3:48 p.m.
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 ?
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
2) Is there a compact text on null keyword?
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.
3) What are main uses of isnull()
Oleh
4:25 p.m.
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
4:39 p.m.
Dear Allin, Thank you for comprehensive answer. I have experience in learning some soft by
experiments, so I hope to be useful in finding undocumented hacks to be excluded
P.S. Sorry for inaccurate line: > eval isnull(varname(somelist)
This works: nulldata 1 list nlist = null z = isnull(varname(nlist)) z
Another hack or legal?
27 жовтня 2015, 22:25:12, від "Allin Cottrell" < cottrell(a)wfu.edu >:
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
_______________________________________________
Gretl-devel mailing list
Gretl-devel(a)lists.wfu.edu
http://lists.wfu.edu/mailman/listinfo/gretl-devel
5:07 p.m.
On Tue, 27 Oct 2015, oleg_komashko(a)ukr.net wrote:
Dear Allin, Thank you for comprehensive answer. I have experience
in learning some soft by experiments, so I hope to be useful in
finding undocumented hacks to be excluded
Thanks, that's helpful!
P.S. Sorry for inaccurate line:
eval isnull(varname(somelist)
This works:
nulldata 1
list nlist = null
z = isnull(varname(nlist))
z
Another hack or legal?
Note, the output from the above is:
<output>
? nulldata 1
periodicity: 1, maxobs: 1
observations range: 1 to 1
? list nlist = null
Generated list nlist
? z = isnull(varname(nlist))
Generated scalar z = 1
? z
1
Done
</output>
Well, it's certainly an oddity but it is legal, and if you think it
through it is consistent with the documentation for isnull(). The
expression "varname(nlist)" produces an empty string -- in
conformity with its documentation, since nlist is empty. But the
empty string is not the name of any existing variable, so
isnull(<empty string>) correctly returns 1 (!).
Allin
5:38 p.m.
Dear Allin, I didn't have any opinion on it. That's why I asked. I do not see any
contradictions there.
Oleh
27 жовтня 2015, 23:07:38, від "Allin Cottrell" < cottrell(a)wfu.edu >:
On Tue, 27 Oct 2015, oleg_komashko(a)ukr.net wrote:
Dear Allin, Thank you for comprehensive answer. I have experience
in learning some soft by experiments, so I hope to be useful in
finding undocumented hacks to be excluded
Thanks, that's helpful!
P.S. Sorry for inaccurate line:
eval isnull(varname(somelist)
This works:
nulldata 1
list nlist = null
z = isnull(varname(nlist))
z
Another hack or legal?
Note, the output from the above is:
<output>
? nulldata 1
periodicity: 1, maxobs: 1
observations range: 1 to 1
? list nlist = null
Generated list nlist
? z = isnull(varname(nlist))
Generated scalar z = 1
? z
1
Done
</output>
Well, it's certainly an oddity but it is legal, and if you think it
through it is consistent with the documentation for isnull(). The
expression "varname(nlist)" produces an empty string -- in
conformity with its documentation, since nlist is empty. But the
empty string is not the name of any existing variable, so
isnull(<empty string>) correctly returns 1 (!).
Allin
_______________________________________________
Gretl-devel mailing list
Gretl-devel(a)lists.wfu.edu
http://lists.wfu.edu/mailman/listinfo/gretl-devel
5:48 p.m.
This is by way of a P.S. (post scriptum).
On Tue, 27 Oct 2015, I quoted Oleh:
> This works:
>
> nulldata 1
> list nlist = null
> z = isnull(varname(nlist))
> z
>
> Another hack or legal?
and replied:
Note, the output from the above is:
<output>
? nulldata 1
periodicity: 1, maxobs: 1
observations range: 1 to 1
? list nlist = null
Generated list nlist
? z = isnull(varname(nlist))
Generated scalar z = 1
? z
1
Done
</output>
Well, it's certainly an oddity but it is legal, and if you think
it through it is consistent with the documentation for isnull().
The expression "varname(nlist)" produces an empty string -- in
conformity with its documentation, since nlist is empty. But the
empty string is not the name of any existing variable, so
isnull(<empty string>) correctly returns 1 (!).
I should perhaps point out that while the above is a perfectly legal
test, it's not what one might be looking for, namely a test for
emptiness of the list argument. Consider the following:
<hansl>
open data4-1
list L = price sqft
eval isnull(varname(L))
</hansl>
In this case, too, "eval" will produce an answer of 1. That's
because varname(L) returns the string "price,sqft", which is not the
name of any existing (or even possible!) hansl variable.
Allin
1:28 a.m.
Very instructive for making experiments! Such things often occur in incomplete
experiments as was the case with me Oleh
27 жовтня 2015, 23:48:26, від "Allin Cottrell" < cottrell(a)wfu.edu >:
This is by way of a P.S. (post scriptum).
On Tue, 27 Oct 2015, I quoted Oleh:
> This works:
>
> nulldata 1
> list nlist = null
> z = isnull(varname(nlist))
> z
>
> Another hack or legal?
and replied:
Note, the output from the above is:
<output>
? nulldata 1
periodicity: 1, maxobs: 1
observations range: 1 to 1
? list nlist = null
Generated list nlist
? z = isnull(varname(nlist))
Generated scalar z = 1
? z
1
Done
</output>
Well, it's certainly an oddity but it is legal, and if you think
it through it is consistent with the documentation for isnull().
The expression "varname(nlist)" produces an empty string -- in
conformity with its documentation, since nlist is empty. But the
empty string is not the name of any existing variable, so
isnull(<empty string>) correctly returns 1 (!).
I should perhaps point out that while the above is a perfectly legal
test, it's not what one might be looking for, namely a test for
emptiness of the list argument. Consider the following:
<hansl>
open data4-1
list L = price sqft
eval isnull(varname(L))
</hansl>
In this case, too, "eval" will produce an answer of 1. That's
because varname(L) returns the string "price,sqft", which is not the
name of any existing (or even possible!) hansl variable.
Allin
_______________________________________________
Gretl-devel mailing list
Gretl-devel(a)lists.wfu.edu
http://lists.wfu.edu/mailman/listinfo/gretl-devel
4:43 p.m.
On Tue, 27 Oct 2015, Allin Cottrell wrote:
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.
Let me join in taking the blame. At the time when isnull() was introduced,
it was pretty much a C-ism, and nobody thought that in fact the semantics
of "null" in hansl are not exactly mappable to the NULL keyword in C. But
then again, hansl has evolved pretty much like a natural language, with
all its quirks and oddities.
-------------------------------------------------------
Riccardo (Jack) Lucchetti
Dipartimento di Scienze Economiche e Sociali (DiSES)
Università Politecnica delle Marche
(formerly known as Università di Ancona)
r.lucchetti(a)univpm.it
http://www2.econ.univpm.it/servizi/hpp/lucchetti
-------------------------------------------------------
3126
days inactive
3127
days old
7 comments
3 participants
participants (3)
-
Allin Cottrell -
oleg_komashko＠ukr.net -
Riccardo (Jack) Lucchetti