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Undefined variable

Undefined variable is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Undefined variable rather than just read about it. In short: 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.

Key takeaways

  • Undefined variable belongs to computer science; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Undefined variable to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Undefined variable from memory before moving on to harder problems.

Reference excerpt

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

*** - EVAL: variable X has no value

C

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.

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

Lua

(no error, continuing)

nil

ML (Standard ML of New Jersey)

stdIn:1.9 Error: unbound variable or constructor: x

MUMPS Set Y=X

<UNDEF>

OCaml

Unbound value x

Perl

(no error)

PHP 5

(no error)

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

Python

Python 3

Python 2.4

REXX

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

Ruby

Tcl

VBScript

(no error)

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

References

Worked examples

Example 1 — a first encounter with Undefined variable

Start with the simplest possible case. Write down what Undefined variable claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Undefined variable before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Undefined variable ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Undefined variable

In research
Undefined variable appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Undefined variable in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Undefined variable is common in secondary-school and first-year university syllabi. It links to neighbouring topics Software bugs, Variable (computer science), so understanding it makes those chapters shorter.
In everyday life
Look for Undefined variable outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Undefined variable in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Undefined variable means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Undefined variable out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Undefined variable in simple terms?

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.

Why does Undefined variable matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Undefined variable?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Undefined variable.

Tags

  • Software bugs
  • Variable (computer science)

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