Hi Friends!

Still making my first feeble steps in TCL so please excuse me if this is
naive or was asked multiple times.

Attempt to fetch by non-existing key in "associative array" results in
error, e.g.

set a(1) 5
puts $a(2) ;# yields error

the workaround seems to be [info exists ::a(2)] which feels a bit remote
from other "array" commands.

Is there some motivation why some command for get-with-default is not implemented, e.g.

puts [array peek $a 2 "default value"]

Popular use-case for this would be creating map where elements are
updated (like counter of words etc) - though I found this is cleverly
covered by "incr" and "append" commands properly behaving
when element to be incremented or appended does not exist yet.

But I suspect there are other situations when such a command may be
handy.

Also why [array exists ...] command does not exist (while [dict exists
..] does)? Perhaps there is something about no good syntax for it due
to how arrays are implemented?

--
to email me substitute github with gmail please

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* RodionGork <rodiongork@github.com>
| Is there some motivation why some command for get-with-default is not
| implemented, e.g.

| puts [array peek $a 2 "default value"]

Most probably because up to now nobody required it so badly that s/he implemented it, since the [info exists] approach is not too bad IMHO.

proc array_peek {arr key default} {
upvar $arr a
if {![array exists a] || ![info exists a($key)]} {
return $default
}
return $a($key)
}
array set a {1 one}
array_peek a 1 "default value"
=> one
array_peek a 2 "default value"
=> default value

Or you could set up a read-trace on the array to provide non-existing values:

array set a {}
trace add variable a read set_array_default
proc set_array_default {name1 name2 op} {
upvar a $name1
# puts stderr "set_array {$name1} {$name2} {$op}"
if {![info exists a($name2)]} {
set a($name2) "default"
}
}
set a(2)
=> default

Though that would alter the array and make the key exists which might
not be what you want.

HTH
R'

--- SoupGate-Win32 v1.05
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On Fri, 16 Aug 2024 07:10:30 +0000
RodionGork <rodiongork@github.com> wrote:

Hi Friends!

Still making my first feeble steps in TCL so please excuse me if this is naive or was asked multiple times.

Attempt to fetch by non-existing key in "associative array" results in
error, e.g.

set a(1) 5
puts $a(2) ;# yields error

the workaround seems to be [info exists ::a(2)] which feels a bit remote
from other "array" commands.

Is there some motivation why some command for get-with-default is not implemented, e.g.

puts [array peek $a 2 "default value"]

Popular use-case for this would be creating map where elements are
updated (like counter of words etc) - though I found this is cleverly
covered by "incr" and "append" commands properly behaving
when element to be incremented or appended does not exist yet.

But I suspect there are other situations when such a command may be
handy.

Already there in 8.7/9.0

% array default set foo NOSUCHVALUE
% set foo(1)
NOSUCHVALUE
% info exists foo(1)
0

8.7/9.0 also adds [dict getwithdefault] (aka [dict getdef]) for dict
values.

Also why [array exists ...] command does not exist (while [dict exists
..] does)? Perhaps there is something about no good syntax for it due
to how arrays are implemented?

The [array exists] command is there since ... forever

% array exists foo
1
% array exists nosucharray
0

Well, at least since 8.4 . See https://www.tcl-lang.org/man/tcl8.4/TclCmd/array.htm

--
Emiliano

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

* Emiliano <emil.g@example.invalid>
| On Fri, 16 Aug 2024 07:10:30 +0000
| RodionGork <rodiongork@github.com> wrote:
| > Also why [array exists ...] command does not exist (while [dict exists
| > ..] does)? Perhaps there is something about no good syntax for it due
| > to how arrays are implemented?

| The [array exists] command is there since ... forever

| % array exists foo
| 1
| % array exists nosucharray
| 0

Obviouosly in this context the OP meant "array exists" with respect to a specific key, i.e.

array exists arrayname key
=> return true if arrayname is an array and the key exists in that array.

R'

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

Friends, thanks for all the replies!

And for suggestions providing insight on techniques of which I wasn't
aware of, especially that "read trace".

Yep, I definitely meant something like [array element_exists ...] rather
than [array exists ...] which seems a bit inconsistent with [dict exists
..] but of course it is not a big issue.

My curiosity was mostly satisfied, thanks! The only small thing I still
haven't grasped well is why it happened that checking for element in
array found its way into [info exists...] rather than some [array ...] subcommands. I suspected there was something hindering implementing it
there, but probably it just, well, so happened historically :)

--
to email me substitute github with gmail please

--- SoupGate-Win32 v1.05
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