'''What's My Line?''' is a game-style exercise where the goal is to guess the
output of the Tcl script
** Description **
The following Tcl scripts are arranged in ascending order roughly according to
the level of challenge the pose. The task is simple: read a script, and then
guess what the output will be. Actually writing down your answer before
looking at the answer is highly recommended. When you get something wrong,
spend some time writing similar scripts in order to explore the subject in more
detail.
Size your browser window so that the answer is not visible while you form your
own answer.
** Scripts **
*** No Soup For You ***
======
proc proc {args} {
puts {No soup for you!}
}
proc hello {} {
puts {Hi there!}
}
catch {hello} res
puts $res
======
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'''Answer:'''
======
No soup for you!
invalid command name "hello"
======
The built-in [[`[proc]`] is placed by a user-defined procedure that, instead of
creating new procedures, prints `No soup for you!`. `proc hello...` then
doesn't create a `hello` procedure, so when an attempt is made to invoke
`hello`, no such command is found, and an error is produced instead.
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*** Source a [[`[proc]`] in a [[`[namespace]`] ***
Prior to running this script create a file named `data` that contains:
======none
#this file should be called "data"
set localvar 99
======
======
namespace eval ns1 {
proc proc1 {} {
upvar 0 [namespace current]::var1 localvar
source data
set var2 Hello
}
proc1
}
catch {set ns1::var1} eres einfo
puts $eres
catch {set ns1::var2} eres einfo
puts $eres
catch {set $var2} eres einfo
puts $eres
======
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'''answer:'''
======
99
can't read "ns1::var2": no such variable
can't read "var2": no such variable
======
[[`[source]`] causes a script to be evaluated in the current scope.
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*** Secret Message ***
======
set generate {apply {length {
while {[incr length -1] > -1} {
set rand [expr {int(rand()*26)}]
set offset [lindex {65 97} [expr {int(rand()*10) % 2}]]
append res [format %c [expr {$rand + $offset}]]
}
return $res
}}}
puts [{*}$generate 50]
======
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'''Answer:'''
A string of 50 random letters, randomly capitalized.
`generate` could be the arguments and body of a procedure. [[`[apply]`]
executes `$generate` directly, no need to actually create a procedure. In
[ASCII], upper-case English letters start at position 65, and lower-case
letters start at position 97. To choose a random letter, a random number
between 0 and 26 is first obtained, and then either 65 (for upper case) or 97
(for lower case) is added to that number. [[`[format]`] is then used to obtain
the [ASCII] letter that corresponds with that number. This process is
repeated until the string of desired length is built up.
<> Example | Games