ios - 计算所有NSManagedObjects属性的平均值

我有一个名称为FoodItem的实体，它具有一个属性price（双精度）。理想情况下，此代码的输出为：

Average Price is: 6.00

虽然我不确定如何访问值6.00。谁能帮助我？谢谢

这是我的代码：

NSEntityDescription *entity = [NSEntityDescription entityForName:@"FoodItem" inManagedObjectContext:self.managedObjectContext];

NSManagedObject *o1 = [NSEntityDescription insertNewObjectForEntityForName:[entity name] inManagedObjectContext:self.managedObjectContext];
[o1 setValue:@(5.00) forKey:@"price"];

NSManagedObject *o2 = [NSEntityDescription insertNewObjectForEntityForName:[entity name] inManagedObjectContext:self.managedObjectContext];
[o2 setValue:@(7.00) forKey:@"price"];


[self.managedObjectContext save:nil];

NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:entity];


[request setSortDescriptors:@[]];

NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:@"price"];
NSExpression *averagePriceExpression = [NSExpression expressionForFunction:@"average:"
                                                              arguments:@[keyPathExpression]];

NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
[expressionDescription setName:@"averagePrice"];
[expressionDescription setExpression:averagePriceExpression];
[expressionDescription setExpressionResultType:NSDecimalAttributeType];

[request setPropertiesToFetch:[NSArray arrayWithObject:expressionDescription]];

NSFetchedResultsController *controller = [[NSFetchedResultsController alloc] initWithFetchRequest:request managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:@"Master"];


[controller performFetch:nil];

NSLog(@"Average price is: %@", @"");

最佳答案

我不明白为什么要为此使用访存结果控制器。 NSFetchedResultsController不能用于此目的。为此使用一个简单的NSFetchRequest。

// Your previous code here

// Execute the fetch.
NSError *error = nil;
NSArray *objects = [managedObjectContext executeFetchRequest:request error:&error];
if (objects == nil) {
    // Handle the error.
}
else {
    if ([objects count] > 0) {
        NSLog(@"Average price is: %@", [[objects objectAtIndex:0] valueForKey:@"averagePrice"]);
    }
}

可以从Apple文档Fetching Specific Values中找到一个简单的示例。

关于ios - 计算所有NSManagedObjects属性的平均值，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/17306968/