Cyclomatic complexity: Definition and limits in understanding code quality

Taylor Bruneaux


Cyclomatic complexity measures a program’s code complexity based on the count of independent paths within the code. This metric helps evaluate a program’s maintainability, understandability, and error potential, serving as a valuable tool for developers and testers to assess code quality and efficiency. 

However, cyclomatic complexity is only one aspect of software quality. It provides insights into potential maintenance and testing challenges but must be balanced with the subjective judgments of experienced developers.

The debate over assessing code quality highlights the limitations of relying solely on quantitative metrics like cyclomatic complexity. For example, Google’s Code Health group emphasizes the importance of qualitative factors such as simplicity and maintainability over numbers. They emphasize that code quality is a human experience, making simple code more accessible to read, understand, and modify.

So, while cyclomatic complexity offers valuable insights, it should not be the sole criterion for evaluating software quality. The broader assessment should include the expertise and wisdom of seasoned developers, focusing on qualities that enhance simplicity, maintainability, and effectiveness. 

Here, we explore the principles, importance, and constraints of cyclomatic complexity and underscore the essential role of human judgment in thoroughly assessing software quality.

What is cyclomatic complexity?

Cyclomatic complexity is a metric used in software engineering to measure the complexity of a program’s source code. It quantifies the number of linearly independent paths through a program, counting each decision point that can affect the execution flow. 

Graph theory derives cyclomatic complexity by representing a program as a control flow graph with nodes (blocks of instructions) and edges (control flow paths).

A higher cyclomatic complexity indicates a more complex and potentially less maintainable codebase, while a lower value suggests simpler, potentially easier-to-understand code. This metric identifies the risk associated with maintenance and software testing, guiding developers and testers toward creating more efficient, reliable, and maintainable software systems.

How to measure cyclomatic complexity

Cyclomatic complexity represents the code as a graph, showing the different blocks of instructions and their connection. The graph helps us identify the number of paths through the code, called the Cyclomatic Complexity Number (CCN).

Cyclomatic complexity is measured using the CC = E—N + 2P formula. Here, “E” refers to the connections between the program’s parts, “N” refers to the parts themselves, and “P” refers to the number of ways to exit or end the program. 

Simply put, this formula shows that a program’s cognitive complexity increases with more decision-making points, such as loops and if-else statements. If a program has more decision points, it will have a higher cyclomatic complexity.

Calculating cyclomatic complexity in software engineering can be done in different ways. You can manually use control flow graphs or tools like cyclomatic complexity calculators. Some tools can even automatically review your code and provide complexity assessments. 

Using and understanding cyclomatic complexity

Cyclomatic complexity can sometimes be a valuable software development metric for developers and testers. It helps in several key areas:

  • For developers: Cyclomatic complexity acts as a guide for making code structure decisions. It indicates when a function’s complexity is too high, suggesting it may be time to refactor or decompose the function to simplify it.
  • For testing: Cyclomatic complexity determines the necessary scope of testing. It directly influences the minimum number of tests required for comprehensive coverage, which is crucial for unit, integration, and regression testing.

However, it’s essential to understand that cyclomatic complexity measures only one dimension of software quality – the complexity of the control flow within a program.

Understanding software complexity and quality requires qualitative aspects that cyclomatic complexity does not capture. Recognizing the subjective nature of code quality and complexity is vital. Factors such as individual experience, the specific context of a project, and each unique software requirement greatly influence what is considered “too complex” or “high quality.”

The role of human judgment

Assessing software quality requires human judgment. Code metrics such as cyclomatic complexity help developers and testers support their judgment. However, experience and insight are essential to make nuanced decisions about code structure, automation testing strategies, and overall maintainability index that metrics alone cannot provide. Therefore, these metrics should only be used to support human judgment, not replace it.

Qualitative factors to consider alongside cyclomatic complexity

To gain a more comprehensive understanding of software quality and complexity, consider the following qualitative factors alongside cyclomatic complexity:

  • Readability: How easily can other developers understand and work with the code? Is the code well-commented and consistently formatted?
  • Maintainability: How easy is it to modify the code? Consider factors like modular design and adherence to coding standards.
  • Testability: Beyond just the number of paths through the code, how easy is it to test those paths? Are there dependencies that make manual testing difficult?
  • Scalability: How well can the codebase accommodate future growth in features, users, or data volume?
  • Performance: How does the complexity of the code affect the software’s runtime performance and resource utilization?
  • Robustness: How well does the code handle unexpected conditions or invalid input? Does it fail gracefully?
  • Compatibility: Consider how changes to reduce cyclomatic complexity affect the system’s compatibility with other software or systems it interacts with.

How to reduce cyclomatic complexity and improve code quality

Crafting code that’s easy to work with, read, and modify is essential. It’s not just about reducing a metric, like cyclomatic complexity. Strategies for simplifying code can also improve its quality and overall understandability. 

By making code feel intuitive, we can ensure that anyone who reads or modifies it can do so easily and without errors. This article’ll explore some effective strategies for reducing cyclomatic complexity while enhancing code quality and understandability.

Refactoring and modularization

Breaking down a big code block into smaller, more focused functions has many benefits. It reduces complexity and makes the code more manageable. Refactored code is easier to understand, test, and maintain because it’s broken down into smaller pieces. This helps developers to keep their codebase clean and more robust.

Simplifying conditional logic 

Simplifying conditional statements can help reduce complexity in code. Minimizing branching makes the code’s behavior more predictable and flow more intuitive. This turns a complicated web of decisions into a clear and straightforward path that is easy to understand and follow. The ultimate result is an improvement in code quality.

Design patterns and best practices

When faced with common problems, using design patterns effectively simplifies the situation. These patterns establish a consistent language of standard solutions that can be easily understood by others. This shared vocabulary helps developers quickly understand the logic of a piece of code, why it’s designed a certain way, and its interactions with other parts of the system. It’s like using building blocks that have a well-known purpose and function. This results in software that’s easier to navigate and maintain.

Iterative refinement and collaborative practices 

Encouraging good coding practices like code reviews and pair programming can improve code quality. These practices shift the focus from individual coding abilities to improving code as a team effort. They help identify areas for reducing complexity and create an environment where multiple people continuously refine, improve, and understand the code. This collective effort ensures that the code is not only easy to understand but also easy to maintain and use.

Reducing cyclomatic complexity is a means to an end, not the end itself. The end goal is to cultivate a codebase that embodies simplicity, clarity, and ease of maintenance. By focusing on these strategies, developers lower complexity metrics and, more importantly, enhance the quality and comprehensibility of their code, making it a pleasure to work with for anyone who crosses its path.

February 20, 2024

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