KX Community

Find answers, ask questions, and connect with our KX Community around the world.
KX Community Guidelines

Home Forums kdb+ .Q.f output unexpected value in kdb4.0

  • .Q.f output unexpected value in kdb4.0

    Posted by baichen on September 20, 2023 at 12:00 am

     

     

    Hello all,

    Came across a problem with .Q.f .

    We expected the first output to be 4194304.98 .

    I saw this issue only in kdb4.0.

    Output in kdb3.5 is expected.

    Any advice is appreciated.

    baichen replied 3 months, 1 week ago 1 Member · 0 Replies
  • 0 Replies
  • rocuinneagain

    Member
    September 20, 2023 at 12:00 am
    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.

     

     

     

Log in to reply.