NSPredicate

NSPredicate is a Foundation class that specifies how data should be fetched or filtered. Its query language, which is like a cross between a SQL WHERE clause and a regular expression, provides an expressive, natural language interface to define logical conditions on which a collection is searched.

It's easier to show NSPredicate in use, rather than talk about it in the abstract, so we're going to revisit the example data set used in the NSSortDescriptor article:

index 0 1 2 3

firstName	Alice	Bob	Charlie	Quentin
lastName	Smith	Jones	Smith	Alberts
age	24	27	33	31

@interface Person : NSObject
@property NSString *firstName;
@property NSString *lastName;
@property NSNumber *age;
@end

@implementation Person

- (NSString *)description {
    return [NSString stringWithFormat:@"%@ %@", self.firstName, self.lastName];
}

@end

#pragma mark -

NSArray *firstNames = @[ @"Alice", @"Bob", @"Charlie", @"Quentin" ];
NSArray *lastNames = @[ @"Smith", @"Jones", @"Smith", @"Alberts" ];
NSArray *ages = @[ @24, @27, @33, @31 ];

NSMutableArray *people = [NSMutableArray array];
[firstNames enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
    Person *person = [[Person alloc] init];
    person.firstName = firstNames[idx];
    person.lastName = lastNames[idx];
    person.age = ages[idx];
    [people addObject:person];
}];

NSPredicate *bobPredicate = [NSPredicate predicateWithFormat:@"firstName = 'Bob'"];
NSPredicate *smithPredicate = [NSPredicate predicateWithFormat:@"lastName = %@", @"Smith"];
NSPredicate *thirtiesPredicate = [NSPredicate predicateWithFormat:@"age >= 30"];

// ["Bob Jones"]
NSLog(@"Bobs: %@", [people filteredArrayUsingPredicate:bobPredicate]);

// ["Alice Smith", "Charlie Smith"]
NSLog(@"Smiths: %@", [people filteredArrayUsingPredicate:smithPredicate]);

// ["Charlie Smith", "Quentin Alberts"]
NSLog(@"30's: %@", [people filteredArrayUsingPredicate:thirtiesPredicate]);

Using NSPredicate with Collections

Foundation provides methods to filter NSArray / NSMutableArray & NSSet / NSMutableSet with predicates.

Immutable collections, NSArray & NSSet, have the methods filteredArrayUsingPredicate: and filteredSetUsingPredicate: which return an immutable collection by evaluating a predicate on the receiver.

Mutable collections, NSMutableArray & NSMutableSet have the method filterUsingPredicate:, which removes any objects that evaluate to FALSE when running the predicate on the receiver.

NSDictionary can use predicates by filtering its keys or values (both NSArray objects). NSOrderedSet can either create new ordered sets from a filtered NSArray or NSSet, or alternatively, NSMutableSet can simply removeObjectsInArray:, passing objects filtered with the negated predicate.

Using NSPredicate with Core Data

NSFetchRequest has a predicate property, which specifies the logical conditions under which managed objects should be retrieved. The same rules apply, except that predicates are evaluated by the persistent store coordinator within a managed object context, rather than collections being filtered in-memory.

Predicate Syntax

Substitutions

• %@ is a var arg substitution for an object value—often a string, number, or date.
• %K is a var arg substitution for a key path.

NSPredicate *ageIs33Predicate = [NSPredicate predicateWithFormat:@"%K = %@", @"age", @33];

// ["Charlie Smith"]
NSLog(@"Age 33: %@", [people filteredArrayUsingPredicate:ageIs33Predicate]);

• $VARIABLE_NAME is a value that can be substituted with NSPredicate -predicateWithSubstitutionVariables:.

NSPredicate *namesBeginningWithLetterPredicate = [NSPredicate predicateWithFormat:@"(firstName BEGINSWITH[cd] $letter) OR (lastName BEGINSWITH[cd] $letter)"];

// ["Alice Smith", "Quentin Alberts"]
NSLog(@"'A' Names: %@", [people filteredArrayUsingPredicate:[namesBeginningWithLetterPredicate predicateWithSubstitutionVariables:@{@"letter": @"A"}]]);

Basic Comparisons

• =, ==: The left-hand expression is equal to the right-hand expression.
• >=, =>: The left-hand expression is greater than or equal to the right-hand expression.
• <=, =<: The left-hand expression is less than or equal to the right-hand expression.
• >: The left-hand expression is greater than the right-hand expression.
• <: The left-hand expression is less than the right-hand expression.
• !=, <>: The left-hand expression is not equal to the right-hand expression.
• BETWEEN: The left-hand expression is between, or equal to either of, the values specified in the right-hand side. The right-hand side is a two value array (an array is required to specify order) giving upper and lower bounds. For example, 1 BETWEEN { 0 , 33 }, or $INPUT BETWEEN { $LOWER, $UPPER }.

Basic Compound Predicates

• AND, &&: Logical AND.
• OR, ||: Logical OR.
• NOT, !: Logical NOT.

String Comparisons

String comparisons are by default case and diacritic sensitive. You can modify an operator using the key characters c and d within square braces to specify case and diacritic insensitivity respectively, for example firstName BEGINSWITH[cd] $FIRST_NAME.

• BEGINSWITH: The left-hand expression begins with the right-hand expression.
• CONTAINS: The left-hand expression contains the right-hand expression.
• ENDSWITH: The left-hand expression ends with the right-hand expression.
• LIKE: The left hand expression equals the right-hand expression: ? and * are allowed as wildcard characters, where ? matches 1 character and * matches 0 or more characters.
• MATCHES: The left hand expression equals the right hand expression using a regex-style comparison according to ICU v3 (for more details see the ICU User Guide for Regular Expressions).

