AND `"\r".count = 1` AND `"\n".count = 1`. But, yes, thank you for the help and the `where: \.isNewline` seems like the right conceptual approach. — Kenny Wyland, May 19 '23 at 20:07

score 2 · Accepted Answer · answered May 19 '23 at 20:41

See this question: Difference between \n and \r?.

TL;DR: \r\n is the newline character on Windows and since Swift strings count characters the way a human perceives them and not in individual bytes, you need to search for the combo character. Or any newline using a predicate instead: rfidstring.firstIndex(where: \.isNewline)

Swift String.firstIndex(of: "\n") returns nil even though string has \n

1 Answers1