Understanding the Weird Array Behaviour in C+ + : A Debugging Guide

Discover the underlying causes of unexpected output in C+ + arrays. Learn how to identify and fix common pitfalls, improving your debugging skills.
---

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: C+ + weird array behaviour

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Introduction

Debugging code can often lead developers down perplexing paths, especially when the output doesn't match expectations. A recent inquiry revolves around a peculiar behaviour observed in a C+ + program involving an array and counting mechanism. The initial output was surprising, leading to confusion that sparked a deeper investigation into the code. Let's take a closer look at the issue and see how we can rectify it.

The Problem

The question posed involves a simple C+ + program that attempts to count occurrences of values in an array. However, when executed, the output displayed was:

[[See Video to Reveal this Text or Code Snippet]]

This output was unexpected because the program seemingly did not modify the input array, yet the second value unexpectedly changed from 1 to 11 during execution. The programmer was puzzled, especially since the issue appeared to originate from the operations applied on the count array.

Here is the key element that generates confusion in the program:

[[See Video to Reveal this Text or Code Snippet]]

The strange behaviour raises a flag and prompts the question: What is going on here?

Investigating the Code

Let's break down the C+ + code to illustrate the problem and identify the root cause.

[[See Video to Reveal this Text or Code Snippet]]

Code Breakdown

Array Initialization:

arr is initialized with ten integers.

count is initialized with seven zeros, meant to count occurrences based on the maximum value in arr.

Counting Elements:

The first for loop increments elements in the count array based on the values in arr. A + 1 offset is applied to avoid using the 0 index.

Accumulative Count:

The second loop is problematic as it attempts to access the count array using count[i + 1], which exceeds the initialized array bounds when i reaches r (7). This leads to undefined behaviour.

Undefined Behaviour Explained

Undefined behaviour in programming refers to the unpredictable nature of code execution when certain conditions aren't met, such as accessing memory outside of allocated bounds. In this specific case, the second loop inadvertently modifies memory locations that overlap with arr, causing the unexpected output.

Compiler Warning

When the code was compiled using g+ + -Wall -Wextra, it revealed the following warning:

[[See Video to Reveal this Text or Code Snippet]]

This warning correctly suggests that you should not be accessing count[i + 1] when i is at its maximum limit (7). Therefore, the addition of count elements possibly leads to overwriting arr, specifically arr[0], which results in the unpredictable outputs observed.

Solution

To fix this issue, we need to ensure that the indexing of the count array remains within valid bounds. Here is the corrected code:

[[See Video to Reveal this Text or Code Snippet]]

Key Changes Made:

Altered the second for loop to iterate only to r, preventing any out-of-bounds access.

Conclusion

In conclusion, the weird behaviour displayed in this C+ + code is a classic case of undefined behaviour stemming from incorrect array indexing. By carefully ensuring that our loops access only valid indices, we can avoid such errors. This case serves as a reminder of the importance of code reviews, compiler warnings, and cautious programming practices when handling arrays and memory in C+ + . Keep honing those debugging skills, as they are essential in becoming a proficient programmer!