# Arrays — Data Structures & Algorithms

Source: https://www.geekswithgeeks.com/en/dsa/arrays

> A contiguous block of memory with O(1) index access.

## Contiguous memory

An array stores elements back-to-back. Because every slot is the same size, `address(i) = base + i * size`, so reading `arr[i]` is `O(1)`. The cost is that inserting in the middle must shift everything after it — `O(n)`.

## Row of numbered lockers

You can walk straight to locker 47 without checking 1–46. But to squeeze a new locker between 10 and 11, every later locker has to move down one.

## Core operations

Access and append (amortised) are cheap; insert/delete at the front are linear.

```python
a = [10, 20, 30, 40]
a[2]            # O(1)  -> 30
a.append(50)   # O(1) amortised
a.insert(0, 5) # O(n)  shifts everything right
a.pop()        # O(1)  from the end
a.pop(0)       # O(n)  from the front
```

## Interview pattern

Clues like "sorted array", "in place", "contiguous subarray", or "fixed extra space" point to arrays with two pointers, prefix sums, or a sliding window.