# floating-WTF

sclytrack sclytrack at fake.com
Tue Jan 24 01:33:19 PST 2012

```t=0.750000, a=-2.000000, b=2.000000, s1=-0.500000, s2=0.000000,
rt1=-0.500000, rt2=1.500000  :::
1.5

t = 0.75	ok
a = -2.0	ok
b = 2.0		ok
s1 = -0.5	ok
s2 = 1.5   //<----different above says s2 = 0.0
rt1= 1.0 = s1 + s2   semi-ok depends on s2.
rt2 = 1.0 = ((1.0 - t) * a) + (t * b);  goes nuts.

On 01/24/2012 10:03 AM, sclytrack wrote:
> On 01/24/2012 04:13 AM, Caligo wrote:
>> alias double Real;
>> //alias float Real;
>>
>> // simple linear interpolation; I partitioned the internals to help me
>> figure out what was happening.
>> Real lerp(Real t, Real a, Real b){
>> Real s1 = (1.0 - t) * a;
>> Real s2 = t * b;
>> Real rt1 = s1 + s2;
>> Real rt2 = ((1.0 - t) * a) + (t * b);
>> writefln("t=%.2f, a=%.2f, b=%.2f, s1=%.2f, s2=%.2f, rt1=%.2f,
>> rt2=%.2f :::", t, a, b, s1, s2, rt1, rt2);
>> return rt2;
>> }
>>
>> unittest{
>>
>> writeln(lerp(0.75, -2.0, 2.0)); // the correct result is 1.0
>> }
>>
>> compile and run with 'dmd -inline -unittest' and the output should be:
>>
>> t=0.75, a=-2.00, b=2.00, s1=-0.50, s2=1.50, rt1=1.00, rt2=1.00 :::
>> 1
>>
>> Now, change 'Real' to float by uncommenting the second line and
>> compile and run with 'dmd -unittest'. This is what I get for the
>> output:
>>
>> t=0.75, a=0.00, b=2.00, s1=-0.50, s2=0.00, rt1=-0.50, rt2=1.50 :::
>> 1.5
>
> This: (with float)
>
> writefln("t=%.2f, a=%.2f, b=%.2f, s1=%.2f, s2=%.2f, rt1=%.2f, rt2=%.2f
> :::", t, a, b, s1, s2, rt1, rt2);
> writefln("t=%.2f, a=%f, b=%.2f, s1=%.2f, s2=%.2f, rt1=%.2f, rt2=%.2f
> :::", t, a, b, s1, s2, rt1, rt2);
>
> Outputs: (with float)
>
> t=0.75, a=0.00, b=2.00, s1=-0.50, s2=0.00, rt1=-0.50, rt2=1.50 :::
> t=0.75, a=-2.000000, b=2.00, s1=-0.50, s2=0.00, rt1=-0.50, rt2=1.50 :::
>
> difference:
>
> %.2f ---> %f
>
>
>
>>
>> I have no idea why 'a' is zero. 'rt1' and 'rt2' do not have the same
>> value, and 'lerp' does not return 1.0. Compiling with 'dmd -inline
>> -unittest' does produce the correct result as before, though.
>>
>> You can play with different compiler options, such as -O, to get the
>> same weird behavior.
>>
>> I presume this is another DMD bug?
>

```