From Newsgroup: comp.lang.tcl

I have a switch command which is doing something I don't expect but I
don't understand what I've done wrong. In this example the default is
always executed but I expect the case before that to be executed.

const UNCOMPRESSED U
const ZLIB_COMPRESSED Z
const SAME_AS_PREV =
set filename somefile.txt
set action "added"
set kind Z
switch $kind {
$::SAME_AS_PREV { puts "unchanged \"$filename\"" }
$::UNCOMPRESSED { puts "$action \"$filename\"" }
$::ZLIB_COMPRESSED { puts "$action \"$filename\" (zlib
compressed)" }
default { puts "!!!!!!!! UNEXPECTED !!!!!!!!" }
}

What am I doing wrong?
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From Newsgroup: comp.lang.tcl

Am 24.06.2025 um 10:23 schrieb Mark Summerfield:

I have a switch command which is doing something I don't expect but I
don't understand what I've done wrong. In this example the default is
always executed but I expect the case before that to be executed.

const UNCOMPRESSED U
const ZLIB_COMPRESSED Z
const SAME_AS_PREV =
set filename somefile.txt
set action "added"
set kind Z
switch $kind {
$::SAME_AS_PREV { puts "unchanged \"$filename\"" }
$::UNCOMPRESSED { puts "$action \"$filename\"" }
$::ZLIB_COMPRESSED { puts "$action \"$filename\" (zlib
compressed)" }
default { puts "!!!!!!!! UNEXPECTED !!!!!!!!" }
}

What am I doing wrong?

Nothing wrong. Tcl is just different, sorry for that.

The "{" always avoids expansion of variables and commands. If you want
to use variables in the switch, you have to avoid the "{".

switch -exact -- $kind [list\
$::SAME_AS_PREV { puts "unchanged \"$filename\"" }\
$::UNCOMPRESSED { puts "$action \"$filename\"" }
$::ZLIB_COMPRESSED { puts "$action \"$filename\" (zlib
compressed)" }\
default { puts "!!!!!!!! UNEXPECTED !!!!!!!!" }\
]

Nevertheless, this is no fun on the quoting level (backslashes at the
end etc). In addition, you have to take care, when the variable
expansion happens. This might be tricky.

I personally would write it like that:

switch -exact -- $kind {
= { # SAME_AS_PREV
puts "unchanged \"$filename\""
}
U { # UNCOMPRESSED
puts "$action \"$filename\""
}
Z { # ZLIB_COMPRESSED
puts "$action \"$filename\" (zlib compressed)"
}
default { puts "!!!!!!!! UNEXPECTED !!!!!!!!" }
}

Harald

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From Newsgroup: comp.lang.tcl

On Tue, 24 Jun 2025 10:37:08 +0200, Harald Oehlmann wrote:

Am 24.06.2025 um 10:23 schrieb Mark Summerfield:

I have a switch command which is doing something I don't expect but I
don't understand what I've done wrong. In this example the default is
always executed but I expect the case before that to be executed.

const UNCOMPRESSED U const ZLIB_COMPRESSED Z const SAME_AS_PREV =
set filename somefile.txt set action "added"
set kind Z switch $kind {
$::SAME_AS_PREV { puts "unchanged \"$filename\"" }
$::UNCOMPRESSED { puts "$action \"$filename\"" }
$::ZLIB_COMPRESSED { puts "$action \"$filename\" (zlib
compressed)" }
default { puts "!!!!!!!! UNEXPECTED !!!!!!!!" }
}

What am I doing wrong?

Nothing wrong. Tcl is just different, sorry for that.

The "{" always avoids expansion of variables and commands. If you want
to use variables in the switch, you have to avoid the "{".

switch -exact -- $kind [list\
$::SAME_AS_PREV { puts "unchanged \"$filename\"" }\
$::UNCOMPRESSED { puts "$action \"$filename\"" }
$::ZLIB_COMPRESSED { puts "$action \"$filename\" (zlib compressed)" }\
default { puts "!!!!!!!! UNEXPECTED !!!!!!!!" }\
]

Nevertheless, this is no fun on the quoting level (backslashes at the
end etc). In addition, you have to take care, when the variable
expansion happens. This might be tricky.

I personally would write it like that:

switch -exact -- $kind {
= { # SAME_AS_PREV
puts "unchanged \"$filename\""
}
U { # UNCOMPRESSED
puts "$action \"$filename\""
}
Z { # ZLIB_COMPRESSED
puts "$action \"$filename\" (zlib compressed)"
}
default { puts "!!!!!!!! UNEXPECTED !!!!!!!!" }
}

Harald

Thanks that works great.

Bit of a disinsentive to use consts though!
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From Newsgroup: comp.lang.tcl

Am 24.06.2025 um 10:46 schrieb Mark Summerfield:

On Tue, 24 Jun 2025 10:37:08 +0200, Harald Oehlmann wrote:

Am 24.06.2025 um 10:23 schrieb Mark Summerfield:

I have a switch command which is doing something I don't expect but I
don't understand what I've done wrong. In this example the default is
always executed but I expect the case before that to be executed.

const UNCOMPRESSED U const ZLIB_COMPRESSED Z const SAME_AS_PREV =
set filename somefile.txt set action "added"
set kind Z switch $kind {
$::SAME_AS_PREV { puts "unchanged \"$filename\"" }
$::UNCOMPRESSED { puts "$action \"$filename\"" }
$::ZLIB_COMPRESSED { puts "$action \"$filename\" (zlib
compressed)" }
default { puts "!!!!!!!! UNEXPECTED !!!!!!!!" }
}

What am I doing wrong?

