proc decr as incr in tcl - input an expression in proc

[cerberus_gr]

Hello,

1)
I want to write a procedure decr as incr command in tcl.

I have already done this:

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proc decr { var } { set ${var} "[expr ${var} - 1]" }

but it stops with error:
Tcl error: can't read "$var": no such variable

set variable_name variable_value
variable_name = ?
variable_value = ?

2)
I want to have a procedure which has as input an expression

examples:
proc chk { expr } { if {$expr} { return "true" } }

[chk "1 = 1"] -> true
[chk "string match a hsagsa'] -> true
[chk "3 >= 4"] -> null

but my code just checks if $expr is/isn't " "

Any help?

[Sir_Fz]

try $var instead of ${var}

[cerberus_gr]

$var will return the name of the variable and not it's value

if ($hello == 10) then "decr hello" will set var = hello and not var = 10
I want to "read" bot name and value, $var returns the name, but HOW could I take the value?

[Sir_Fz]

your code should be:

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proc decr { var } { set var "[expr $var -1]" }

[user]

1) To work with the actual variable you'll have to import it using 'upvar' or execute the expr/incr in the scope your variable exists in using 'uplevel'

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proc decr var {
	upvar 1 $var bla
	incr bla -1
}

...or:

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proc decr var {
	uplevel 1 [list incr $var -1]
}

2) use 'expr':

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proc chk expr {
	if {[expr $expr]} {return "true"} {return "null"}
}

...or use the C-style "if-then-else" structure:

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proc chk expr {
	expr $expr?"true":"null"
}

...but then you might aswell use 'expr' directly where you want the check imo. Oh, and btw: "1 = 1" is not a valid expression in tcl and the string "null" doesn't mean anything speacial.

[cerberus_gr]

Thx a lot both

1) user I haven't taught tcl and I know only a few commands and I didn't know that in incr command u would use numbers < 0, so my problem has sold

2) Perfect

P.S. in "1 = 1" expression I forgot one more "=" and in my code I didn't returned anything, so that's why I wrote null Thanks a lot