How to create a NSString from a format string like @"xxx=%@, yyy=%@" and a NSArray of objects?

Question

Is there any way to create a new NSString from a format string like @"xxx=%@, yyy=%@" and a NSArray of objects?

In the NSSTring class there are many methods like:

- (id)initWithFormat:(NSString *)format arguments:(va_list)argList
- (id)initWithFormat:(NSString *)format locale:(id)locale arguments:(va_list)argList
+ (id)stringWithFormat:(NSString *)format, ...

but non of them takes a NSArray as an argument, and I cannot find a way to create a va_list from a NSArray...

Answer 1

A va_list isn't really an argument so much as an indicator that it actually takes a variable number of arguments. So if you have 5 parameters in your string, just pass them as 5 separate arguments.

Answer 2

I found some code on the web that claims that this is possible however I haven't managed to do it myself, however if you don't know the number of arguments in advance you also need to build the format string dynamically so I just don't see the point.

You better off just building the string by iterating the array.

You might find the stringByAppendingString: or stringByAppendingFormat: instance method handy .

Answer 3

No, you won't be able to. Variable argument calls are solved at compile time, and your NSArray has contents only at runtime.

Answer 4

There is no general way to pass an array to a function or method that uses varargs. In this particular case, however, you could fake it by using something like:

for (NSString *currentReplacement in array)
    [string stringByReplacingCharactersInRange:[string rangeOfString:@"%@"] 
            withString:currentReplacement];

Answer 5

@Chuck is correct about the fact that you can't convert an NSArray into varargs. However, I don't recommend searching for the pattern %@ in the string and replacing it each time. (Replacing characters in the middle of a string is generally quite inefficient, and not a good idea if you can accomplish the same thing in a different way.) Here is a more efficient way to create a string with the format you're describing:

NSArray *array = ...
NSAutoreleasePool *pool = [NSAutoreleasePool new];
NSMutableArray *newArray = [NSMutableArray arrayWithCapacity:[array count]];
for (id object in array) {
    [newArray addObject:[NSString stringWithFormat:@"x=%@", [object description]]];
}
NSString *composedString = [[newArray componentsJoinedByString:@", "] retain];
[pool drain];

I included the autorelease pool for good housekeeping, since an autoreleased string will be created for each array entry, and the mutable array is autoreleased as well. You could easily make this into a method/function and return composedString without retaining it, and handle the autorelease elsewhere in the code if desired.

Answer 6

One solution that came to my mind is that I could create a method that works with a fixed large number of arguments like:

+ (NSString *) stringWithFormat: (NSString *) format arguments: (NSArray *) arguments {
    return [NSString stringWithFormat: format ,
          (arguments.count>0) ? [arguments objectAtIndex: 0]: nil,
          (arguments.count>1) ? [arguments objectAtIndex: 1]: nil,
          (arguments.count>2) ? [arguments objectAtIndex: 2]: nil,
          ...
          (arguments.count>20) ? [arguments objectAtIndex: 20]: nil];
}

I could also add a check to see if the format string has more than 21 '%' characters and throw an exception in that case.

Answer 7

Yes, it is possible. In GCC targeting Mac OS X, at least, va_list is simply a C array, so you'll make one of ids, then tell the NSArray to fill it:

NSArray *argsArray = [[NSProcessInfo processInfo] arguments];
va_list args = malloc(sizeof(id) * [argsArray count]);
NSAssert1(args != nil, @"Couldn't allocate array for %u arguments", [argsArray count]);

[argsArray getObjects:(id *)args];

//Example: NSLogv is the version of NSLog that takes a va_list instead of separate arguments.
NSString *formatSpecifier = @"\n%@";
NSString *format = [@"Arguments:" stringByAppendingString:[formatSpecifier stringByPaddingToLength:[argsArray count] * 3U withString:formatSpecifier startingAtIndex:0U]];
NSLogv(format, args);

free(args);

You shouldn't rely on this nature in code that should be portable. iPhone developers, this is one thing you should definitely test on the device.

Answer 8

It is actually not hard to create a va_list from an NSArray. See Matt Gallagher's excellent article on the subject.

Here is an NSString category to do what you want:

@interface NSString (NSArrayFormatExtension)

+ (id)stringWithFormat:(NSString *)format array:(NSArray*) arguments;

@end

@implementation NSString (NSArrayFormatExtension)

+ (id)stringWithFormat:(NSString *)format array:(NSArray*) arguments;
{
    char *argList = (char *)malloc(sizeof(NSString *) * [arguments count]);
    [arguments getObjects:(id *)argList];
    NSString* result = [[[NSString alloc] initWithFormat:format arguments:argList] autorelease];
    free(argList);
    return result;
}

@end

Then:

NSString* s = [NSString stringWithFormat:@"xxx=%@, yyy=%@" array:[NSArray arrayWithObjects:@"XXX", @"YYY", nil]];
NSLog( @"%@", s );