Swift - String removal code not performing as expected

Question

I'm working on a macOS Swift-app where I need to perform folder/file traversal. In this specific instance, I need to remove the first part of a file path...i.e., if a file has the path of MyFolder/MyItem, I need it to just read as MyItem for display purposes.

Based off the responses in this answer, I wrote the following code:

if fileString.hasPrefix("/") {
    fileString.remove(at: fileString.startIndex)
    print(fileString)
}

...where I remove any part of the fileString before and including "/".

However, this doesn't seem to work in practice....MyFile/MyItem doesn't get changed to MyItem, it remains as MyFile/MyItem.

The code you posted would change `/MyFile/MyItem` to `MyFile/MyItem`. Why do you expect your code to remove a `/` in the middle? — rmaddy, Jan 04 '18 at 19:50
Swift doesn't modify your string, I believe it creates a new one. Try var s = (your line) and then see what s has — Stephen J, Jan 04 '18 at 19:52
@rmaddy I see, makes sense. How would I then remove "/" and whatever is before it? — narner, Jan 04 '18 at 20:12
use the substring after getting the index of the /. Just remember Swift does copy-on-change — Stephen J, Jan 04 '18 at 20:26
Actually, the `remove(at:)` method is a mutating function: it _will_ modify the value of fileString in-place, without requiring you to re-assign the variable. In fact, the return value of that method is _not_ the modified string, but the character that was removed from the string. Copy-on-write is an important concept in Swift but it's not really relevant to this question. — Mike Mertsock, Jan 04 '18 at 22:20

Martin R · Accepted Answer · 2018-01-04T21:07:07.420

You can locate the first (or last) slash character and remove everything from the beginning of the string up to and including that separator, for example:

var fileString = "My Volume/My Folder/My Item"
if let r = fileString.range(of: "/", options: .backwards) {
    fileString.removeSubrange(..<r.upperBound)
}
print(fileString) // My Item

But if your intention is to extract the file name (last path component) for display then there is a dedicated method for that purpose:

let fileString = "My Volume/My Folder/My Item"
let displayName = FileManager.default.displayName(atPath: fileString)
print(displayName) // My Item

Another method is

let fileString = "My Volume/My Folder/My Item"
let baseName = URL(fileURLWithPath: fileString).lastPathComponent
print(baseName) // My Item

which is similar to the previous one, but does not localize the file name.

Swift - String removal code not performing as expected

1 Answers1