Undefined variable

An undefined variable in the source code of a computer program is a variable that is accessed in the code but has not been previously declared by that code.

In some programming languages an implicit declaration is provided the first time such a variable is encountered at compile time or run time error. In other languages such a usage is considered to be a fatal error, resulting an a diagnostic being issued.

Some languages have started out with the implicit declaration behavior but as they matured they provided an option to disable it (e.g. Perl's "use strict" or Visual Basic's "Option Explicit").

Examples

Examples of how various programming languages respond to undefined variables are given below. Each code example is followed by an error message (if any).

CLISP (GNU CLISP 2.35):

 (setf y x)

*** - EVAL: variable X has no value

C (GNU GCC 3.4):

 int main() {
   int y = x;
   return 0;
 }

foo.c: In function `main':
foo.c:2: error: `x' undeclared (first use in this function)
foo.c:2: error: (Each undeclared identifier is reported only once
foo.c:2: error: for each function it appears in.)

JavaScript (Mozilla Firefox 1.0):

  y = x

 Error: x is not defined
 Source File: file:///c:/temp/foo.js

ML (Standard ML of New Jersey v110.55):

val y = x;

stdIn:1.9 Error: unbound variable or constructor: x

MUMPS

Set Y=X
<UNDEF>

OCaml 3.08

 let y = x;;

Unbound value x

Perl 5.8:

 my $y = $x;

(no error)

 use strict;
 my $y = $x;

Global symbol "$x" requires explicit package name at foo.pl line 2.
Execution of foo.pl aborted due to compilation errors.

Python 2.4:

  x = y

Traceback (most recent call last):
  File "foo.py", line 1, in ?
    x = y
NameError: name 'y' is not defined

Ruby 1.8

 y = x
</code>
<code>
 NameError: undefined local variable or method `x' for main:Object             
 from (irb):1 
</code>

[[VBScript]] (WSH 5.6)
<source lang="vb">
 Dim y
 y = x

(no error)

>
 Option Explicit
 Dim y
 y = x

(3, 1) Microsoft VBScript runtime error: Variable is undefined: 'x'