直接貼代碼了

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    NSLog(@"----%@---",[self decimalwithFormat:@"0.0000" floatV:0.334]);
    NSLog(@"----%@---",[self decimalwithFormat:@"0.0000" floatV:0.334569]);
    NSLog(@"----%@---",[self decimalwithFormat:@"0.0000" floatV:0.334519]);
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

//格式話小數 四捨五入類型
- (NSString *) decimalwithFormat:(NSString *)format  floatV:(float)floatV
{
    NSNumberFormatter *numberFormatter = [[NSNumberFormatter alloc] init];
    
    [numberFormatter setPositiveFormat:format];
    
    return  [numberFormatter stringFromNumber:[NSNumber numberWithFloat:floatV]];
}

@end

最後控制台打印結果：

2014-03-19 15:44:45.262 TestNumberFormatter[3056:60b] ----0.3340---
2014-03-19 15:44:45.263 TestNumberFormatter[3056:60b] ----0.3346---
2014-03-19 15:44:45.263 TestNumberFormatter[3056:60b] ----0.3345---