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Unsetting variables in GNU Awk
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Janis Papanagnou
Jan 17, 2021, 3:31:33 AM1/17/21
to
Scalar [user-]variables are unset by default so that a test
var==0 && var=="" can be used to check that 'unset' state.
Whenever one assigns a value, e.g. var=0 or var="" that test
will further return 'false'. - Is there some way to unset a
variable again?
Janis
var==0 && var=="" can be used to check that 'unset' state.
Whenever one assigns a value, e.g. var=0 or var="" that test
will further return 'false'. - Is there some way to unset a
variable again?
Janis
Kaz Kylheku
Jan 17, 2021, 3:41:17 AM1/17/21
to
Firstly, if you can substitute an array element for the variable, then
you can delete the containing array.
E.g. here our variable is z[0]:
$ gawk 'BEGIN { z[0] = 1; delete z ; print z[0] == 0 && z[0] == "" }'
1
See what I mean?
Secondly, we can return a scalar to to he unset state by assigning
to it the value of such a scalar.
$ gawk 'BEGIN { a = 1; print a == 0 && a == "";
a = b; print a == 0 && a == "" }'
0
1
a was able to return to the unset state by copying the value of b,
which was never set.
Not vouching for the portability of that without additional research.
--
TXR Programming Language: http://nongnu.org/txr
Janis Papanagnou
Jan 17, 2021, 5:50:48 AM1/17/21
to
On 17.01.2021 09:41, Kaz Kylheku wrote:
> On 2021-01-17, Janis Papanagnou <janis_pa...@hotmail.com> wrote:
>> Scalar [user-]variables are unset by default so that a test
>> var==0 && var=="" can be used to check that 'unset' state.
>> Whenever one assigns a value, e.g. var=0 or var="" that test
>> will further return 'false'. - Is there some way to unset a
>> variable again?
>
> Yes.
>
> [...]
> On 2021-01-17, Janis Papanagnou <janis_pa...@hotmail.com> wrote:
>> Scalar [user-]variables are unset by default so that a test
>> var==0 && var=="" can be used to check that 'unset' state.
>> Whenever one assigns a value, e.g. var=0 or var="" that test
>> will further return 'false'. - Is there some way to unset a
>> variable again?
>
> Yes.
>
>
> Secondly, we can return a scalar to to he unset state by assigning
> to it the value of such a scalar.
>
> $ gawk 'BEGIN { a = 1; print a == 0 && a == "";
> a = b; print a == 0 && a == "" }'
> 0
> 1
>
> a was able to return to the unset state by copying the value of b,
> which was never set.
This is a great idea. Thanks!
> Secondly, we can return a scalar to to he unset state by assigning
> to it the value of such a scalar.
>
> $ gawk 'BEGIN { a = 1; print a == 0 && a == "";
> a = b; print a == 0 && a == "" }'
> 0
> 1
>
> a was able to return to the unset state by copying the value of b,
> which was never set.
> Not vouching for the portability of that without additional research.
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