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

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:@[@"XXX", @"YYY"]];
NSLog( @"%@", s );

Unfortunately, for 64-bit, the va_list format has changed, so the above code no longer works. And probably should not be used anyway given it depends on the format that is clearly subject to change. Given there is no really robust way to create a va_list, a better solution is to simply limit the number of arguments to a reasonable maximum (say 10) and then call stringWithFormat with the first 10 arguments, something like this:

+ (id)stringWithFormat:(NSString *)format array:(NSArray*) arguments
{
    if ( arguments.count > 10 ) {
        @throw [NSException exceptionWithName:NSRangeException reason:@"Maximum of 10 arguments allowed" userInfo:@{@"collection": arguments}];
    }
    NSArray* a = [arguments arrayByAddingObjectsFromArray:@[@"X",@"X",@"X",@"X",@"X",@"X",@"X",@"X",@"X",@"X"]];
    return [NSString stringWithFormat:format, a[0], a[1], a[2], a[3], a[4], a[5], a[6], a[7], a[8], a[9] ];
}

Answer 2

Based on this answer using Automatic Reference Counting (ARC): https://stackoverflow.com/a/8217755/881197

Add a category to NSString with the following method:

+ (id)stringWithFormat:(NSString *)format array:(NSArray *)arguments
{
    NSRange range = NSMakeRange(0, [arguments count]);
    NSMutableData *data = [NSMutableData dataWithLength:sizeof(id) * [arguments count]];
    [arguments getObjects:(__unsafe_unretained id *)data.mutableBytes range:range];
    NSString *result = [[NSString alloc] initWithFormat:format arguments:data.mutableBytes];
    return result;
}

Answer 3

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.