Re: [Gretl-devel] some questions on null keyword
On Tue, 27 Oct 2015, oleg_komashko(a)ukr.net wrote:
Dear all, There are several ways to check whether a list is empty,
e.g.
eval isnull(varname(somelist)
eval somelist=null
eval nelem(list)
1) Which way is the most efficient/ recommended ?
If you want to check whether a variable of type list has 0 members
the only sane way is the third, using nelem(). The first "eval" is
invalid syntax and the second, if it works, is some sort of
undocumented hack.
Also, the behavior of null keyword is different with lists and
strings, for example
somelist = null
is evaluated but
eval somestring = null
outputs
Data types not conformable for operation
Like I said, an undocumented hack.
2) Is there a compact text on null keyword?
No. That's the whole problem here. We (well, I take most of the
responsibility) did not properly think through what precise meaning
we wanted to attach to "null", across the various contexts in which
we have allowed it to occur.
I can imagine the following way to use isnull(): Suppose, I want
to write a function which creates new variables with automated
names. isnull() could be used for checking duplicated names.
Yes, that's true. As it stands, isnull() returns 1 if and only if
there is no object with the name given as an argument; so it could
be used to avoid collision of names.
3) What are main uses of isnull()
The main intended use of isnull() is for writers of functions.
Suppose you have written a function which has a parameter specified
like
matrix *m[null]
or
bundle *b[null]
that is, a "pointerized" variable with a default of "null", so that
the caller is OK with skipping the argument, or giving a dummy
argument of "null". Now you want to know if you actually got a
existing variable as an argument: isnull() will do the job. However,
it won't do the job for list arguments with a "null" default,
because (as things stand) gretl will manufacture an empty list as a
pseudo-argument.
Allin