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

Returns '''1''' if the variable named ''varName'' exists in the current context (either as a global or local variable) and has been defined by being given a value, returns '''0''' otherwise.

<<discussion>>

Called with a variable or array element name (no $ in front!), returns 1 if the variable or array element exists, else 0.

  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

Notice the last one - if the user thought they were testing to see if the
a variable exists, they used the wrong syntax - saying $a means ''read
the contents of the a variable, and see if there is a variable by
that name.''

-- It doesn't have to be wrong syntax, actually: it's often useful to have
one variable reference others, like this:

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

Then again -

  set a b
 b
  info exists $a
 1

Unintentionally setting a to a value that equals the name of an existing 
variable yields a false positive when erroneously (in this case) checking
the value of a.

[huck.] Shouldn't this be 0, since variable b 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. 
 % 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

----
2003-12-21 [VI] I remember reading that info exists is slow on some versions
of Tcl on large arrays.  Anybody have more info on that?
----
2007-12-29 [DKF]: From 8.5 onwards, '''info exists''' is byte-compiled.


<<discussion>>
----
**See also**
   * [info]

<<categories>> Command | Tcl syntax