Re: No difference between %f, %lf, and %Lf?

On Jun 3, 2011, at 18:28, Nathan Sims wrote:

> It would seem the only way I can display 8 significant digits after the decimal place using floats, doubles, or long doubles is to get the stringValue of an NSNumber (OSX 10.6.7, Xcode 3.2.6). I know internally the full precision is being used, but it appears there's no difference whatever between format specifiers %f, %lf, and %Lf. Or am I somehow using these incorrectly?
> 
> #import <Foundation/Foundation.h>
> int main (int argc, const char *argv[]) {
>    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
> 
>    float ftest = -175.12345678;
>    double dtest = -175.12345678;
>    long double ldtest = -175.12345678;
>    char fteststr[32], dteststr[32], ldteststr[32];
> 
>    // try converting using C functions
>    sprintf(fteststr,"%f",ftest);
>    sprintf(dteststr,"%lf",dtest);
>    sprintf(ldteststr,"%Lf",ldtest);
>    NSLog(@"C values:   Float: %f   Double: %lf   Long Double: %Lf",ftest,dtest,ldtest);
>    NSLog(@"C strings:  Float: %s   Double: %s   Long Double: %s",fteststr,dteststr,ldteststr);
> 
>    // try converting using NSNumbers
>    float f2test;
>    double d2test;
>    NSNumber *fNumb = [NSNumber numberWithFloat:ftest];
>    NSNumber *dNumb = [NSNumber numberWithDouble:dtest];
>    f2test = [fNumb floatValue];
>    d2test = [dNumb doubleValue];
>    NSString *sStr = [dNumb stringValue];
>    NSLog(@"NSNumber:   Float: %f   Double: %lf   String: %@",ftest,dtest,sStr);
> 
>    // try converting using NSStrings
>    NSString *fStr = [NSString stringWithFormat:@"%f",ftest];
>    NSString *dStr = [NSString stringWithFormat:@"%lf",dtest];
>    NSString *ldStr = [NSString stringWithFormat:@"%Lf",ldtest];
>    NSLog(@"NSString:   Float: %@   Double: %@   String: %@",fStr,dStr,ldStr);
> 
>    [pool drain];
>    return 0;
> }
> 
> run
>   C values:   Float: -175.123459   Double: -175.123457   Long Double: -175.123457
>   C strings:  Float: -175.123459   Double: -175.123457   Long Double: -175.123457
>   NSNumber:   Float: -175.123459   Double: -175.123457   String: -175.12345678  <--- the only correct result
>   NSString:   Float: -175.123459   Double: -175.123457   String: -175.123457

The difference between %f, %lf, %Lf are about the size of the argument, not the representation as a string. For the representation such as number of decimals you have to provide flags and/or precision in the specifier IEEE printf specification.

Christiaan