Undefined variable

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

(setf y x) *** - EVAL: variable X has no value

C

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

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

y = x

(no error, continuing)

print(y) nil

ML (Standard ML of New Jersey)

val y = x; stdIn:1.9 Error: unbound variable or constructor: x

MUMPS

Set Y=X <UNDEF>

OCaml

let y = x;; Unbound value x

Perl

my $y = ($x // 0) + 1; # defined-or operator (no error)

PHP 5

$y = $x; (no error) $y=""; $x=""; error_reporting(E_ALL); $y = $x; PHP Notice: Undefined variable: x in foo.php on line 3

Python

Python 3

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

>>> x = y Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'y' is not defined

REXX

signal on novalue y = x +++ Error 30 in line 2: Label not found

Ruby

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

Tcl

% set y $x can't read "x": no such variable

VBScript

Dim y y = x (no error) Option Explicit Dim y y = x (3, 1) Microsoft VBScript runtime error: Variable is undefined: 'x'