iPhone：如何截斷未經四捨五入的浮動？

Question

我需要簡單地截斷浮點數（截斷而不是圓）。

float FloatNum = 43.6823921; 
NSString *numString = [NSString stringWithFormat:@"%.1f", FloatNum]; // yields 43.7 

Answer 1

這會將float轉換爲字符串，將substring轉換爲索引4，轉換回float。我認爲這是截斷浮動的常用方法，不用四捨五入：float myTruncatedFloat = [[[[NSNumber numberWithFloat:43.6823921] stringValue] substringToIndex:4] floatValue];。這會將43.6823921轉換爲43.68。

float myfl = 43.6823921; 
NSNumber *num = [NSNumber numberWithFloat:myfl]; 
NSString *numStr = [num stringValue]; 
NSLog(@"%@", numStr); 
NSLog(@"%@", [numStr substringToIndex:2]); 
NSLog(@"%@", [numStr substringToIndex:3]); 
NSLog(@"%@", [numStr substringToIndex:4]); 
NSLog(@"%@", [numStr substringToIndex:5]);  
NSLog(@"%@", [numStr substringToIndex:6]); 
NSLog(@"%@", [numStr substringToIndex:7]); 
NSLog(@"%@", [numStr substringToIndex:8]); 
float newMyFloat = [[numStr substringToIndex:4] floatValue]; 
NSLog(@"%.1f", newMyFloat); 
newMyFloat = [[numStr substringToIndex:5] floatValue]; 
NSLog(@"%.2f", newMyFloat); 

打印：

[AppDelegate application:didFinishLaunchingWithOptions:]: 43.68239 
[AppDelegate application:didFinishLaunchingWithOptions:]: 43 
[AppDelegate application:didFinishLaunchingWithOptions:]: 43. 
[AppDelegate application:didFinishLaunchingWithOptions:]: 43.6 
[AppDelegate application:didFinishLaunchingWithOptions:]: 43.68 
[AppDelegate application:didFinishLaunchingWithOptions:]: 43.682 
[AppDelegate application:didFinishLaunchingWithOptions:]: 43.6823 
[AppDelegate application:didFinishLaunchingWithOptions:]: 43.68239 
[AppDelegate application:didFinishLaunchingWithOptions:]: 43.6 
[AppDelegate application:didFinishLaunchingWithOptions:]: 43.68 

Answer 2

有很多方法可以做，但您的解決方案將取決於多種因素。

這將永遠截斷到十分之一的地方。

float floatNum = 43.6823921; 
float truncatedFloat = truncf(floatNum * 10)/10; 

正如CHOWN提到的，可以將其轉換爲字符串，並採取子。如果您不總是處理雙位數字，您可能需要使用rangeOfString: os來查找小數點。

另一種選擇是使用NSDecimalNumber，其方法decimalNumberByRoundingAccordingToBehavior:明確設置舍入。我已經多次使用這個選項來處理精確度非常重要的貨幣操縱。

float num = 43.6894589; 
NSDecimalNumber *decimalNumber = [NSDecimalNumber decimalNumberWithDecimal:[[NSNumber numberWithFloat:num] decimalValue]]; 
float truncatedFloat = [[decimalNumber decimalNumberByRoundingAccordingToBehavior:self] floatValue]; 

// NSDecimalNumberBehaviors 
- (NSDecimalNumber *)exceptionDuringOperation:(SEL)method error:(NSCalculationError)error leftOperand:(NSDecimalNumber *)leftOperand rightOperand:(NSDecimalNumber *)rightOperand { 
    return [NSDecimalNumber notANumber]; 
} 

- (short)scale { 
    return 1; 
} 

- (NSRoundingMode)roundingMode { 
    return NSRoundDown; 
}