Printf() anomaly?

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#1
What would cause this command,

Code:
Printf(L"Estimated local time offset: %s%3.3f seconds\r\n\r\n", fOffset > 0 ? L"+" : L"", fOffset);
to produce this output:

Code:
Estimated local time offset: -0.00000 seconds
(five characters after the decimal point) It doesn't happen often.

Notice that I'm forcing the "+" on positive values. Can I get Printf to do that automatically?
 
#2
I can get it to happen every time. This code

Code:
#define N 100000
    LONGLONG x;
    for ( INT i=0; i<10; i++ )
    {
        x = -10 + (rand()%20);
        float f = (float) x/N;
        Printf(L"%s%3.3f\r\n", f > 0 ? L"+" : L"", f);

    }
produces:

Code:
-0.0000
-0.0000
+0.0000
-0.0000
-0.00000
-0.0000
+0.0000
+0.0000
-0.0000
-0.0000
If I change N to 10000000, it produces this:

Code:
-0.000000
-0.000000
+0.000000
-0.000000
-0.000000
-0.000000
+0.000000
+0.000000
-0.000000
-0.000000
 

rconn

Administrator
Staff member
May 14, 2008
10,103
85
#3
> What would cause this command,
>
> Code:
> ---------
> Printf(L"Estimated local time offset: %s%3.3f seconds\r\n\r\n", fOffset
> > 0 ? L"+" : L"", fOffset);
> ---------
> to produce this output:
>
>
> Code:
> ---------
> Estimated local time offset: -0.00000 seconds
> ---------
> (five characters after the decimal point) It doesn't happen often.
>
> Notice that I'm forcing the "+" on positive values. Can I get Printf
> to do that automatically?
First, Printf() doesn't support floats, only doubles. Any %f argument is
assumed to actually be a double; if it's not, then unpleasantness will
probably soon erupt.

And no, there's no way to force a '+' on positive values.
 
#4
First, Printf() doesn't support floats, only doubles. Any %f argument is
assumed to actually be a double; if it's not, then unpleasantness will
probably soon erupt.
That doesn't seem to matter.

Code:
#define N 10000000
    LONGLONG x;
    for ( INT i=0; i<10; i++ )
    {
        x = -10 + (rand()%5000);
        double f = (double) x/N;
        Printf(L"%s%3.3f\r\n", f > 0 ? L"+" : L"", f);

    }

+0.00000
+0.000
+0.000
+0.000
+0.000
+0.0000
+0.000
+0.000
+0.000
+0.000
Should I **not** expect it to honor my "3.3" specificartion?
 

rconn

Administrator
Staff member
May 14, 2008
10,103
85
#5
> ---Quote (Originally by rconn)---
> First, Printf() doesn't support floats, only doubles. Any %f argument
> is
> assumed to actually be a double; if it's not, then unpleasantness will
> probably soon erupt.
> ---End Quote---
> That doesn't seem to matter.
> ---------
> Should I **not** expect it to honor my "3.3" specificartion?
I have no idea; I've never used the floating point code. I won't have time
to look at it until the weekend.