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Control Flow Graph In Software Testing

April 16, 2024Yamini Priya
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control flow graph in software testing
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Control flow graph(CFG) in software testing is the key to unlocking the hidden pathways of a program’s logic, allowing testers to uncover potential pitfalls and ensure robust functionality. Just like an explorer relies on a map to chart their course through unknown territory, a tester leans on the CFG to guide them through the twists and turns of code execution. 

Just as a well-drawn map can make a trip more enjoyable and less prone to wrong turns, a well-constructed CFG simplifies the testing process, making it more efficient and effective. 

Not just that, CFG has more to do with software testing. This blog will discuss everything about control flow graphs and control flow testing in software testing.

Before we get into how control flow testing works, let’s see what control flow graph is and how control flow graph plays an important role in software testing.

What is a Control Flow Graph?

Control Flow Graph is a visual representation of a program’s control flow, showing how the program progresses through various statements and decision points.

The control flow graph was developed by Frances E. Allen, inspired by Reese T. Prosser’s use of boolean connectivity matrices for flow analysis.

Now, let’s see how this is related to the control flow testing.

What is Control Flow Testing?

Control flow testing is a software testing technique that helps ensure that the different paths a program can take are tested. It is a white-box testing technique. The goal of control flow testing is to verify that all possible execution paths of a program behave as expected.

Need more clarity? Let me explain it in a way that is as easy as ABC. 

Imagine you have a computer program with a set of instructions for the computer to follow. Sometimes, the program can go in different ways based on certain conditions (like if-else statements in code).

Now, the testers will test all these different paths to ensure the program works correctly. Simply put, it is like checking that different roads on a map lead to the right destination.

So, How is control flow testing done?

Control flow testing is usually performed using a control flow graph (CFG), which is a graphical representation of the flow of a program. 

The CFG shows the different nodes in the program, such as statements, branches, and loops, and the edges between them.

The software tester will start by creating CFG, which will help them identify all the possible execution paths of a program. Then, the tester can create test cases to cover each path.

Purpose of Control Flow Testing

A study states that control flow testing helps detect 50% of bugs during unit testing. And that is about 33% of all bugs.

But, control flow testing cannot find all bugs in a program. For example, it can not find interface issues.

Here are some examples of control flow testing,

  • Testing all branches of an if statement.
  • Testing all cases of a switch statement.
  • Checking all possible iterations of a loop.
  • Testing the error handling.
  • Testing the performance of the application for different inputs.

By testing all possible paths through a program, testers can ensure that the program is high quality and meets user requirements.

Control Testing Example

Let’s look at an example program in Java and see how control flow testing works.

Control Flow Testing Example

Control flow testing will help testers test all the possible paths the above program can take. That way, they can make sure the program works as expected with any path.

Uses of Control Flow Testing

Control flow testing, a type of white box testing, involves analyzing program code by executing test cases. Another approach involves executing data flow.

Here are the key uses of control flow testing:

  • Unit Testing – Control flow testing is mainly in unit testing. It aims to identify the different paths of execution in the code. Test cases are then created based on these paths and executed.
  • Control Flow Graphs – This testing involves creating control flow graphs. These graphs provide a visual representation of how the program flows. By analyzing these paths, testers will be able to write various test cases.
  • High Test Coverage – When testing with CFGs, every node and edge in the graph is covered. Thus, it helps achieve high test coverage.

Notation of Control Flow Testing

Control flow testing consists of four nodes, namely,

  1. Node
  2. Edge
  3. Decision Node
  4. Junction node

Node

A node represents a basic block of code. It is a section of code that runs from start to end. The nodes show the order of procedure in a program. They help testers to identify what step comes next, making it easy to understand the sequence of actions.

Edge

An edge represents a possible transition or flow of control from one node to another.

Decision Node

A decision node, also known as a conditional node, represents a point in the program where a decision is made based on a condition (e.g., an if-else statement).

Junction Node

A junction node is a point in the program where at least three nodes meet. It is used to represent points where multiple branches merge back into a single path or split into multiple paths. Simply put, junction nodes are where different nodes lead to one node.

Notations of Control Flow Graph

Control Flow Testing Notations – Example

Let’s understand these notations using a simple program. The below program checks if a person is eligible to vote or not.

Control flow testing in software testing

What will this program do?

Simple, it takes two paths, as follows,

  • If x is greater than or equal to 18, it prints the message “You are eligible to vote.”
  • Else, when x is less than 18, it prints the message “You aren’t eligible to vote.”

Here, the input value ‘x’ denotes the age.

Sample Control Flow Graph

Now, the control flow graph for the above program will be as follows,

Control Flow Graph

Let’s see which are what notations.

  • Nodes – Start, Age, Eligible, Not Eligible, Stop
  • Decision node – x>=18
  • The last node that leads to the stop is the Junction node.
  • Edge – All the direction arrows that connect the nodes

Control Flow Testing Process

The control flow testing process involves the following steps,

Control flow testing process

1. Control Flow Graph Creation

Start by creating a control flow graph, a visual representation of how the program runs based on the source code. It can be created manually or with a software.

2. Coverage Target

A coverage target is defined over the CFG, which includes nodes, paths, edges, branches, etc. Here, testers will decide what part of the graph has to be tested.

3. Test Case Creation

Develop specific test scenarios and test cases based on the control flow graph to cover the designed coverage target.

4. Test Case Execution

Now, execute the test cases that were created to target specific parts of the program.

5. Analysis

Analyze the test results and find out if the program behaves as expected or not.

Advantages

Control flow testing offers several benefits as follows,

  • It identifies nearly half of the defects found during unit testing.
  • It also pinpoints around one-third of the total program defects.
  • Control flow testing can be performed manually or using test automation platforms. With automated testing, you can make your testing super fast and efficient.

Learn More about Automated Testing

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  • It helps detect at least one-third of the defects of the overall program. Thus, it increases the efficiency of testing.
  • It helps test every edge and node in a program, thereby helping achieve high test coverage.

Disadvantages

Though it offers advantages, it has other drawbacks as follows,

  • Identifying missing paths in a large control flow graph can be challenging, mainly when one person handles the program and its model.
  • Control flow testing may not detect all errors during the variable’s instantiation.
  • The errors identified during specification may not always be detected during testing.
  • Detecting mistakes or discrepancies in the interface is particularly difficult during testing.
  • It is not possible to detect flaws in the user interface. So, you will have to do comprehensive GUI testing to find interface issues.

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Summary

Just like following a roadmap on a journey, control flow testing guides us through the intricate pathways of code, helping us to identify potential issues and verify the correctness of the software.

It allows testers to design test cases that cover all possible scenarios, making sure that no stone is left unturned. Whether you manually perform testing or automate it, the control flow testing ensures that the software behaves as expected and handles different inputs.

Frequently Asked Questions

What is the importance of a control flow graph?

The control flow graph identifies all possible paths through the code, ensuring comprehensive test coverage. It helps us identify inaccessible areas of the program and is great for identifying loops and other conditional statements in the code.

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