objective-c - 仅在需要时提供精确的数字 - 这可能吗？

Question

是否有可能以不同的精度呈现相同的变量？
例如，如果我要呈现两个整数的加法，则结果如下:

result = 2+3;
NSLog(@"%.0f", result);

但是如果有一个除法，它会像:

result = 2/3;
NSLog(@"%.1f", result);

虽然呈现 π (pi)，但它会像:

result = M_PI;
NSLog(@"%.19f", result); //And it would be 3.141592653589793238

Answer 1

NSNumber 将为你做一些快速而肮脏的格式化，

NSNumber * five = [NSNumber numberWithDouble:3+2];
NSNumber * twoThirds = [NSNumber numberWithDouble:2/3.0];
NSNumber * mPi = [NSNumber numberWithDouble:M_PI];

NSLog(@"%@", [five stringValue]);
NSLog(@"%@", [twoThirds stringValue]);
NSLog(@"%@", [mPi stringValue]);

2011-11-17 21:52:12.621 Precision[13359:903] 5
2011-11-17 21:52:12.624 Precision[13359:903] 0.6666666666666666
2011-11-17 21:52:12.625 Precision[13359:903] 3.141592653589793

但是你真的应该使用NSNumberFormatter:

NSNumberFormatter * formatter = [[NSNumberFormatter alloc] init];

[formatter setUsesSignificantDigits:YES];

NSLog(@"%@", [formatter stringFromNumber:five]);
NSLog(@"%@", [formatter stringFromNumber:twoThirds]);

[formatter setMaximumSignificantDigits:16];
NSLog(@"%@", [formatter stringFromNumber:mPi]);

2011-11-17 21:52:12.629 Precision[13359:903] 5
2011-11-17 21:52:12.630 Precision[13359:903] 0.666667
2011-11-17 21:52:12.630 Precision[13359:903] 3.14159265358979

string format specifiers还允许可变精度；使用星号代替小数点后的数字:

int precision = 3;
NSLog(@"%.*f", precision, 2/3.0);