Objective-C Fundamentals

216 CHAPTER 11 Key-Value Coding and NSPredicate
11.1.4 Aggregating and collating values
While working with one-to-many relationships in key paths, you may sometimes prefer an
aggregate result that summarizes the raw data instead of an individual result for each
value a key path matches. For example, rather than a list of tenants, you may just need the
age of the oldest tenant, the average age of all tenants, or a list of all unique last names.
Key-value paths can perform such calculations by introducing a collection operator
into the key path using the @ prefix. In practice, most key paths consist of two
parts; see table 11.1.
Table 11.1
Common key path collection operators that aggregate and summarize items in a collection
Operator
Description
@avg
@count
@max
@min
@sum
@distinctUnionOfObjects
Converts the key path to the right of the operator to a collection
of doubles and returns the average value
Returns the number of objects in the key path to the left of
the operator
Converts the key path to the right of the operator to a collection
of doubles and returns the maximum value
Converts the key path to the right of the operator to a collection
of doubles and returns the minimum value
Converts the key path to the right of the operator to a collection
of doubles and returns their sum
Returns an array containing the distinct objects in the property
specified by the key path to the right of the operator
As an example, the following code snippet uses the @count operator to determine the
number of rental properties located in the rentalProperties array:
NSNumber *count = [rentalProperties valueForKeyPath:"@count"];
NSLog(@"There are %d rental properties for available", [count intValue]);
Internally, KVC queries a key path as normal, but when it detects an aggregate function,
it creates a loop that iterates over all matching elements and performs the
required calculation. The neat thing is that this process is all hidden from you: you
express in a string the result you need. For example, the following two queries return
the maximum and average length of all rental property addresses:
NSNumber *avg = [rentalProperties valueForKeyPath:@"@avg.address.length"];
NSNumber *max = [rentalProeprties valueForKeyPath:@"@max.address.length"];
Another thing you might need is a set of unique values assigned to a given property,
such as a list of all unique rental prices. You may be tempted to generate the list via
the KVC query:
NSArray *rentalPrices = [rentalProperties valueForKeyPath:@"rentalPrice"];