I solved even if the Attributes are String Type as follows:
NSArray *allValues = [array valueForKeyPath:@"@unionOfObjects.entityAttribute"];
NSMutableArray *allNumberValues = [[NSMutableArray alloc] init];
for (int i=0; i<valoriVotoEsame.count; i++) {
if ( ![[allValues objectAtIndex:i] isEqual:@""])
{
[allNumberValues addObject:[allValues objectAtIndex:i]];
}
}