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.[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 edit
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 edit
(setf y x)
*** - EVAL: variable X has no value
C edit
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 edit
A ReferenceError only happens if the same piece of executed code has a let
or a const
(but not var
) declaration later on, or if the code is executed in strict mode. In all other cases, the variable will have the special value undefined
.
"use strict";
let y = x;
let y = x;
let x; // causes error on line 1
ReferenceError: x is not defined Source File: file:///c:/temp/foo.js
Lua edit
y = x
(no error, continuing)
print(y)
nil
ML (Standard ML of New Jersey) edit
val y = x;
stdIn:1.9 Error: unbound variable or constructor: x
MUMPS edit
Set Y=X
<UNDEF>
OCaml edit
let y = x;;
Unbound value x
Perl edit
my $y = ($x // 0) + 1; # defined-or operator
(no error)
PHP 5 edit
$y = $x;
(no error)
$y="";
$x="";
error_reporting(E_ALL);
$y = $x;
PHP Notice: Undefined variable: x in foo.php on line 3
Python edit
Python 3 edit
x = y
Traceback (most recent call last):
File "<pyshell#0>", line 1, in <module>
x = y
NameError: name 'y' is not defined
Python 2.4 edit
>>> x = y
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'y' is not defined
REXX edit
signal on novalue
y = x
+++ Error 30 in line 2: Label not found
Ruby edit
irb(main):001:0> y = x
NameError: undefined local variable or method `x' for main:Object
from (irb):1
Tcl edit
% set y $x
can't read "x": no such variable
VBScript edit
Dim y
y = x
(no error)
Option Explicit
Dim y
y = x
(3, 1) Microsoft VBScript runtime error: Variable is undefined: 'x'
References edit
- ^ "undefined variable." YourDictionary, n.d. Web. 24 July 2013. <http://computer.yourdictionary.com/undefined-variable>.