.Q.f output unexpected value in kdb4.0

[rocuinneagain]

From 3.6 Readme

2018.09.26 NEW added -27! as a more precise, builtin version of .Q.f. n.b. It is atomic and doesn't take P into account. e.g.  q)("123456789.457";"123456790.457")~-27!(3i;0 1+2#123456789.4567)

The definition of .Q.f changed at this time also – when comparing q.k file definitions:

3.6.0 2018.09.10

f:{$[^y;"";y<0;"-",f[x;-y];y<1;1_f[x;10+y];9e15>j:"j"$y*/x#10;(x_j),".",(x:-x)#j:$j;$y]}

3.6.0 2018.10.03

f:{$[^y;"";y<0;"-",f[x;-y];y<1;1_f[x;10+y];9e15>j:"j"$y*prd x#10f;(x_j),".",(x:-x)#j:$j;$y]}

 

Floating point numbers are not exact and such behaviours should be expected in certain cases:

https://code.kx.com/q/basics/precision/

 

They are approximations:

q)P 
0 

q)4194303.975 
4194303.9750000001 

q)4194304.975 
4194304.9749999996

 

The same in C:

int main() {         double x = 4194303.975;         printf("%10.10fn",x); }

Result:

4194303.9750000001

 

If you find that you need exact decimal rounding, consider whether you actually need to operate in fixed-point, rather than floating-point.
e.g. keep monetary values in integral millicents etc.