Filter NSArray based on another array using a predicate
Consider the arrays below. Arrays contain objects of type "Alpha". We just have to know the property username
that has a type NSString
.
NSArray *some_usernames = @[ <multiple values of type Alpha> ]
NSArray *all_usernames = @[ <multiple values of type Alpha> ]
I basically want a list of all usernames that are not in the array some_usernames
, i.e.
NSArray *remaining_usernames = @[ <all_usernames but not in some_usernames> ];
What I would like to do is:
NSPredicates *predicates;
for (Alpha *alpha in some_usernames)
{
predicate = [predicate with @"username != %@", alpha.username];
predicates.add(predicate)
}
create compound predicate
filter all_usernames
But this seems like a bad way to do it. Is there a way to do this in two lines? I've seen this before, but I can't link to the code link anymore.
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NSPredicate *predicate = [NSPredicate predicateWithFormat:@"not (self.username IN %@)", [some_usernames valueForKey:@"username"]];
NSArray *remaining_usernames = [all_usernames filteredArrayUsingPredicate:predicate];
complete example
@interface Alpha : NSObject
@property (nonatomic, copy) NSString *username;
-(instancetype) initWithUsername:(NSString *)username;
@end
@implementation Alpha
-(instancetype) initWithUsername:(NSString *)username
{
self = [super init];
if (self) {
self.username = username;
}
return self;
}
-(NSString *)description{
return [NSString stringWithFormat:@"%@: %@", NSStringFromClass([self class]), self.username];
}
@end
NSArray *all_usernames = @[[[Alpha alloc] initWithUsername:@"a"], [[Alpha alloc] initWithUsername:@"b"], [[Alpha alloc] initWithUsername:@"z"], [[Alpha alloc] initWithUsername:@"f"], [[Alpha alloc] initWithUsername:@"e"]];
NSArray *some_usernames = @[[[Alpha alloc] initWithUsername:@"b"], [[Alpha alloc] initWithUsername:@"f"]];
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"not (self.username IN %@)", [some_usernames valueForKey:@"username"]];
NSArray *remaining_usernames = [all_usernames filteredArrayUsingPredicate:predicate];
NSLog(@"%@", remaining_usernames);
prints
(
"Alpha: a",
"Alpha: z",
"Alpha: e"
)
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I want to add another answer:
If ordering of objects is not required (and most likely unwanted objects), you can use Sets and set arithmetic instead of using predicate filtering on arrays. To do this, we must teach Alpha
what equality means and provide a hash method. In this case, we just use the NSStrings implementation:
@implementation Alpha
-(instancetype) initWithUsername:(NSString *)username
{
self = [super init];
if (self) {
self.username = username;
}
return self;
}
-(NSString *)description{
return [NSString stringWithFormat:@"%@: %@", NSStringFromClass([self class]), self.username];
}
-(BOOL)isEqual:(id)object
{
return [self.username isEqual:[object username]];
}
-(NSUInteger)hash
{
return [self.username hash];
}
@end
NSArray *all_usernames = @[[[Alpha alloc] initWithUsername:@"a"],
[[Alpha alloc] initWithUsername:@"b"],
[[Alpha alloc] initWithUsername:@"z"],
[[Alpha alloc] initWithUsername:@"f"],
[[Alpha alloc] initWithUsername:@"e"]];
NSArray *some_usernames = @[[[Alpha alloc] initWithUsername:@"b"],
[[Alpha alloc] initWithUsername:@"f"]];
NSSet *allSet = [NSSet setWithArray:all_usernames];
NSSet *someSet = [NSSet setWithArray:some_usernames];
NSMutableSet *remainingSet = [allSet mutableCopy];
[remainingSet minusSet:someSet];
NSLog(@"%@", remainingSet);
prints
{(
Alpha: z,
Alpha: e,
Alpha: a
)}
This code should be much faster for more data. Please see WWDC 2013: Developing Code for Performance
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