Nspredicate to Test for Null, and Blank Strings

NSPredicate to test for NULL, and blank strings

If you don't use Core Data, you could do:

NSPredicate *predicateName = [NSPredicate predicateWithFormat:@"name.length > 0"];

If the string is empty, this will fail (because 0 == 0). Similarly, if name is nil, it will also fail, because [nil length] == 0.

NSPredicate to check for NULL, and blank strings in Core Data

Please go through CoreData predicate: string property length?

or you can do like this

NSString *attributeName = @"name";
NSString *attributeValue = [NSString stringWithFormat:@".{%d}", 5];
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"%K MATCHES %@", attributeName, attributeValue];

Reference - NSPredicate for an NSManagedObject's string attribute's length

NSPredicate in swift for empty string

You can simply compare to the empty String, "". The %@ placeholder represents a String, so the crash happens because you supply an Int to the NSPredicate instead of a String.

You should also use the %K placeholder for variable names instead of appending strings.

let predicate = NSPredicate(format: " %K != %@", remoteAttributes.lineOwner, "")

If you also want to filter out nil values, you can use a compound predicate:

let predicate = NSPredicate(format: " %K != %@ AND %K != nil", remoteAttributes.lineOwner, "")

Generic is null predicate]

Since isNull() is generic, and the compiler cannot infer the generic parameter when combined like that, you need to explicitly specify the type parameter.

To do that, you must qualify with the class name, e.g. Test:

Test.<String>isNull().or(isEmpty())

Full example:

public class Test {
    public static void main(String[] args) {
        Predicate<String> isNullOrEmpty = Test.<String>isNull().or(isEmpty());
        System.out.println(isNullOrEmpty.test(null));   // true
        System.out.println(isNullOrEmpty.test(""));     // true
        System.out.println(isNullOrEmpty.test("Foo"));  // false
    }
    private static <T> Predicate<T> isNull(){
        return Objects::isNull;
    }
    private static Predicate<String> isEmpty(){
        return string -> string.isEmpty();
    }
}

You can also resolved it by assigning each part to a variable:

Predicate<String> isNull = isNull(); // String is inferred from assignment operator
Predicate<String> isEmpty = isEmpty();
Predicate<String> isNullOrEmpty = isNull.or(isEmpty);

Or just the first part:

Predicate<String> isNull = isNull();
Predicate<String> isNullOrEmpty = isNull.or(isEmpty());

How to create NSCompoundPredicate with empty predicate

Instead of creating “empty” predicates you can dynamically build an array containing only the needed predicates:

var predicates = [NSPredicate]() // Start with empty list.
if isGenreFilterOn {
    // Add genre predicate:
    predicates.append(NSPredicate(format: "genID == \(genreID)"))
}
if !searchFor.isEmpty {
    // Add book predicate:
    predicates.append(NSPredicate(format: "bokName CONTAINS[cd] %@", searchFor))
}
// ... more criteria ...

// Combine all predicates:
let predicate = NSCompoundPredicate(type: .and, subpredicates: predicates)

More optional (or non-optional) criteria can easily be added. This works correctly even if the predicates array is empty.

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Remark: Be careful with string interpolation in predicate format strings.

NSPredicate(format: "genID == \(genreID)")

is fine if genreID is a number, but can crash at runtime if it is a string and contains any special characters or reserved keywords for predicates.

NSPredicate for an NSManagedObject's string attribute's length

Not sure how this code snippet is performance wise but here is my answer to your question:

NSString *attributeName = @"letters";
NSString *attributeValue = [NSString stringWithFormat:@"'.{%d,%d}'", 5, 20];
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"%K MATCHES %@", attributeName, attributeValue];
[fetchRequest setPredicate:predicate];

Joss.

iOS - NSPredicate string.length always evaluates to 0 when string starts with arithmetic operators + - * /

You cannot use Objective-C functions like length in a Core Data fetch request
(and the ".length" part is simply ignored when Core Data translates the fetch request
to a SQLite query). But you can simply compare with an empty string instead:

 [NSPredicate predicateWithFormat:@"comment != ''"]

For other queries involving the length, you can use the MATCHES operator with
a regular expression as shown here: CoreData predicate: string property length?.

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