Why is longLongValue returning the incorrect value

Question

I have a NSDictionary that contains a key with a value of 4937446359977427944. I try and get the value of it as a long long and get 4937446359977427968 back?

Answer 1

I'm not sure if your are interested in a simple solution or just looking into the details of why the loss of precision takes place.

If you are interested in a simple answer: -[NSDecimalNumber description] products a string with the value, and -[NSString longLongValue] converts a string into a long long

NSDecimalNumber *decimalNumber = [NSDecimalNumber decimalNumberWithString:@"4937446359977427944"];
long long longLongNumber = [[decimalNumber description] longLongValue];
NSLog(@"decimalNumber %@ -- longLongNumber %lld", decimalNumber, longLongNumber);

outputs

2014-04-16 08:51:21.221 APP_NAME[30458:60b] decimalNumber 4937446359977427944 -- longLongNumber 4937446359977427944

Final Note

[decimalNumber descriptionWithLocale:[[NSLocale alloc] initWithLocaleIdentifier:@"en_US"]] may be more reliable is your app supports multiple locales.