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Going out of an array's bounds means the program tries to access an element **at an index that does not exist** - for example, if the array has 5 elements, but the access is to `A[5]` or `A[-2]` (where only indices 0-4 are valid).

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### **1. What this means technically**

An array is stored **in a contiguous memory area**, and when it goes out of bounds the program starts accessing **cells that do not belong to it**.
This means it reads or changes someone else's data in memory - **memory allocated for other variables or system data**.

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### **2. What happens in different languages**

| Language | Reading `arr[i]` out of range | Writing `arr[i] = x` out of range | Comment |
|---|---|---|---|
| **JavaScript (Array)** | `undefined` | Usually extends the array (if `i >= length`), creates "holes" | `arr[-1]` is **not an index**, but an object property (like `arr["-1"]`) |
| **TypeScript** | Same as JS at runtime | Same as JS at runtime | TS can flag it with types, but doesn't change the behavior |
| **Python (list)** | `IndexError` | `IndexError` | Negative indices are **allowed**: `a[-1]` is the last element |
| **Java** | `ArrayIndexOutOfBoundsException` | `ArrayIndexOutOfBoundsException` | Bounds are always checked for arrays |
| **C# (.NET)** | `IndexOutOfRangeException` | `IndexOutOfRangeException` | Also checked for `Span<T>`/`List<T>`; can differ in `unsafe` |
| **Go (slice/array)** | `panic: runtime error: index out of range` | `panic` | Slices are strictly checked |
| **Rust (Vec/array)** | `panic` (with `v[i]`) / `None` (with `v.get(i)`) | `panic` (with `v[i]=...`) | Idiomatic: use `get()` to avoid a panic |
| **C (plain array)** | **Undefined Behavior** (may "work", may crash, may corrupt memory) | **Undefined Behavior** | No guarantees, possible vulnerabilities |
| **C++ (operator[])** | **Undefined Behavior** | **Undefined Behavior** | `std::vector`'s `at()` throws `std::out_of_range` |
| **Swift (Array)** | Runtime trap (crash) | Runtime trap (crash) | Safe wrappers/checks can be built |
| **Kotlin (Array/List)** | `IndexOutOfBoundsException` | `IndexOutOfBoundsException` | On the JVM, essentially the same as Java |

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### **3. Possible consequences**

- **Program crash**
The system stops execution when it tries to access someone else's memory.
- **Data corruption**
The program can silently change variables in neighboring memory areas.
- **Vulnerabilities**
In low-level languages this can be exploited for hacking (buffer overflow).

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### **4. Example**

```c
int a[3] = {10, 20, 30};
printf("%d", a[5]);  // accessing a non-existent element
```

→ the program might print a random value, crash, or corrupt memory.

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**Summary:**

> When an array goes out of bounds, the program accesses memory it doesn't own.
> In "safe" languages this raises an error; in low-level languages it leads to unpredictable behavior or even a vulnerability.

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