There appears to be zero documentation about the SUBQUERY keyword from Apple and I can't find a simple explanation about it on SO or on Google. It's a conspiracy! ;)
Please, could someone from the inner-circle please just provide a quick explanation of its syntax so I can use it?
SUBQUERY(Bs, $x, $x IN %@)
Thanks
And for people who don't quite get what the documentation is saying, a SUBQUERY
is essentially this:
SUBQUERY(collection, variableName, predicateFormat)
And could (simplistically) be implemented like this:
id resultingCollection = ...; //a new collection, either a mutable set or array
NSMutableDictionary * substitutions = [NSMutableDictionary dictionary];
NSPredicate * p = [NSPredicate predicateWithFormat:predicateFormat];
for (id variable in collection) {
[substitutions setObject:variable forKey:variableName];
NSPredicate * filter = [p predicateWithSubstitutionVariables:substitutions];
if ([filter evaluateWithObject:collection] == YES) {
[resultingCollection addObject:variable];
}
}
return resultingCollection;
So in a nutshell, a SUBQUERY
is basically taking a collection of objects and filtering out various objects based on the predicate expression of the SUBQUERY
, and returning the resulting collection. (And the predicate itself can contain other SUBQUERY
s)
Example:
NSArray * arrayOfArrays = [NSArray arrayWithObjects:
[NSArray arrayWithObjects:....],
[NSArray arrayWithObjects:....],
[NSArray arrayWithObjects:....],
[NSArray arrayWithObjects:....],
[NSArray arrayWithObjects:....],
[NSArray arrayWithObjects:....],
nil];
NSPredicate * filter = [NSPredicate predicateWithFormat:@"SUBQUERY(SELF, $a, $a.@count > 42)"];
NSArray * filtered = [arrayOfArrays filteredArrayUsingPredicate:filter];
//"filtered" is an array of arrays
//the only arrays in "filtered" will have at least 42 elements each