Funny issue with casting double to ulong

[Basile B via Digitalmars-d-learn]

On Monday, 3 July 2017 at 03:50:14 UTC, Saurabh Das wrote:
> Consider this snippet:
>
> void main()
> {
>     import std.stdio;
>     auto a = 6.2151;
>     auto b = a * 10000;
>     auto c = cast(ulong)b;
>     writeln("a: ", typeof(a).stringof, " ", a);
>     writeln("b: ", typeof(b).stringof, " ", b);
>     writeln("c: ", typeof(c).stringof, " ", c);
>
>     auto x = 62151.0;
>     auto y = cast(ulong)x;
>     writeln("x: ", typeof(x).stringof, " ", x);
>     writeln("y: ", typeof(y).stringof, " ", y);
> }
>
> The output is:
> a: double 6.2151
> b: double 62151
> c: ulong 62150
> x: double 62151
> y: ulong 62151
>
> Why does c round off from 62151 to 62150 when casting to ulong?
>
> Thanks,
> Saurabh

6.251 has no perfect double representation. It's real value is:

     6.21509999999999962483343551867E0

Hence when you cast to ulong after the product by 10_000, this is 
the equivalent of

     trunc(62150.9999999999962483343551867E0)

which gives 62150

CQFD ;-]