Re: misunderstaning of switch command

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