How to Fix Array Out of Bounds Access Errors

In this tutorial, you'll learn about How to Fix Array Out of Bounds Access Errors. We cover key concepts, practical examples, and best practices.

Array out of bounds errors occur when code accesses an index less than 0 or greater than or equal to the array length. In C and C++, this causes undefined behavior. In Java, Python, and Rust, it throws an exception or panics at runtime.

Quick Fix

Wrong

int arr[5] = {10, 20, 30, 40, 50};
for (int i = 0; i <= 5; ++i) {
    std::cout << arr[i] << " ";
}

10 20 30 40 50 32767

The loop condition i <= 5 accesses index 5, which is past the end. Output may include garbage or crash.

Right

int arr[5] = {10, 20, 30, 40, 50};
for (int i = 0; i < 5; ++i) {
    std::cout << arr[i] << " ";
}

10 20 30 40 50

Fix with bounds checking

#include <vector>
#include <stdexcept>

int safeAccess(const std::vector<int>& arr, size_t index) {
    if (index >= arr.size()) {
        throw std::out_of_range("Index " + std::to_string(index)
            + " out of range for array size " + std::to_string(arr.size()));
    }
    return arr[index];
}

Fix for off-by-one in C strings

char buf[10];
strcpy(buf, "hello");  // space for 10 is fine

// Wrong: no null terminator room
char buf2[5];
strcpy(buf2, "hello"); // overflow! needs 6 chars for null terminator

// Right:
char buf3[6] = "hello";

Prevention

• Use std::vector::at() instead of operator[] for bounds-checked access.
• Use range-based for loops where possible.
• Use std::array with size() method.
• In C, use snprintf instead of sprintf, strncpy instead of strcpy.
• Enable address sanitizer (-fsanitize=address) during testing.

DodaTech Tools

Doda Browser's memory safety checker detects out-of-bounds access patterns in codebases. DodaZIP archives sanitizer reports for debugging. Durga Antivirus Pro detects buffer overflow exploits targeting array bounds.

Common Mistakes with array out of bounds

1. Using foldl instead of foldl' causing stack overflow on large lists
2. Forgetting deriving (Show, Eq) on custom data types needed for debugging
3. Placing the wildcard pattern first in case expressions, making all subsequent patterns unreachable

These mistakes appear frequently in real-world DS code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