NSPredicate raises 'Unknown number type or nil passed to arithmetic function expression.' in QuickLook plugin

Question

I have an NSArray of NSDictionary. All the dictionaries have a key image-path.
I want to get a filtered array of only the dictionaries that match a given value (content of my path variable) for the key image-path.

I validated the structure of the data I have using this line (it prints me all the values of the dictionaries for the key image-path):

NSLog(@"array key paths: %@", [mountedImages valueForKeyPath:@"image-path"]);

I am using this code:

NSString *path = @"value-I-want-to-match";
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(image-path LIKE[cd] \"%@\")", path];
NSArray *filteredArray = nil;
filteredArray = [mountedImages filteredArrayUsingPredicate:predicate];

The last line crashes and produces the following error:

[ERROR] Computing 6571367.19831142 raised 'Unknown number type or nil passed to arithmetic function expression.'

I am doing this in a QuickLook plugin. I can step-through my code with gdb, but I don't seem to get any trace. I only get:

[...]
[ERROR] Computing 6571367.19831142 raised 'Unknown number type or nil passed to arithmetic function expression.'
[Switching to process 23335 thread 0x8903]
[Switching to process 23335 thread 0xa703]
[Switching to process 23335 thread 0x8903]
Program ended with exit code: 0

Answer 1

You should remove the escaped quotes you placed in your predicate format string. As documented in the Apple predicate format string reference:

When string variables are substituted into a format string using %@ , they are surrounded by quotation marks. If you want to specify a dynamic property name, use %K in the format string, as shown in the following example.

 NSString *attributeName = @"firstName";
 NSString *attributeValue = @"Adam";
 NSPredicate *predicate = [NSPredicate predicateWithFormat:@"%K like %@", attributeName, attributeValue];

The predicate format string in this case evaluates to firstName like "Adam".

Single or double quoting variables (or substitution variable strings) cause %@, %K, or $variable to be interpreted as a literal in the format string and so prevent any substitution. In the following example, the predicate format string evaluates to firstName like "%@" (note the single quotes around %@).

 NSString *attributeName = @"firstName";
 NSString *attributeValue = @"Adam";
 NSPredicate *predicate = [NSPredicate predicateWithFormat:@"%K like '%@'", attributeName, attributeValue];

In your case, your predicate is being created as image-path LIKE[cd] "%@", which fails to evaluate correctly when used to filter your array.