使用谓词基于另一个数组过滤NSArray [英] Filter NSArray based on another array using predicate

问题描述

考虑下面的数组。数组包含Alpha类型的对象。我们只关心属性用户名，其类型为 NSString 。

Consider the arrays below. The arrays contain objects of type 'Alpha'. We only care about the property username which is of type NSString.

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 ，即

I basically want a list of all the usernames that are not in the array some_usernames, i.e.

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

我打算这样做的方式是：

The way I would intend 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

但这感觉就像这样做的坏方法。有两种方法可以做到这一点吗？我以前见过它但我不能再指向代码引用了。

But this feels like a bad way of doing this. Is there a way to do this in two lines? I have seen it before but I can't point to the code reference anymore.

推荐答案

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

---

完整示例

---

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);

打印

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

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