How can I get the average value of a column with Core Data?

Question

Hi Guys,

If I have the following NSManagedObject, how can I get the average of number1 and number2.

@interface Log :  NSManagedObject  
{


}


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

Thanks :D

Answer 1

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];