Aggregate Operations

Relational Operations

• ANY, SOME: Specifies any of the elements in the following expression. For example, ANY children.age < 18.
• ALL: Specifies all of the elements in the following expression. For example, ALL children.age < 18.
• NONE: Specifies none of the elements in the following expression. For example, NONE children.age < 18. This is logically equivalent to NOT (ANY ...).
• IN: Equivalent to an SQL IN operation, the left-hand side must appear in the collection specified by the right-hand side. For example, name IN { 'Ben', 'Melissa', 'Nick' }.

Array Operations

• array[index]: Specifies the element at the specified index in array.
• array[FIRST]: Specifies the first element in array.
• array[LAST]: Specifies the last element in array.
• array[SIZE]: Specifies the size of array.

Boolean Value Predicates

• TRUEPREDICATE: A predicate that always evaluates to TRUE.
• FALSEPREDICATE: A predicate that always evaluates to FALSE.

NSCompoundPredicate

We saw that AND & OR can be used in predicate format strings to create compound predicates. However, the same can be accomplished using an NSCompoundPredicate.

For example, the following predicates are equivalent:

[NSCompoundPredicate andPredicateWithSubpredicates:@[[NSPredicate predicateWithFormat:@"age > 25"], [NSPredicate predicateWithFormat:@"firstName = %@", @"Quentin"]]];

[NSPredicate predicateWithFormat:@"(age > 25) AND (firstName = %@)", @"Quentin"];

While the syntax string literal is certainly easier to type, there are occasions where you may need to combine existing predicates. In these cases, NSCompoundPredicate -andPredicateWithSubpredicates: & -orPredicateWithSubpredicates: is the way to go.

NSComparisonPredicate

Similarly, if after reading last week's article you now find yourself with more NSExpression objects than you know what to do with, NSComparisonPredicate can help you out.

Like NSCompoundPredicate, NSComparisonPredicate constructs an NSPredicate from subcomponents—in this case, NSExpressions on the left and right hand sides. Analyzing its class constructor provides a glimpse into the way NSPredicate format strings are parsed:

+ (NSPredicate *)predicateWithLeftExpression:(NSExpression *)lhs
                             rightExpression:(NSExpression *)rhs
                                    modifier:(NSComparisonPredicateModifier)modifier
                                        type:(NSPredicateOperatorType)type
                                     options:(NSUInteger)options

Parameters

• lhs: The left hand expression.
• rhs: The right hand expression.
• modifier: The modifier to apply. (ANY or ALL)
• type: The predicate operator type.
• options: The options to apply. For no options, pass 0.

NSComparisonPredicate Types

enum {
   NSLessThanPredicateOperatorType = 0,
   NSLessThanOrEqualToPredicateOperatorType,
   NSGreaterThanPredicateOperatorType,
   NSGreaterThanOrEqualToPredicateOperatorType,
   NSEqualToPredicateOperatorType,
   NSNotEqualToPredicateOperatorType,
   NSMatchesPredicateOperatorType,
   NSLikePredicateOperatorType,
   NSBeginsWithPredicateOperatorType,
   NSEndsWithPredicateOperatorType,
   NSInPredicateOperatorType,
   NSCustomSelectorPredicateOperatorType,
   NSContainsPredicateOperatorType,
   NSBetweenPredicateOperatorType
};
typedef NSUInteger NSPredicateOperatorType;

NSComparisonPredicate Options

• NSCaseInsensitivePredicateOption: A case-insensitive predicate. You represent this option in a predicate format string using a [c] following a string operation (for example, "NeXT" like[c] "next").
• NSDiacriticInsensitivePredicateOption: A diacritic-insensitive predicate. You represent this option in a predicate format string using a [d] following a string operation (for example, "naïve" like[d] "naive").
• NSNormalizedPredicateOption: Indicates that the strings to be compared have been preprocessed. This option supersedes NSCaseInsensitivePredicateOption and NSDiacriticInsensitivePredicateOption, and is intended as a performance optimization option. You represent this option in a predicate format string using a [n] following a string operation (for example, "WXYZlan" matches[n] ".lan").
• NSLocaleSensitivePredicateOption: Indicates that strings to be compared using <, <=, =, =>, > should be handled in a locale-aware fashion. You represent this option in a predicate format string using a [l] following one of the <, <=, =, =>, > operators (for example, "straße" >[l] "strasse").

Block Predicates

Finally, if you just can't be bothered to learn the NSPredicate format syntax, you can go through the motions with NSPredicate +predicateWithBlock:.

NSPredicate *shortNamePredicate = [NSPredicate predicateWithBlock:^BOOL(id evaluatedObject, NSDictionary *bindings) {
            return [[evaluatedObject firstName] length] <= 5;
        }];

// ["Alice Smith", "Bob Jones"]
NSLog(@"Short Names: %@", [people filteredArrayUsingPredicate:shortNamePredicate]);

...Alright, that whole dig on predicateWithBlock: as being the lazy way out wasn't entirely charitable.

Actually, since blocks can encapsulate any kind of calculation, there is a whole class of queries that can't be expressed with the NSPredicate format string (such as evaluating against values dynamically calculated at run-time). And while its possible to accomplish the same using an NSExpression with a custom selector, blocks provide a convenient interface to get the job done.

One important note: NSPredicates created with predicateWithBlock: cannot be used for Core Data fetch requests backed by a SQLite store.

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NSPredicate is, and I know this is said a lot, truly one of the jewels of Cocoa. Other languages would be lucky to have something with half of its capabilities in a third-party library—let alone the standard library. Having it as a standard-issue component affords us as application and framework developers an incredible amount of leverage in working with data.

Together with NSExpression, NSPredicate reminds us what a treat Foundation is: a framework that is not only incredibly useful, but meticulously architected and engineered, to be taken as inspiration for how we should write our own code.