'''`[info] exists`''' tests whether a [variable] exists and is defined.



** Synopsis **

    :   '''info exists''' ''varName''



** Description **

Returns `1` if the variable named ''varName'' exists in the current context according the the rules of [namespace%|%name resolution],  and has been defined by being given a value, returns `0` otherwise.

`info exists` returns `0` for variables that exist but are undefined.  This can
happen, for example, with `[trace]`, because if a trace is set on a nonexisting
variable, `[trace]` will create that variable, but leave it undefined.
Although `info exists` doesn't detect undefined variables, `[namespace which]
-variable` does.  In that case, it's usually a good idea to pass a fully-qualified variable name to `[namespace which] -variable`.


'''Examples:'''


======
info exists a
# -> 0
set a 1
# -> 1
info exists a
# -> 1
info exists b
# -> 0
set b(2) 2
# -> 2
info exists b
# -> 1
info exists b(1)
# -> 0
info exists b(2)
# -> 1
info exists $a
# -> 0
======

That last command is an example of a common mistake: using `$varname` instead
of just `varname`.  Since the value of `$a` is `1`, the command is asking if a
variable named `1` exists.

It can be useful to store the name of a variable in another variable: 

======
foreach var {a b c} {
    if {[info exists $var]} {
        puts "$var does indeed exist"
    } else {
        puts "$var sadly does not exist"
    }
}
======

----
[KPV]: Another thing to keep in mind is that linking variable with 
`[upvar]` can do funny things with the existence of variables. 

======none
% set a 1
1
% upvar #0 a b
% info exists b
1
% unset a
% info exists b
0
% set a 1
1
% info exists b
1
======

----

[VI] 2003-12-21: I remember reading that `info exists` is slow on some versions
of Tcl on large arrays.  Anybody have more info on that?

----

[DKF] 2007-12-29: From 8.5 onwards, '''info exists''' is byte-compiled.


 
** See Also **

   `[info]`:   

   `[namespace which] -variable`:   

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