NSURL file extension addition etc

I have received a URL from the savePanel sheet and I would like to do following:

• Check to see if it has an extension
• if it does, remove it
• add a custom extension
• if it does not, then add a custom extension
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Any simple way to do this.. I tried something like following but it does not work

if ( [tmp pathExtension] != @"xxx" )
   path = [tmp stringByAppendingFormat:@"xxx"];

OK... A possible solution as follows

NSString *path;
NSURL *filepath;
fileurl = [sheet URL];
fileurl = [fileurl URLByDeletingPathExtension];
fileurl = [fileurl URLByAppendingPathExtension:@"yyy"];
path = [fileurl path];

This can be achieved using NSString's methods. Note that for string comparison, you have to use isEqualToString:, not ==, which tests for pointers equality.

About the extension use : -(NSString *)pathExtension;. To remove the extension use -(NSString *)stringByDeletingPathExtension;.

In all cases to add an extension, compose a new string using for example : +(NSString *)stringWithFormat:.

So :

NSString *finalString;
if([[tmp pathExtension] isEqualToString:@"xxx"]) {
    finalString = [tmp stringByDeletingPathExtension];
}
finalString = [NSString stringWithFormat:@"%@.yyy", finalString];

An alternate approach to the workable solution given on this same page by @user756245 but using different NSString methods:

NSString *finalString;
if([[tmp pathExtension] isEqualToString:@"xxx"]) {
    finalString = [tmp stringByDeletingPathExtension];
}

finalString = [finalString stringByAppendingPathExtension:@"yyy"];

Here is an update for Swift 4.1 using URL

// assuming you are building an URL from string
let url = URL(string: "file.abc")!
let finalUrl = url.deletingPathExtension().appendingPathExtension("mp3")
let finalString = finalUrl.path // output "file.mp3"