Nothing wrong. Tcl is just different, sorry for that.

The "{" always avoids expansion of variables and commands. If you want
to use variables in the switch, you have to avoid the "{".

switch -exact -- $kind [list\
$::SAME_AS_PREV { puts "unchanged \"$filename\"" }\
$::UNCOMPRESSED { puts "$action \"$filename\"" }
$::ZLIB_COMPRESSED { puts "$action \"$filename\" (zlib
compressed)" }\
default { puts "!!!!!!!! UNEXPECTED !!!!!!!!" }\
]

Nevertheless, this is no fun on the quoting level (backslashes at the
end etc). In addition, you have to take care, when the variable
expansion happens. This might be tricky.

I personally would write it like that:

switch -exact -- $kind {
= { # SAME_AS_PREV
puts "unchanged \"$filename\""
}
U { # UNCOMPRESSED
puts "$action \"$filename\""
}
Z { # ZLIB_COMPRESSED
puts "$action \"$filename\" (zlib compressed)"
}
default { puts "!!!!!!!! UNEXPECTED !!!!!!!!" }
}

Harald

Thanks that works great.

Bit of a disinsentive to use consts though!

Great, that it works for you.

If I want to compare something with multiple variables, I would use an
if chain:

if {$kind eq $::SAME_AS_PREV} {
puts "unchanged \"$filename\""
} else if {$kind eq $::UNCOMPRESSED} {
puts "$action \"$filename\""
} else if {$kind eq $::ZLIB_COMPRESSED} {
puts "$action \"$filename\" (zlib >> compressed)"
} else {
puts "!!!!!!!! UNEXPECTED !!!!!!!!"
}

The "if" command takes his first argument and passes it to "expr".
Then, eval will do the variable expansion.
This does not happen with "switch".

I never used const. But its use may help for clarity.

Harald

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From Newsgroup: comp.lang.tcl

Sorry, typo in my last post:

The "if" command takes his first argument and passes it to "expr".
Then, eval will do the variable expansion.

Correct:

Then, expr will do the variable expansion.

--- Synchronet 3.21a-Linux NewsLink 1.2

From Newsgroup: comp.lang.tcl

On 24/06/2025 10:08, Harald Oehlmann wrote:

Sorry, typo in my last post:

The "if" command takes his first argument and passes it to "expr".
Then, eval will do the variable expansion.

Correct:

Then, expr will do the variable expansion.

Just one additional trick for a Tcl newbie to consider...

The OP had several lines like

$::SAME_AS_PREV { puts "unchanged \"$filename\"" }

which have now morphed (typically) into

if {$kind eq $::SAME_AS_PREV} {
puts "unchanged \"$filename\""

I have got into the habit of using the [format] command to build strings
that have fixed and variable parts - it's kind of analagous to sprintf
in C. This would lead me to change the output command to

puts [format {unchanged "%s"} $filename]

I believe it's easier to make out what's going on without all the
backslashes that are otherwise needed around the quotes that surround
the filename. (You can't exchange the outer quotes for curly brackets
because that would prevent the Tcl interpreter from expanding $filename.]

Hope you find this helpful.

Alan
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From Newsgroup: comp.lang.tcl

On 24/06/2025 10:23, Mark Summerfield wrote:

I have a switch command which is doing something I don't expect but I
don't understand what I've done wrong. In this example the default is
always executed but I expect the case before that to be executed.

const UNCOMPRESSED U
const ZLIB_COMPRESSED Z
const SAME_AS_PREV =
set filename somefile.txt
set action "added"
set kind Z
switch $kind {
$::SAME_AS_PREV { puts "unchanged \"$filename\"" }
$::UNCOMPRESSED { puts "$action \"$filename\"" }
$::ZLIB_COMPRESSED { puts "$action \"$filename\" (zlib
compressed)" }
default { puts "!!!!!!!! UNEXPECTED !!!!!!!!" }
}

What am I doing wrong?

Switch allows for two different ways to specify the pattern/body pairs:
As as a single argument or as individual arguments. If you want the
patterns to be in variables (or consts) you may prefer the latter
method. This can be written as:

switch $kind \
$::SAME_AS_PREV {
puts "unchanged \"$filename\""
} $::UNCOMPRESSED {
puts "$action \"$filename\""
} $::ZLIB_COMPRESSED {
puts "$action \"$filename\" (zlib compressed)"
} default {
puts "!!!!!!!! UNEXPECTED !!!!!!!!"
}


Schelte.

