using a @min in predicate

问题

My current version for get @min or @max value is:

    for (NSManagedObject *destination in allSpecifics)
    {
        [allRates addObject:[destination valueForKey:@"rate"]];
    }

    NSExpression *arrayExpression = [NSExpression expressionForConstantValue: allRates];
    NSArray *argumentArray = [NSArray arrayWithObject: arrayExpression];

    NSExpression* expression = [NSExpression expressionForFunction:@"min:" arguments:argumentArray];
    id result = [expression expressionValueWithObject: nil context: nil];

    NSNumber *rateForAdd = result;

allSpecific is array of NSManagedObjects. Looks like is a not good way to fill array of objects before (memory and process time cost). Probably somebody can help refactor it?

回答1:

So you have an array (allSpecifics) of NSManagedObjects, and these objects have a rate property? If that's correct then:

To find the:

• minimum rate value:
  NSNumber *minimumRate = [allSpecifics valueForKeyPath:@"@min.rate"];
• maximum rate value:
  NSNumber *minimumRate = [allSpecifics valueForKeyPath:@"@max.rate"];
• average rate value:
  NSNumber *minimumRate = [allSpecifics valueForKeyPath:@"@avg.rate"];
• summed rate value:
  NSNumber *minimumRate = [allSpecifics valueForKeyPath:@"@sum.rate"];

来源：https://stackoverflow.com/questions/5023017/using-a-min-in-predicate