Data Structures: Core Concepts: 31 flashcards
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1. What does `ds` most commonly stand for in computer science?
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**Data structures** — the ways data is organised in memory so operations on it are efficient.
2. What does `DS` often stand for in statistics and analytics?
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**Data Science** — extracting insight from data using statistics, programming, and domain knowledge.
3. Why is `ds` described as an 'overloaded' abbreviation?
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Because the same two letters map to several different fields: data structures, Data Science, Decision Stump, Dataset, and distribution system.
4. What is a data structure, in one sentence?
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A **contract about cost** — it defines which operations are cheap and which are expensive.
5. What is the time complexity of indexed access in an array?
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$O(1)$ — constant time, because the address is computed directly from the index.
6. What is the time complexity of inserting into the middle of an array?
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$O(n)$ — elements after the insertion point must be shifted.
7. What is the time complexity of inserting into a linked list once you hold the node?
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$O(1)$ — you simply rewire the pointers.
8. What is the time complexity of searching a linked list?
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$O(n)$ — you must walk node by node from the head.
9. What is the average lookup time of a hash table?
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$O(1)$ average — but it can degrade under collisions.
10. What invariant must a binary search tree maintain?
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**left < node < right** — every value in the left subtree is smaller, every value in the right subtree is larger.
11. What three things define every data structure?
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**Storage layout**, **invariants**, and **operations**.
12. What is an invariant in a data structure?
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A rule that must hold after every insert or delete for the speed of operations to survive.
13. Which data structure powers the browser back button?
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A **stack** — last in, first out.
14. Which data structure powers print jobs?
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A **queue** — first in, first out.
15. Which data structure underlies contact lists and dictionary lookups?
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A **hash map** — keyed by name or word.
16. Which data structure underlies file-system directories?
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A **tree** — directories nest inside directories.
17. Which data structure underlies route finding in maps apps?
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A **graph** — nodes are places, edges are roads.
18. What is the data science workflow, in order?
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**Collect → Clean → Explore → Model → Communicate.**
19. In data science, which stage takes more time than modelling?
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**Cleaning and framing the question** — deciding what to measure and whether the measurement is trustworthy.
20. What is the disciplined habit when you meet an ambiguous abbreviation like `ds`?
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Ask: **which field, which course, which chapter?** Resolve the context before memorising.
21. Why is treating an abbreviation as a definition a mistake?
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An abbreviation is a **label**, and labels only mean something inside a context — without context, any definition is a guess.
22. In Wi-Fi terminology, what does `ds` stand for?
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**Distribution system** — the wired backbone that connects access points.
23. In machine learning, what can `DS` stand for?
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**Decision Stump** or **Dataset**.
24. Why does no single data structure win on every operation?
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Because each one trades off search, insert, delete, and memory costs differently — the right choice depends on the workload.
25. What is the cost profile of a sorted array?
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Search $O(\log n)$ via binary search, but insert in the middle $O(n)$ because of shifting.
26. Why is choosing the wrong data structure a common cause of slow software?
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Because an $O(n^2)$ structure where an $O(n \log n)$ one existed turns milliseconds into seconds that users feel.
27. What does the BST invariant buy you at search time?
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It lets you **discard half the tree** at each step, giving $O(\log n)$ search.
28. What is the first step when studying an under-specified topic?
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**Anchor the context** — find the course, chapter, or syllabus that names the topic.
29. Why do exam questions about `ds` always carry context?
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Because the abbreviation is ambiguous — the context is what disambiguates it, and misreading it is a common way to lose marks.
30. What is the overlap between data structures and data science?
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Data science still needs efficient storage, but its centre of gravity is **inference**, not cost.
31. What is the 'meta-skill' the topic recommends for overloaded terms?
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**Resolving ambiguity before memorising** — it applies to every overloaded term you will ever meet.
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