# Dictionaries and Maps

Dictionaries (also known as Maps, Hash Maps, or Associative Arrays) are data structures that store data in **key-value pairs**. Unlike arrays which use numeric indices, dictionaries use unique keys to look up values.

## 1. Concept

*   **Key:** A unique identifier for the data (e.g., "username", "id", 42).
*   **Value:** The data associated with the key.

Think of a real-world dictionary: you look up a word (Key) to find its definition (Value).

## 2. Implementation in Languages

### Python (Dictionary)
Python uses the `dict` type, defined with curly braces `{}`.
```python
user = {
    "name": "Alice",
    "age": 30,
    "is_admin": True
}
```

### JavaScript (Object & Map)
JavaScript traditionally uses **Objects** for key-value pairs, but modern JS also has a dedicated **Map** class.

**Object:**
```javascript
let user = {
    name: "Alice",
    age: 30
};
```

**Map:**
```javascript
let inventory = new Map();
inventory.set("apple", 10);
inventory.set("banana", 5);
```

### Java (HashMap)
Java uses the `Map` interface, commonly implemented by `HashMap`.
```java
import java.util.HashMap;

HashMap<String, Integer> scores = new HashMap<>();
scores.put("Alice", 95);
scores.put("Bob", 82);
```

## 3. Common Operations

### Accessing Values
*   **Python:** `value = data["key"]`
*   **JavaScript:** `value = data.key` or `data["key"]` (Object), `data.get("key")` (Map)
*   **Java:** `value = map.get("key");`

### Adding/Updating
*   **Python:** `data["new_key"] = "value"`
*   **JavaScript:** `data.newKey = "value"` (Object), `map.set("key", "value")` (Map)
*   **Java:** `map.put("key", "value");`

[[programming/common-syntax]]
[[programming/arrays-and-lists]]