Writing an NSPredicate that returns true if condition is not met

Question

I currently have the following piece of code

NSPredicate *pred = [NSPredicate predicateWithFormat:@"SELF contains '-'"];
[resultsArray filterUsingPredicate:pred];

This returns an array with the elements which contain '-'. I want to do the inverse of this so all elements not containing '-' are returned.

Is this possible?

I've tried using the NOT keyword in various locations but to no avail. (I didn't think it would work anyway, based on the Apple documentation).

To further this, is it possible to provide the predicate with an array of chars that I don't want to be in the elements of the array? (The array is a load of strings).

Answer 1

I'm not an Objective-C expert, but the documentation seems to suggest this is possible. Have you tried:

predicateWithFormat:"not SELF contains '-'"

Answer 2

You can build a custom predicate to negate the predicate that you already have. In effect, you're taking an existing predicate and wrapping it in another predicate that works like the NOT operator:

NSPredicate *pred = [NSPredicate predicateWithFormat:@"SELF contains '-'"];
NSPredicate *notPred = [NSCompoundPredicate notPredicateWithSubpredicate:pred];
[resultsArray filterUsingPredicate:pred];

The NSCompoundPredicate class supports AND, OR, and NOT predicate types, so you could go through and build a large compound predicate with all the characters you don't want in your array, then filter on it. Try something like:

// Set up the arrays of bad characters and strings to be filtered
NSArray *badChars = [NSArray arrayWithObjects:@"-", @"*", @"&", nil];
NSMutableArray *strings = [[[NSArray arrayWithObjects:@"test-string", @"teststring", 
                   @"test*string", nil] mutableCopy] autorelease];

// Build an array of predicates to filter with, then combine into one AND predicate
NSMutableArray *predArray = [[[NSMutableArray alloc] 
                                    initWithCapacity:[badChars count]] autorelease];
for(NSString *badCharString in badChars) {
    NSPredicate *charPred = [NSPredicate 
                         predicateWithFormat:@"SELF contains '%@'", badCharString];
    NSPredicate *notPred = [NSCompoundPredicate notPredicateWithSubpredicate:pred];
    [predArray addObject:notPred];
}
NSPredicate *pred = [NSCompoundPredicate andPredicateWithSubpredicates:predArray];

// Do the filter
[strings filterUsingPredicate:pred];

I make no guarantees as to its efficiency, though, and it's probably a good idea to put the characters which are likely to eliminate the most strings from the final array first so that the filter can short-circuit as many comparisons as possible.

Answer 3

I'd recomment NSNotPredicateType as described in the Apple documentation.