如何獲得具有核心數據的列的平均值？

如果我有以下NSManagedObject，我怎樣才能得到number1的值的平均值和number2的值的平均值？如何獲得具有核心數據的列的平均值？

@interface Log : NSManagedObject 
{ 


} 


@property (nonatomic, retain) NSNumber * number1; 
@property (nonatomic, retain) NSNumber * number2;

感謝：d

來源

2010-10-18 Travis

NSManagedObjectContext *managedObjectContext = [(AppDelegate_Shared*)[UIApplication sharedApplication].delegate managedObjectContext]; 

NSFetchRequest *request = [[NSFetchRequest alloc] init]; 
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Log" inManagedObjectContext:managedObjectContext]; 
[request setEntity:entity]; 

// Specify that the request should return dictionaries. 
[request setResultType:NSDictionaryResultType]; 

// Create an expression for the key path. 
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:@"systolic"]; 

// Create an expression to represent the minimum value at the key path 'creationDate' 
NSExpression *avgExpression = [NSExpression expressionForFunction:@"average:" arguments:[NSArray arrayWithObject:keyPathExpression]]; 

// Create an expression description using the minExpression and returning a date. 
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init]; 

// The name is the key that will be used in the dictionary for the return value. 
[expressionDescription setName:@"averageSystolicPressure"]; 
[expressionDescription setExpression:avgExpression]; 
[expressionDescription setExpressionResultType:NSInteger32AttributeType]; 

// Set the request's properties to fetch just the property represented by the expressions. 
[request setPropertiesToFetch:[NSArray arrayWithObject:expressionDescription]]; 

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

    NSLog(@"Average systolic pressure: %d", [[[objects objectAtIndex:0] valueForKey:@"averageSystolicPressure"] integerValue]); 
    } 
} 

[expressionDescription release]; 
[request release];

來源

2010-10-29 05:10:22 Travis

使用集合操作@avg。

比方說，您已經爲您的Log對象提取了一個對象並將結果NSSet存儲在logs中。然後，你可以簡單地說：

NSNumber *avg1 = [logs valueForKeyPath:@"@avg.number1"]; 
NSNumber *avg2 = [logs valueForKeyPath:@"@avg.number2"];

@avg是，你可以在關鍵路徑集合使用運營商的極少數之一。其他一些是@max,@min和@sum。

來源

2014-01-07 21:30:21 Caleb

如何獲得具有核心數據的列的平均值？

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