Question

I have a (to me) curious case with NSPredicate’s predicateWithFormat: method.

Using the following I log the description of two NSPredicate instances to the console:

NSNumber *myNumber = [NSNumber numberWithInt:1];

NSString *predicateFormatByHand = [NSString stringWithFormat:@"self MATCHES 'chp%@_img[0-9]+\\.png'", myNumber];
NSPredicate *firstPredicate = [NSPredicate predicateWithFormat:predicateFormatByHand];

NSLog(@"firstPredicate description: %@", firstPredicate);

NSPredicate *secondPredicate = [NSPredicate predicateWithFormat:@"self MATCHES 'chp%@_img[0-9]+\\.png'", myNumber];

NSLog(@"secondPredicate description: %@", secondPredicate);

This outputs:

 firstPredicate description: SELF MATCHES "chp1_img[0-9]+.png"
secondPredicate description: SELF MATCHES "chp%@_img[0-9]+.png"

I would expect these descriptions to be the same.

Can someone explain why they are not?

(Following this question I’ve played with various escape sequences for the embedded single-quotes but when doing so keep having NSPredicate complain that it cannot then parse the format string. I’d be grateful to know what’s going on.)

UPDATE: one answer suggested it’s an issue with using NSNumber rather than an int, so:

NSPredicate *thirdPredicate = [NSPredicate predicateWithFormat:@"self MATCHES 'chp%d_img[0-9]+\\.png'", [myNumber intValue]];

NSLog(@"thirdPredicate description: %@", thirdPredicate);   

I began with this originally, but alas the output is the same:

thirdPredicate description: SELF MATCHES "chp%d_img[0-9]+.png"

(Something means the format specifier is not evaluated.)

Answer 1

The answer is simple: the parser used by NSPredicate assumes that anything inside the quote marks is a string literal, and does not attempt to do any substitutions on its contents. It you need to have a dynamic string value, you will have to build the string before substituting it into the predicate format string, as in your first example.