.Q.f output unexpected value in kdb4.0

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1

https://learninghub.kx.com/forums/topic/q-f-output-unexpected-value-in-kdb4-0

 

 

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.

2
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.