How to create a UDF used for a SQL where clause in DolphinDB?

Question

I have created a UDF and used it in a where clause, which is shown in the following figure.

def isZero(number,precision=4){
    unit =1\pow(10,precision)
    ngUnit =-unit
    if(ngUnit < number && number < unit){
        return true
    }
    return false
}

res= select * from diff where isZero(openDiff)

But an error is reported as follows:

Why is that happening? Is there anything I should be aware of when creating a UDF in such a clause?

Answer 1

DolphinDB supports vector operation. In the where clause, the parameter openDiff of your UDF is a column (ie, a vector), so you need to ensure that your function returns a vector of the same length. But the output you designed for your UDF is a Boolean scalar. To solve your problem, refer to the following methods:

(1) change your UDF to a vector function

(2) use each(isZero, openDiff)

(3) Use iif : where iif((openDiff<1.0/pow(10,4) && openDiff>-1.0/pow(10,4)),true,false) == true

(4) use eqFloat(precision=4)