Использование NSNumberFormatter
[numberFormatter numberFromString:[NSString stringWithFormat:@"%.8f",0]]
Образец:
NSNumberFormatter *numberFormatter = [[NSNumberFormatter alloc] init];
NSLog(@"1: %@",[numberFormatter numberFromString:[NSString stringWithFormat:@"%.8f",2.344383933]]);
NSLog(@"2: %@",[numberFormatter numberFromString:[NSString stringWithFormat:@"%.8f",2.0]]);
Результаты:
1: 2.344383933
2: 2