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From Newsgroup: comp.lang.tcl

Am 24.06.2025 um 12:53 schrieb Schelte:

On 24/06/2025 10:23, Mark Summerfield wrote:

I have a switch command which is doing something I don't expect but I
don't understand what I've done wrong. In this example the default is
always executed but I expect the case before that to be executed.

     const UNCOMPRESSED U
     const ZLIB_COMPRESSED Z
     const SAME_AS_PREV =
     set filename somefile.txt
     set action "added"
     set kind Z
     switch $kind {
         $::SAME_AS_PREV { puts "unchanged \"$filename\"" }
         $::UNCOMPRESSED { puts "$action \"$filename\"" }
         $::ZLIB_COMPRESSED { puts "$action \"$filename\" (zlib
compressed)" }
         default { puts "!!!!!!!! UNEXPECTED !!!!!!!!" }
     }

What am I doing wrong?

Switch allows for two different ways to specify the pattern/body pairs:
As as a single argument or as individual arguments. If you want the
patterns to be in variables (or consts) you may prefer the latter
method. This can be written as:

switch $kind \
  $::SAME_AS_PREV {
    puts "unchanged \"$filename\""
} $::UNCOMPRESSED {
    puts "$action \"$filename\""
} $::ZLIB_COMPRESSED {
    puts "$action \"$filename\" (zlib compressed)"
} default {
    puts "!!!!!!!! UNEXPECTED !!!!!!!!"
}

Schelte.

RIGHT YOU ARE !
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From Newsgroup: comp.lang.tcl

At Tue, 24 Jun 2025 08:23:45 -0000 (UTC) Mark Summerfield <m.n.summerfield@gmail.com> wrote:

I have a switch command which is doing something I don't expect but I
don't understand what I've done wrong. In this example the default is
always executed but I expect the case before that to be executed.

const UNCOMPRESSED U
const ZLIB_COMPRESSED Z
const SAME_AS_PREV > set filename somefile.txt
set action "added"
set kind Z
switch $kind {
$::SAME_AS_PREV { puts "unchanged \"$filename\"" }
$::UNCOMPRESSED { puts "$action \"$filename\"" }
$::ZLIB_COMPRESSED { puts "$action \"$filename\" (zlib
compressed)" }
default { puts "!!!!!!!! UNEXPECTED !!!!!!!!" }
}

What am I doing wrong?

switch does NOT evaluate the case labels. That is, the case labels are *literally*:
{$::SAME_AS_PREV}, {$::UNCOMPRESSED}, {$::ZLIB_COMPRESSED}


--
Robert Heller -- Cell: 413-658-7953 GV: 978-633-5364
Deepwoods Software -- Custom Software Services
http://www.deepsoft.com/ -- Linux Administration Services
heller@deepsoft.com -- Webhosting Services
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From Newsgroup: comp.lang.tcl

At Tue, 24 Jun 2025 08:46:57 -0000 (UTC) Mark Summerfield <m.n.summerfield@gmail.com> wrote:

On Tue, 24 Jun 2025 10:37:08 +0200, Harald Oehlmann wrote:

Am 24.06.2025 um 10:23 schrieb Mark Summerfield:

I have a switch command which is doing something I don't expect but I
don't understand what I've done wrong. In this example the default is
always executed but I expect the case before that to be executed.

const UNCOMPRESSED U const ZLIB_COMPRESSED Z const SAME_AS_PREV > >> set filename somefile.txt set action "added"
set kind Z switch $kind {
$::SAME_AS_PREV { puts "unchanged \"$filename\"" }
$::UNCOMPRESSED { puts "$action \"$filename\"" }
$::ZLIB_COMPRESSED { puts "$action \"$filename\" (zlib
compressed)" }
default { puts "!!!!!!!! UNEXPECTED !!!!!!!!" }
}

What am I doing wrong?

Nothing wrong. Tcl is just different, sorry for that.

The "{" always avoids expansion of variables and commands. If you want
to use variables in the switch, you have to avoid the "{".

switch -exact -- $kind [list\
$::SAME_AS_PREV { puts "unchanged \"$filename\"" }\
$::UNCOMPRESSED { puts "$action \"$filename\"" }
$::ZLIB_COMPRESSED { puts "$action \"$filename\" (zlib compressed)" }\
default { puts "!!!!!!!! UNEXPECTED !!!!!!!!" }\
]

Nevertheless, this is no fun on the quoting level (backslashes at the
end etc). In addition, you have to take care, when the variable
expansion happens. This might be tricky.

I personally would write it like that:

switch -exact -- $kind {
= { # SAME_AS_PREV
puts "unchanged \"$filename\""
}
U { # UNCOMPRESSED
puts "$action \"$filename\""
}
Z { # ZLIB_COMPRESSED
puts "$action \"$filename\" (zlib compressed)"
}
default { puts "!!!!!!!! UNEXPECTED !!!!!!!!" }
}

Harald

Thanks that works great.

Bit of a disinsentive to use consts though!

Yes. You really can't use them the way you do in C/C++. Tcl does not really have "const". It also really does not have "statements". Tcl is like LISP in many ways.


--
Robert Heller -- Cell: 413-658-7953 GV: 978-633-5364
Deepwoods Software -- Custom Software Services
http://www.deepsoft.com/ -- Linux Administration Services
heller@deepsoft.com -- Webhosting Services
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