How to filter records in CoreData using NSPredicate

I have amount attribute in coredata model with string datatype. I need to find records by comparing amount. I have tried NSPredicate with following but nothing is coming and result is always empty.

NSDecimal result;
NSScanner *theScanner = [[NSScanner alloc] initWithString:@"10.00"];
[theScanner scanDecimal:&result];
[theScanner release];



 [request setPredicate:[NSPredicate predicateWithFormat:@"amount > %@",[NSDecimalNumber decimalNumberWithDecimal:result]]];
 NSError* error = nil;
 NSArray* records = [managedObjectContext executeFetchRequest:request error:&error];
 NSLog(@"%@",开发者_开发技巧records);

Please remember that i am storing amount value in string data type in coredata. following are some examples of what i am storting into data.

Examples: -12.00, +30.00 etc.

I know storing decimal values in string type is not good, but i didn't find a good way to store amount with negative or positive mark within coredata attribute. let me if there is any good way to do that.

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Just use Decimal data type for amounts (corresponds with NSDecimalNumber class in Objective-C).