Round of in KDB

Community Support
1

Hi All,

Here is the information i want to share,

To round of up to any decimal place,Use below formula

Formula ---->  n*floor 0.5+m*AnyFloatValue

Eg.

  1. One decimal place:

n = 1/10 = 0.1

m = 1*10 = 10

Expression Example ---->  0.1*floor 0.5+10*34.45

Output -----> 34.5f

  1. Two decimal place:

n = 1/100 = 0.01

m = 1*100 = 100

Expression Example ---->  0.01*floor 0.5+100*34.456

Output -----> 34.46f

2

you can find this functionality in the ‘.util.rnd’ function provided in the code that accompanies “Q Tips: Fast, Scalable and Maintainable Kdb+”: http://q-tips.net

https://github.com/psaris/qtips/blob/master/util.q

.util.rnd:{x*“j”$y%x}

q).util.rnd[.1]34.45
34.5
q).util.rnd[.01]34.456
34.46

note: you can cast the number instead of adding .5 and taking the floor

3

If you need to be as accurate as possible, I’d suggest using division
rather than multiplication after the casting:

q)rnd:{x*“j”$y%x}
q)rnd2:{(“j”$x*y)%x}
q)0=1.4-rnd[0.1;1.41]
0b
q)0=1.4-rnd2[10;1.41]
1b

4

be careful when relying on exact comparisons with floats. a slight change to the inputs, might have unexpected results.

q)rnd2[10;4.1]
4.0999999999999996
q)rnd2[10;1#4.1]
,4.1000000000000005

q optimizes division of a vector by scalar. it first taking the reciprocal of the scalar and then multiplies.

q)41%10
4.0999999999999996
q)enlist[41]%10
,4.1000000000000005
q)41*1%10
4.1000000000000005

if you need exact values, it is better to store (and round) your data as integers