computer science

Parent pointer tree

Parent pointer tree 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 Parent pointer tree rather than just read about it. In short: In computer science, an in-tree or parent pointer tree is an N-ary tree data structure in which each node has a pointer to its parent node, but no pointers to child nodes. When used to implement a set of stacks, the structure is called a spaghetti stack, cactus stack or saguaro stack (after the saguaro, a kind of cactus).

Key takeaways

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

Reference excerpt

In computer science, an in-tree or parent pointer tree is an N-ary tree data structure in which each node has a pointer to its parent node, but no pointers to child nodes. When used to implement a set of stacks, the structure is called a spaghetti stack, cactus stack or saguaro stack (after the saguaro, a kind of cactus). Parent pointer trees are also used as disjoint-set data structures. The structure can be regarded as a set of singly linked lists that share part of their structure, in particular, their tails. From any node, one can traverse to ancestors of the node, but not to any other node.

Use in compilers A compiler for a language such as C creates a spaghetti stack as it opens and closes symbol tables representing block scopes. When a new block scope is opened, a symbol table is pushed onto a stack. When the closing curly brace is encountered, the scope is closed and the symbol table is popped. But that symbol table is remembered, rather than destroyed, and it remembers its higher level "parent" symbol table and so on. Thus when the compiler is later performing translations over the abstract syntax tree, for any given expression, it can fetch the symbol table representing that expression's environment and can resolve references to identifiers. If the expression refers to a variable X, it is first sought after in the leaf symbol table representing the inner-most lexical scope, then in the parent and so on.

Use as call stacks The term spaghetti stack is closely associated with implementations of programming languages that support continuations. Spaghetti stacks are used to implement the actual run-time stack containing variable bindings and other environmental features. When continuations must be supported, a function's local variables cannot be destroyed when that function returns: a saved continuation may later re-enter into that function, and will expect not only the variables there to be intact, but it will also expect the entire stack to be present so the function is able to return again. To resolve this problem, stack frames can be dynamically allocated in a spaghetti stack structure, and simply left behind to be garbage collected when no continuations refer to them any longer. This type of structure also solves both the upward and downward funarg problems, as a result first-class lexical closures are readily implemented in that substrate. Examples of languages that use spaghetti stacks are:

Languages having first-class continuations such as Scheme and Standard ML of New Jersey Languages where the execution stack can be inspected and modified at runtime such as Smalltalk Felix Cilk Mainframe computers using the instruction set architecture of the Burroughs B6500 and its successors, running the MCP operating system, can spawn multiple tasks within the same program. Since these were originally ALGOL-based systems they have to support nested functions and the result is that task creation results in a fork in the stack, which Burroughs informally described as a "saguaro stack.".

Illustrations

Worked examples

Example 1 — a first encounter with Parent pointer tree

Start with the simplest possible case. Write down what Parent pointer tree 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 Parent pointer tree 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 Parent pointer tree 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 Parent pointer tree

In research: Parent pointer tree 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 Parent pointer tree 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: Parent pointer tree is common in secondary-school and first-year university syllabi. It links to neighbouring topics Continuations, Metaphors referring to spaghetti, Trees (data structures), so understanding it makes those chapters shorter.
In everyday life: Look for Parent pointer tree 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.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Parent pointer tree in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Parent pointer tree 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 Parent pointer tree out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Parent pointer tree in simple terms?

Why does Parent pointer tree 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 Parent pointer tree?

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 Parent pointer tree.