Core Data Xcode 中的随机函数

Question

我是一名新程序员，正在尝试使用 Core Data 制作随机函数。我想要的是标签从我的实体返回一个随机 managedObject 。这是我的代码

- (IBAction)randomize{
    NSEntityDescription *entitydesc = [NSEntityDescription entityForName:@"Restraunt" inManagedObjectContext:context ];
    NSFetchRequest *request = [[NSFetchRequest alloc]init];
    [request setEntity:entitydesc];
    NSError *error;
    NSArray *matchingData = [context executeFetchRequest:request error:&error];
    NSString *textString;
    int RandomNumber;
    RandomNumber = arc4random() %[matchingData count];
    textString = [matchingData objectAtIndex:RandomNumber];
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"restraunt_name like %@",textString];
    [request setPredicate:predicate];

    if(matchingData.count <= 0){
        self.label.text = @"doesnt exist";}
    else {
        NSString *string;
        for (NSString *obj in matchingData){
            string = [obj valueForKey:@"restraunt_name"];

        }
        self.label.text = [NSString stringWithFormat:@" %@", string] ;
    }
}

Answer 1

这段代码有几处错误。例如，“textString”不是字符串，它是您的 Restraunt 实体之一。此外，您在 else 子句中的 for 循环只是遍历所有实体（使您的随机选择无关紧要），因此“字符串”将是您数据中的最后一个餐厅名称。你的代码应该是这样的，

- (IBAction)randomize{
    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Restraunt"];
    NSError *error;
    NSArray *matchingData = [context executeFetchRequest:request error:&error];
    int randomNumber = arc4random() %[matchingData count];
    NSManagedObject *aRestraunt = matchingData[randomNumber]; // aRestraunt might not be an NSManagedObject depending on whether you've subclassed NSManagedObject or not -- if you have, then that should probably be Restraunt instead.

    self.label.text = [aRestaurant valueForKey:@"restraunt_name"];
}

编辑后：

我上面提供的答案基本上是对您的代码进行“修复”以使其正常工作。但是，这并不是获取单个随机对象的最有效方法。没有必要为了得到一个对象而将所有对象都提取到内存中。相反，您可以按照 Hal Mueller 在他的评论中建议的那样获取您的记录数，然后将 fetchOffset 与 fetchLimit 结合使用来获取一条记录，

NSError *error;
NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Restraunt"];
NSUInteger count = [context countForFetchRequest:request error:&error];
if (! error) {
    NSUInteger rand = arc4random_uniform() % count;
    [request setFetchOffset:rand];
    [request setFetchLimit:1];
    NSError *fetchError;
    NSManagedObject *aRestraunt = [[context executeFetchRequest:request error:&fetchError] lastObject];
    if (! fetchError) {
        self.label.text = [aRestaurant valueForKey:@"restraunt_name"];
    }else{
        NSLog(@"There was an error in fetching a Restraunt entity");
}