RoundOffTest() seems to round UP and DOWN

Question

void RoundOffTest(double number)
{
    // What value would you pass to RoundOffTest() in order to get this output:
    // BEFORE number=1.785000 round=178.000000 (round down)
    // AFTER  NUMBER=1.785000 round=179.000000 (round up)
    //
    // It seems to round it DOWN.
    // But the 2nd line seems to round it UP.
    // Isn't that impossible?  Wouldn't there be NO possible number you could pass
    // this function, and see that output?  Or is there?

    NSLog(@"BEFORE number=%f round=%f (round down)", number, round(number * 100));
    double NUMBER = 1.785000;  
    NSLog(@"AFTER  NUMBER=%f round=%f (round up)  ", NUMBER, round(NUMBER * 100));

}

Answer 1

Set number to 1.7849995 and you'll get the result you see. Note that %f is printing 6 decimal places, and that the output is rounded to that number of places - so 1.7849994 will not work.

Formatting without rounding

To answer your comment question: use NSNumberFormatter. I think all the printf style formatters round. NSNumberFormatter provides 7 different rounding modes. Here is a modified version of your test function:

void RoundOffTest(double number)
{
    NSNumberFormatter *formatter = [NSNumberFormatter new];
    [formatter setFormat:@"0.000000"];  // see docs for format strings
    [formatter setRoundingMode:NSNumberFormatterRoundFloor]; // i.e. truncate
    NSString *formatted = [formatter stringFromNumber:[NSNumber numberWithDouble:number]];
    NSLog(@"%@\n", formatted);
    [formatter release];
}