Getting the Average Value of a column in Core Data.

gistfile1.txt

	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];