Array vs Dictionary: How Do They Differ?
💡 Concept Name
Array vs Dictionary — Arrays use numbered slots for fast, ordered access. Dictionaries match custom keys to values for rapid lookups, but don’t care about order.
📘 Quick Intro
Arrays and dictionaries are both containers, but they solve different problems. Arrays store items in a sequence using numbers (indices) to find each value. Dictionaries work more like a mini-database, matching unique keys (like names or codes) to their associated values.
🧠 Analogy / Short Story
If an array is a row of theater seats (each seat has a number), a dictionary is the ticket desk at the door—just say your name (key) and you instantly get your seat (value), regardless of where it is in the hall.
🔧 Technical Explanation
- 🔢 Arrays: Items are accessed by their position (integer index, zero-based).
- 🔑 Dictionaries: Items are stored as key-value pairs; keys can be almost any type, not just numbers.
- 📐 Memory: Arrays use continuous blocks; dictionaries use hash tables under the hood.
- ⚡ Speed: Both can give O(1) lookup time—arrays by index, dictionaries by key. But worst-case for dictionaries (due to hash collisions) is O(n).
- 📝 Order: Arrays keep their order; dictionaries do not guarantee order of elements.
- 🔄 Duplication: Arrays can store repeated values; dictionary keys must be unique.
🎯 Purpose & Use Case
- ✅ Use arrays when your data is naturally sequential or you need super-fast access by position (like daily sales, pixel colors, etc.).
- ✅ Use dictionaries when you need to look things up by a unique identifier (such as a username, code, or ID number).
- ✅ Arrays are best for sorting, iteration, and scenarios where order matters. Dictionaries excel when mapping real-world data—like associating product codes with prices or names with phone numbers.
💻 Real Code Example
// Array Example: Numbered collection
string[] colors = { "Red", "Green", "Blue" };
Console.WriteLine(colors[1]); // Output: Green
// Dictionary Example: Key-value pairs
var phoneBook = new Dictionary<string, string> {
{ "Alice", "+91-999001111" },
{ "Bob", "+91-888002222" }
};
Console.WriteLine(phoneBook["Bob"]); // Output: +91-888002222
❓ Interview Q&A
Q1: How does an array differ from a dictionary in .NET?
A: Arrays use numeric indices; dictionaries use keys of any type to locate values.
Q2: Which structure is better for sequential data?
A: Arrays—they keep elements in order and support fast iteration.
Q3: What happens if you try to add a duplicate key to a dictionary?
A: An exception is thrown—dictionary keys must be unique.
Q4: Can you access dictionary values by numeric index?
A: No, you must use keys.
Q5: What’s the average lookup time for a dictionary?
A: O(1), thanks to hash-based key lookup.
Q6: When is an array more efficient than a dictionary?
A: When you have a fixed list and access by index is all you need.
Q7: What type of structure does Dictionary<TKey,TValue> use internally?
A: A hash table.
Q8: Can arrays have duplicate values?
A: Yes, arrays can store the same value multiple times.
Q9: Are dictionaries ordered by key?
A: No, not by default.
Q10: Which is better for mapping user IDs to names?
A: Dictionary—user ID is a perfect unique key.
📝 MCQs
Q1. How are values accessed in an array?
- By key
- By index
- By hash
- By reference
Q2. Which data structure allows string keys for lookup?
- Array
- Dictionary
- Stack
- Queue
Q3. Can arrays store duplicate values?
- Yes
- No
- Only in C#
- Only if sorted
Q4. What must be unique in a dictionary?
- Values
- Keys
- Indices
- Types
Q5. Which is better for random ordered access?
- Dictionary
- Array
- Set
- Map
Q6. What type of data structure is a dictionary in .NET?
- Array
- Linked list
- Hash table
- Tree
Q7. Is element order guaranteed in a dictionary?
- Yes
- No
- Only with sorting
- Only for integers
Q8. What is the main benefit of a dictionary?
- Low memory use
- Fast lookup by key
- Guaranteed order
- Built-in sorting
Q9. Which is more memory efficient for fixed-size data?
- Dictionary
- Array
- Both equal
- Depends on type
Q10. Which structure should you choose for key-value storage?
- Array
- Dictionary
- List
- Stack
💡 Bonus Insight
Choosing between arrays and dictionaries depends on your data needs. If you care about order and memory use, stick to arrays. If you want super-fast access using unique labels (keys), dictionaries will make your life easier.
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