Building an NSString based on a variadic number of arguments - ios

Question

My function takes a dictionary argument and a variadic number of NSString variables. All this combined is put in an [NSString stringWithFormat:] method, and is returned as a NSURLRequest. The method looks like this:
- (NSURLRequest *)buildPath:(NSString *)stringPath attributes:(NSString *)attribute, ...
{
va_list list;
NSString *eachObject;
NSMutableArray *args = [NSMutableArray array];
[args addObject:attribute];
va_start(list, attribute);
while ((eachObject = va_arg(list, NSString *))) {
[args addObject:eachObject];
}
va_end(list);
NSString *listOfAttributes = [args componentsJoinedByString:#", "];
NSString *pathURL = _requestString[stringPath];
NSString *path = [NSString stringWithFormat:pathURL, listOfAttributes];
NSURL *url = [NSURL URLWithString:path];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
return request;
}
This is what it looks like when I call the method:
NSURLRequest *request = [_venueService buildPath:#"categories"
attributes:_venueService.clientID, _venueService.clientSecret, _venueService.todaysDate, nil];
When I run the program, it crashes. When I log out listOfAttributes it gives me:
client_id, client_secret, 20140507
This is my 3 arguments, which is correct, and the stringPath (when I actually call it in my program I write stringPath[#"categories"]) which, when I NSLog gives me:
https://api.foursquare.com/v2/venues/categories?client_id=%#&client_secret=%#&v=%#
So, my question is, why would these two strings, combined in an [NSString stringWithFormat:] cause problems?
Any help would be greatly appreciated!

Answer 1

As Justin pointed out, there is a much simpler way of doing this. NSString has a -initWithFormat:arguments: method that does exactly what you want.
Also, your method name has a few issues:
Naming convention - you should indicate in the method name its purpose (creating a URL request)
You are passing in an (NSDictionary *) for the path, but casting it to an (NSString *) when you use it. The two objects are not type compatible. I'm supposing this might be a typo when you copy-pasted your code?
Might as well use the same calling convention as NSString's +stringWithFormat: method.
Given all of the above, the method becomes something like (without error checking):
- (NSURLRequest *)URLRequestWithFormat:(NSString *)format, ... {
va_list arguments;
va_start(arguments, format);
NSString *urlPath = [[NSString alloc] initWithFormat:format arguments:arguments];
va_end(arguments);
NSURL *url = [NSURL URLWithString:urlPath];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
return request;
}
This worked fine with a call like:
NSURLRequest *request = [self URLRequestWithFormat:#"https://api.foursquare.com/v2/venues/categories/client_id=%#&client_secret=%#&v=%#", #"One",#"Two",#"Three"];
NSLog(#"Request: %#", request);
With output:
2014-05-07 09:52:30.645 Test[5888:60b] Request: <NSURLRequest: 0x8c64f30> { URL: https://api.foursquare.com/v2/venues/categories/client_id=One&client_secret=Two&v=Three }

Answer 2

You may want to read the documentation for -[NSString initWithFormat:arguments:]. That method accepts a va_list parameter and will probably do what you want.
The reason your sample code doesn't work is because stringWithFormat needs a separate argument for each placeholder that appears in the format string. Your format string looks like it contains three %# placeholders, but you're only passing one argument, listOfAttributes.

Answer 3

The format within stringPath is specifying that there should be 3 arguments, but you are only supplying one - listOfAttributes.
listOfAttributes is one argument not 3.