Re: Breaking a table of record rows into an array

On 3/13/2024 4:49 PM, Kaz Kylheku wrote:

On 2024-03-13, Keith Thompson <Keith.S.Thompson+u@gmail.com> wrote:

Kaz Kylheku <433-929-6894@kylheku.com> writes:

On 2024-03-13, Keith Thompson <Keith.S.Thompson+u@gmail.com> wrote:

arnold@freefriends.org (Aharon Robbins) writes:

In article <usqkgn$he7u$2@dont-email.me>,
Ed Morton  <mortonspam@gmail.com> wrote:

the effect of setting `NF` is
undefined behavior per POSIX and so will do different things in
different awk variants and even in 1 awk variant can behave differently
depending on whether you're setting it to a higher or lower than
original value

>
This is not true. The effect of setting NF was well defined
by the original awk book and also in POSIX.
>
Decreasing NF throws away fields. Increasing NF adds the
intervening fields with the null string as their values
and rebuilds the record.

>
I don't see that in the POSIX specification.

>
The key is this:
>
   References to nonexistent fields (that is, fields after $NF), shall
   evaluate to the uninitialized value.
>
NF is assignable, and fields after $NF do not exist. Thus if we
have four fields and set NF = 3, then $4 doesn't exist.

That's a bit like the argument from an old episode of the comedy TV show "Yes, Prime Minister" in the UK where his aide says (paraphrased) "Some country has done X, we must go something. War is something, therefore we must go to war".
Being able to set NF to 3 does not mean you must delete $4. Why not delete $1 or $2 instead? You'd still end up with 3 fields to satisfy the value of NF. Lots of things you can do are undefined by POSIX despite how sensible some impacts may seem, assigning a value to NF is just 1 more of them.
You could say that "$0 holds the last record read, you can use $0 in the END section, therefore in the END section $0 must contain the value of the last record read". Except that's not true. From the gawk manual (https://www.gnu.org/software/gawk/manual/html_node/I_002fO-And-BEGIN_002fEND.html#I_002fO-And-BEGIN_002fEND):
----
Most probably due to an oversight, the standard does not say that $0 is also preserved, although logically one would think that it should be. In fact, all of BWK awk, mawk, and gawk preserve the value of $0 for use in END rules. Be aware, however, that some other implementations and many older versions of Unix awk do not.
----

>
That describes what happens if NF is modified by assignment, but I don't
see that it implies that such an assignment is allowed.

 "The left-hand side of an assignment and the target of increment and
decrement operators can be one of a variable, an array with index, or a
field selector."
 NF is described as a variable.  Some unique remarks are made about NF,
but none deny that it's assignable like any other variable.
 

But I can imagine a hypothetical awk-like language in which assigning to
NF has undefined behavior.  My question is, how does the POSIX
specification not describe that language?

 That language is failing to support an instance of a variable
being the left operand of an assignment, which a variable "can be".
 It looks like the violation of a requirement.
 

On the other hand, it also implies that `foo = 42` is valid where `foo`
is the name of a user-defined function (gawk disallows it).

 POSIX does say that "[t]he same name shall not be used as both a
function parameter name and as the name of a function or a special awk
variable." So foo = 42 isn't valid if foo is already a function.
 Also: "The same name shall not be used both as a variable name with
global scope and as the name of a function. The same name shall not be
used within the same scope both as a scalar variable and as an array."
 All that said, the business of the NF tail wagging the $1, $2, ...
legs of the dog should be the target of at least one clarifying remark,
and the other defects should also be corrected:
 - In a BEGIN clause NF should be undefined unless any action
   whatsoever is executed that sets its value: direct assignment,
   use of getline or assignment to $0.
 - At the start of the execution of an END clause, NF retains
   its current value (or undefined status, if it was never set);
   the END clause has no implicit effect on NF.
 

All of the above claims that POSIX states you can assign a value to NF. That may or may not be correct, I expect it is but I don't care because nothing above nor in the POSIX spec states what the IMPACT is of assigning a value to NF. As far as I can see there is absolutely nothing in the POSIX spec that says anything like "if you set NF to a higher value fields will be created and if you set NF to a lower value fields will be removed" but I'd honestly love to be proven wrong and shown the section that does defined the impact of assigning a higher or lower value to NF.
      Ed.