NSComparisonPredicate to find empty strings - string

Question

I need to check if a string in my array is empty using the NSComparisonPredicate. The predicate that is being executed is: string MATCHES[c] ""
But nothing shows up in the results, it does not list my empty strings.
Is my predicate wrong or there is just another way of dealing with empty strings with NSPredicate?

Answer 1

I'm not sure why you're using NSComparisonPredicate ,I've never used that one, so I'm not familiar with it. Have you tried predicateWithFormat?
NSPredicate *pred = [NSPredicate predicateWithFormat:#"self.length == 0"];
It's not clear how you want to use the predicate, but this should work if you're using it to filter an array.
If you want to know the indexes of strings in your array that are empty, then you could use indexesOfObjectsPassingTest: like so:
NSIndexSet *indxs = [array indexesOfObjectsPassingTest:^BOOL(NSString *aString, NSUInteger idx, BOOL *stop) {
return aString.length == 0;
}];
NSLog(#"%#",indxs);

Answer 2

As rdelmar mentions above, for such a task you should just check the length of the string by string.length==0

Answer 3

You can filter your array using blocks. I would recommend that approach against using NSPredicate. Here is some sample code:
//creating array
NSArray *myArray = [NSArray arrayWithObjects:#"a", #"b", #"", #"c", nil];
//filtering
NSIndexSet *iset = [myArray indexesOfObjectsPassingTest:^BOOL(id obj, NSUInteger idx, BOOL *stop) {
NSString *str = obj;
return !str.length;
}
];
//creating my result array with empty strings
NSArray *emptyStrings = [myArray objectsAtIndexes:iset];
//logging all strings
for (NSString *str in myArray) {
NSLog(#"string: %#", str);
}
//logging empty strings
for (NSString *str in emptyStrings) {
NSLog(#"empty string: %#", str);
}
EDIT:
If you really need to use NSComparisonPredicate, here it is:
NSExpression *left = [NSExpression expressionForKeyPath:#"length"];
NSExpression *right = [NSExpression expressionForConstantValue:[NSNumber numberWithInt:0]];
NSComparisonPredicateModifier modifier = NSDirectPredicateModifier;
NSPredicateOperatorType operator = NSEqualToPredicateOperatorType;
NSPredicate *predicate = [NSComparisonPredicate predicateWithLeftExpression:left rightExpression:right modifier:modifier type:operator options:0];
NSArray *filtered = [myArray filteredArrayUsingPredicate:predicate];