computer science

Non-local variable

Non-local 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 Non-local variable rather than just read about it. In short: In programming language theory, a non-local variable is a variable that is not defined in the local scope. While the term can refer to global variables, it is primarily used in the context of nested and anonymous functions where some variables can be in neither the local nor the global scope.

Key takeaways

Non-local 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 Non-local variable to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Non-local variable from memory before moving on to harder problems.

Reference excerpt

In programming language theory, a non-local variable is a variable that is not defined in the local scope. While the term can refer to global variables, it is primarily used in the context of nested and anonymous functions where some variables can be in neither the local nor the global scope. In Lua they are called the upvalues of the function.

Examples

Nested functions In the Python 3 example that follows there is a nested function inner defined in the scope of another function outer. The variable x is local to outer, but non-local to inner (nor is it global):

In JavaScript, the locality of a variable is determined by the closest var statement for this variable. In the following example, x is local to outer as it contains a var x statement, while inner doesn't. Therefore, x is non-local to inner:

Anonymous functions In the Haskell example that follows the variable c is non-local in the anonymous function \x -> x + c:

Implementation issues

Non-local variables are the primary reason it is difficult to support nested, anonymous, higher-order and thereby first-class functions in a programming language. If the nested function or functions are (mutually) recursive, it becomes hard for the compiler to know exactly where on the call stack the non-local variable was allocated, as the frame pointer only points to the local variable of the nested function itself and there can be an arbitrary number of activation records on the stack in between. This is generally solved using access links or display registers. If the nested function is passed as an argument to a higher-order function a closure needs to be built in order to locate the non-local variables. If the nested function is returned as a result from its outer function (or stored in a variable) the non-local variables will no longer be available on the stack. They need to be heap allocated instead, and their lifetime extends beyond the lifetime of the outer function that declared and allocated them. This generally requires garbage-collection.

Notes

References Aho, Lam, Sethi, and Ullman. "7.3 Access to Nonlocal Data on the Stack". Compilers: Principles, Techniques, & Tools. Second edition.

Worked examples

Example 1 — a first encounter with Non-local variable

Start with the simplest possible case. Write down what Non-local 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 Non-local 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 Non-local 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 Non-local variable

In research: Non-local 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 Non-local 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: Non-local variable is common in secondary-school and first-year university syllabi. It links to neighbouring topics Programming language theory, Variable (computer science), so understanding it makes those chapters shorter.
In everyday life: Look for Non-local 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 Non-local variable in 20 minutes

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

Frequently asked questions

What is Non-local variable in simple terms?

Why does Non-local 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 Non-local 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 Non-local variable.