Undefined variable

An undefined variable in a computer program is a variable that is accessed by the program but which has not been previously declared by that program. This often results in a compile time or run time error such as undefined variable or undeclared variable.

Some languages do not trigger this error but rather implicitly declare the variable. This may be convient (such as for short programs), but it can also be error-prone since typos inadvertently introduce new variables without warning. Some languages have started out with this 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).

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

JavaScript (Mozilla Firefox 1.0):

 y = x

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

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.)

OCaml 3.08

let y = x;;

Unbound value x

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'