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.^[1]

In some programming languages an implicit declaration is provided the first time such a variable is encountered at compile time. In other languages such a usage is considered to be sufficiently serious that a diagnostic being issued and the compilation fails.

Some language definitions initially used the implicit declaration behavior and as they matured provided an option to disable it (e.g. Perl's "use warnings" or Visual Basic's "Option Explicit").

Examples

The following provides some examples of how various programming language implementations respond to undefined variables. Each code snippet 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

Lua

y = x

(no error, continuing)

print(y)

nil

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 + 1;

(no error)

use warnings;
my $y = defined($x) ? $x + 1 : 1; # avoid operating on undef
my $z = $x + 1;

Use of uninitialized value in addition (+) at foo.pl line 3.

Perl 5.10:

my $y = ($x // 0) + 1; # defined-or operator

(no error)

PHP 5:

$y = $x;

(no error)

error_reporting(E_ALL);
$y = $x;

PHP Notice: Undefined variable: x in foo.php on line 3

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

NameError: undefined local variable or method `x' for main:Object             
from (irb):1 

VBScript (WSH 5.6)

Dim y
y = x

(no error)

Option Explicit
 
Dim y
y = x

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

References