使用谓词根据另一个数组过滤 NSArray

Question

考虑下面的数组。数组包含 'Alpha' 类型的对象。我们只关心类型为 NSString.

的属性 username

NSArray *some_usernames = @[ <multiple values of type Alpha> ]
NSArray *all_usernames = @[ <multiple values of type Alpha> ]

我基本上想要一个不在数组 some_usernames 中的所有用户名的列表，即

NSArray *remaining_usernames = @[ <all_usernames but not in some_usernames> ];

我打算采用的方式是：

NSPredicates *predicates;
for (Alpha *alpha in some_usernames)
{
    predicate = [predicate with @"username != %@", alpha.username];
    predicates.add(predicate)
}

create compound predicate
filter all_usernames

但这感觉不太好。有没有办法在两行中做到这一点？我以前看过，但是我不能再指向代码参考了。

Answer 1

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"not (self.username IN %@)", [some_usernames valueForKey:@"username"]];
NSArray *remaining_usernames = [all_usernames filteredArrayUsingPredicate:predicate];

完整示例

@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);

打印

(
    "Alpha: a",
    "Alpha: z",
    "Alpha: e"
)

Answer 2

我想添加另一个答案：

如果不需要对象的排序（并且很可能不需要相等的对象），您可以使用集合和集合算术来代替对数组使用谓词过滤。为此，我们必须教导 Alpha 相等的含义并提供哈希方法。在这种情况下，我们只使用 NSStrings 实现：

@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);

打印

{(
    Alpha: z,
    Alpha: e,
    Alpha: a
)}

此代码对于更多数据应该更快。请收看WWDC 2013: Designing Code for Performance

使用谓词根据另一个数组过滤 NSArray

Filter NSArray based on another array using predicate

ios

objective-c

predicate