Is there a way to specify argument position/index in NSString stringWithFormat?

Question

C# has syntax that allows you to specify the argument index in a string format specifier, e.g.:

string message = string.Format("Hello, {0}. You are {1} years old. How does it feel to be {1}?", name, age);

You can use arguments more than once and also omit arguments that are provided from being used. Another question mentions the same formatting for C/C++ in the form of %[index]$[format], e.g. %1$i. I have not been able to get NSString to fully respect this syntax, because it does behave well when omitting arguments from the format. The following does not work as expected (EXC_BAD_ACCESS because it attempts to dereference the age parameter as a NSObject*):

int age = 23;
NSString * name = @"Joe";
NSString * message = [NSString stringWithFormat:@"Age: %2$i", name, age];

The positional arguments are respected only if there are no missing arguments from the format (which seems to be an odd requirement):

NSString * message = [NSString stringWithFormat:@"Age: %2$i; Name: %1$@", name, age];

All these calls work properly in OS X:

printf("Age: %2$i", [name UTF8String], age);
printf("Age: %2$i %1$s", [name UTF8String], age);

Is there a way of accomplishing this using NSString in Objective-C / Cocoa? It would be useful for localization purposes.

Answer 1

NSString and CFString support reorderable/positional arguments.

NSString *string = [NSString stringWithFormat: @"Second arg: %2$@, First arg %1$@", @"<1111>", @"<22222>"];
NSLog(@"String = %@", string);

Also, see the documentation at Apple: String Resources

Answer 2

The following code fixes the bug specified in this issue. It is a workaround and renumbers the placeholders to fill gaps.

+ (id)stringWithFormat:(NSString *)format arguments:(NSArray*) arguments 
{
    NSMutableArray *filteredArguments = [[NSMutableArray alloc] initWithCapacity:arguments.count];
    NSMutableString *correctedFormat = [[NSMutableString alloc ] initWithString:format];
    NSString *placeHolderFormat = @"%%%d$";

    int actualPlaceholderIndex = 1;

    for (int i = 1; i <= arguments.count; ++i) {
        NSString *placeHolder = [[NSString alloc] initWithFormat:placeHolderFormat, i];
        if ([format rangeOfString:placeHolder].location != NSNotFound) {
            [filteredArguments addObject:[arguments objectAtIndex:i - 1]];

            if (actualPlaceholderIndex != i) {
                NSString *replacementPlaceHolder = [[NSString alloc] initWithFormat:placeHolderFormat, actualPlaceholderIndex];
                [correctedFormat replaceAllOccurrencesOfString:placeHolder withString:replacementPlaceHolder];    
                [replacementPlaceHolder release];
            }
            actualPlaceholderIndex++;
        }
        [placeHolder release];
    }

    if (filteredArguments.count == 0) {
        //No numbered arguments found: just copy the original arguments. Mixing of unnumbered and numbered arguments is not supported.
        [filteredArguments setArray:arguments];
    }

    NSString* result;
    if (filteredArguments.count == 0) {
        //Still no arguments: don't use initWithFormat in this case because it will crash: just return the format string
        result = [NSString stringWithString:format];
    } else {
        char *argList = (char *)malloc(sizeof(NSString *) * [filteredArguments count]);
        [filteredArguments getObjects:(id *)argList];
        result = [[[NSString alloc] initWithFormat:correctedFormat arguments:argList] autorelease];
        free(argList);    
    }

    [filteredArguments release];
    [correctedFormat release];

    return result;
}

Answer 3

After doing more research, it appears Cocoa respects positional syntax in printf. Therefore an alternate pattern would be:

char msg[512] = {0};
NSString * format = @"Age %2$i, Name: %1$s"; // loaded from resource in practice
sprintf(msg, [format UTF8String], [name UTF8String], age);
NSString * message = [NSString stringWithCString:msg encoding:NSUTF8StringEncoding];

However, it would be nice if there was an implementation of this on NSString.