Objective-C Fundamentals

Filtering and matching with predicates
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Table 11.4 Set-based operators that can be used in NSPredicate-based expressions when a
collection of objects is present in a key path
Operator Description Example
ANY
Do one or more objects satisfy the key path
expression?
ANY address.length > 50
ALL Do all objects satisfy the key path expression? ALL address.length < 20
NONE
Do all objects not match the key path
expression?
NONE address.length < 20
11.3.6 Parameterizing and templating predicate expressions
So far, all examples of creating an NSPredicate object have used the predicateWith-
Format: class message and consisted of a simple string. As the name of the message
alludes to, the predicateWithFormat: message accepts NSString stringWith-
Format:–style format arguments as well. For example, the following code snippet produces
the predicate expression rentalPrice > 500 || address ENDSWITH 'Sumner':
NSPredicate *predicate = [NSPredicate predicateWithFormat:
@"rentalPrice > %f || address ENDSWITH %@", 500.0f, @"Sumner"];
Notice that, unlike the original example in which the string Sumner had to be escaped
with quotation marks, this example uses the %@ format specifier, and NSPredicate is
smart enough to introduce the required quote marks. The following code sample
emits the final predicate expression generated by the previous code snippet:
NSLog(@"The predicate expression is %@", [predicate predicateFormat]);
This feature and others like it can be handy, and they help avoid common problems in
generating dynamic expressions, such as improperly escaping strings that contain
internal quotation marks.
If you parameterize a predicate expression because you want to dynamically
adjust its values, perhaps by hooking up a numeric value to a UISlider control, the
NSPredicate class can go one better. Altering the previous code snippet, you can use
the following predicate template definition
NSPredicate *template = [NSPredicate predicateWithFormat:
@"rentalPrice > $MINPRICE || address ENDSWITH $SUBURB"];
where names beginning with a dollar sign indicate placeholders in the templates
where you can substitute different values at runtime. If you attempt to use this template
predicate directly, an NSInvalidArgumentException is thrown, indicating that
you haven’t provided values for MINPRICE and SUBURB. To specify these values, you use
the predicateWithSubstitutionVariables: message, passing in an NSDictionary of
variable name/value pairs, as follows: