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Beginner Topic 2 of 52 Data Structures

Dynamic Arrays

Resizable arrays that grow by reallocating with amortized cost.

How it works

A dynamic array wraps a fixed array with a size and a capacity. When the capacity is full, it allocates a larger block, typically double, copies the elements over and frees the old block. Because the cost of each copy is spread across many appends, appending takes amortized O(1) time even though a single append can occasionally take O(n).

Step-by-step walkthrough

  1. The array has capacity 8 and holds 4 items.
  2. Appending the 5th item uses spare capacity in constant time.
  3. Items 6 to 8 fill the remaining slots.
  4. When it is full, a new array of double size is allocated and everything is copied.
  5. Copies are rare, so append is amortized O(1).

Where it is used

Python lists, Java ArrayList, C++ vector and JavaScript arrays are dynamic arrays. They are the standard sequence type for collecting results, building output, buffering input and implementing stacks.

Common pitfall

Resizing invalidates pointers and iterators into the old block. Deleting from the middle or front is still O(n), and over-allocation wastes memory. Reserve capacity when the final size is known to avoid repeated reallocations, and shrink only when memory is tight.

Where it fits in the course

Dynamic Arrays is topic 2 of 52 in Data Structures, rated Beginner. It follows Arrays. The next topic is Linked Lists.

Practice and explore

How to study this topic

  1. Read the explanation above and restate the definition in your own words.
  2. Open the interactive lesson and answer its mini quiz.
  3. Read the common pitfall, then write a short example that avoids it.
  4. Check your understanding in the Data Structures quiz, and revisit the topic if you miss a question.

Frequently asked questions

What is Dynamic Arrays?
Resizable arrays that grow by reallocating with amortized cost. It belongs to Data Structures and is a beginner-level topic.
Where do people use Dynamic Arrays?
Python lists, Java ArrayList, C++ vector and JavaScript arrays are dynamic arrays.
What is the most common mistake with Dynamic Arrays?
Resizing invalidates pointers and iterators into the old block.
What should I learn before this topic?
Start with Arrays, the topic just before it in Data Structures.
How can I practice it?
Answer the mini quiz in the interactive lesson, then take the Data Structures quiz.
Is Dynamic Arrays hard to learn?
It is a Beginner topic, so it works as an entry point. If the terms are new, read the explanation twice and try the visualizer before the quiz.
How long does it take to learn Dynamic Arrays?
Most learners can read the explanation and finish the interactive lesson in under an hour. Fluency comes from applying the idea, so plan a few short practice sessions with the quiz.
Which topics are related to Dynamic Arrays?
Arrays, Linked Lists, Doubly Linked Lists are closely connected. The full list is in the related topics section of this page.
What should I study after Dynamic Arrays?
Continue with Linked Lists, the next topic in Data Structures.

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