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 declared by that code.

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

<syntaxhighlight lang="lisp"> (setf y x) </syntaxhighlight>

*** - EVAL: variable X has no value

C

<syntaxhighlight lang="c"> int main() {

 int y = x;
 return 0;

} </syntaxhighlight>

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

A ReferenceError only happens if the same piece of executed code has a <syntaxhighlight lang="js" class="" style="" inline="1">let</syntaxhighlight> or a <syntaxhighlight lang="js" class="" style="" inline="1">const</syntaxhighlight> (but not <syntaxhighlight lang="js" class="" style="" inline="1">var</syntaxhighlight>) declaration later on, or if the code is executed in strict mode. In all other cases, the variable will have the special value <syntaxhighlight lang="js" class="" style="" inline="1">undefined</syntaxhighlight>.

<syntaxhighlight lang="javascript"> "use strict"; let y = x; </syntaxhighlight> <syntaxhighlight lang="javascript"> let y = x; let x; // causes error on line 1 </syntaxhighlight>

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

Lua

<syntaxhighlight lang="lua"> y = x </syntaxhighlight> (no error, continuing) <syntaxhighlight lang="lua"> print(y) </syntaxhighlight>

nil

ML (Standard ML of New Jersey)

<syntaxhighlight lang="sml"> val y = x; </syntaxhighlight>

stdIn:1.9 Error: unbound variable or constructor: x

MUMPS

Set Y=X

<UNDEF>

OCaml

<syntaxhighlight lang="ocaml"> let y = x;; </syntaxhighlight>

Unbound value x

Perl

<syntaxhighlight lang="perl"> my $y = ($x // 0) + 1; # defined-or operator </syntaxhighlight>

(no error)

PHP 5

<syntaxhighlight lang="php"> $y = $x; </syntaxhighlight>

(no error)

<syntaxhighlight lang="php"> $y=""; $x=""; error_reporting(E_ALL); $y = $x; </syntaxhighlight>

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

Python

Python 3

<syntaxhighlight lang="python3"> x = y Traceback (most recent call last):

 File "<pyshell#0>", line 1, in <module>
   x = y

NameError: name 'y' is not defined </syntaxhighlight>

Python 2.4

<syntaxhighlight lang="pycon"> >>> x = y Traceback (most recent call last):

 File "<stdin>", line 1, in <module>

NameError: name 'y' is not defined </syntaxhighlight>

REXX

<syntaxhighlight lang="rexx"> signal on novalue y = x </syntaxhighlight>

+++ Error 30 in line 2: Label not found

Ruby

<syntaxhighlight lang="irb"> irb(main):001:0> y = x NameError: undefined local variable or method `x' for main:Object from (irb):1 </syntaxhighlight>

Tcl

<syntaxhighlight lang="tcl"> % set y $x can't read "x": no such variable </syntaxhighlight>

VBScript

<syntaxhighlight lang="vbscript"> Dim y y = x </syntaxhighlight>

(no error)

<syntaxhighlight lang="vbscript"> Option Explicit

Dim y y = x </syntaxhighlight>

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