On 2011-07-08 04:31, Loopback wrote:
On 2011-07-08 02:28, bearophile wrote:
Loopback:
I do want to ask though what an alternative would be in this case, to
compare the two different values. You mentioned something about
"epsilons" but I have no experience within this field. I would really
appreciate an example or something similar so I could understand your
statement.
If you want to use floating point values in your programs then you
probably need to know something about floating point representation.
This is a good starting point:
http://en.wikipedia.org/wiki/Floating_point
For your problem there is the feqrel function:
http://www.digitalmars.com/d/2.0/phobos/std_math.html#feqrel
From what I can see these are two identical values, I would be more than
glad if someone could explain just what is the difference between these
two "non-equal" values and how make them "equal". Perhaps I should use
ints and long instead since they don't seem to suffer from this
"problem"?
Generally in a program you use floating point values only if you can't
use integral values (and you don't want to go toward fixed point
values, rationals, etc).
Bye,
bearophile
I've looked up both approxEqual and feqrel, and the last one seems most
appropriate, though I cannot use this function without issuing any
errors. If I use the following code:
writefln("Equality: %s", feqrel(m, 1.73205f));
(From the previous example)
I receive this error with the code:
Error: function std.math.feqrel!(float).feqrel has no return statement,
but is expected to return a value of type int
Error: template instance std.math.feqrel!(float) error instantiating
I weren't able to solve this error so if it's of anyone's interest I
used this function instead;
int feqrel(real a, real b)
{
if (a==b) return real.mant_dig;
real diff = fabs(a-b);
ushort *pa = cast(ushort *)(&a);
ushort *pb = cast(ushort *)(&b);
ushort *pd = cast(ushort *)(&diff);
int bitsdiff = ( ((pa[4]&0x7FFF) + (pb[4]&0x7FFF)-1)>>1) - pd[4];
if (pd[4]== 0)
{
diff*=0x1p+63;
return bitsdiff + real.mant_dig - pd[4];
}
if (bitsdiff>0) return bitsdiff+1;
return bitsdiff==0 ? pa[4]==pb[4] : 0;
}
From: http://www.digitalmars.com/d/archives/digitalmars/D/27873.html
