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White Box Testing

White box testing focuses on the internal functioning of the product. For this different procedures are tested. White box testing tests the following
  • Loops of the procedure
  • Decision points
  • Execution paths

For performing white box testing, basic path testing technique is used. We will illustrate how to use this technique, in the following section.

Basis Path Testing

Basic path testing a white box testing technique .It was proposed by Tom McCabe. These tests guarantee to execute every statement in the program at least one time during testing. Basic set is the set of all the execution path of a procedure.

Flow graph Notation

Before basic path procedure is discussed, it is important to know the simple notation used for the repres4enttation of control flow. This notation is known as flow graph. Flow graph depicts control flow and uses the following constructs.

These individual constructs combine together to produce the flow graph for a particular procedure

Sequence -                      Until -

If -              While -              Case -

Basic terminology associated with the flow graph

Node: Each flow graph node represents one or more procedural statements. Each node that contains a condition is called a predicate node.

Edge: Edge is the connection between two nodes. The edges between nodes represent flow of control. An edge must terminate at a node, even if the node does not represent any useful procedural statements.

Region: A region in a flow graph is an area bounded by edges and nodes. Cyclomatic complexity: Independent path is an execution flow from the start point to the end point. Since a procedure contains control statements, there are various execution paths depending upon decision taken on the control statement. So Cyclomatic complexity provides the number of such execution independent paths. Thus it provides a upper bound for number of tests that must be produced because for each independent path, a test should be conducted to see if it is actually reaching the end point of the procedure or not.

Cyclomatic Complexity

Cyclomatic Complexity for a flow graph is computed in one of three ways:

  1. The numbers of regions of the flow graph correspond to the Cyclomatic complexity.

  2. Cyclomatic complexity, V(G), for a flow graph G is defined as

    V(G) = E – N + 2

    where E is the number of flow graph edges and N is the number of flow graph nodes.

  3. Cyclomatic complexity, V(G), for a graph flow G is also defined as

    V(G) = P + 1

    Where P is the number of predicate nodes contained in the flow graph G.

Example: Consider the following flow graph

Region, R= 6
Number of Nodes = 13
Number of edges = 17
Number of Predicate Nodes = 5

Cyclomatic Complexity, V( C) :

V( C ) = R = 6;

V(C) = Predicate Nodes + 1
=5+1 =6

V( C)= E-N+2
= 17-13+2

Deriving Test Cases

The main objective of basic path testing is to derive the test cases for the procedure under test. The process of deriving test cases is following

  1. From the design or source code, derive a flow graph.

  2. Determine the Cyclomatic complexity, V(G) of this flow graph using any of the formula discussed above.

    · Even without a flow graph, V(G) can be determined by counting the number of conditional statements in the code and adding one to it.

  3. Prepare test cases that will force execution of each path in the basis set.

    · Each test case is executed and compared to the expected results.

Graph Matrices

Graph matrix is a two dimensional matrix that helps in determining the basic set. It has rows and columns each equal to number of nodes in flow graph. Entry corresponding to each node-node pair represents an edge in flow graph. Each edge is represented by some letter (as given in the flow chart) to distinguish it from other edges. Then each edge is provided with some link weight, 0 if there is no connection and 1 if there is connection.

For providing weights each letter is replaced by 1 indicating a connection. Now the graph matrix is called connection matrix. Each row with two entries represents a predicate node. Then for each row sum of the entries is obtained and 1 is subtracted from it. Now the value so obtained for each row is added and 1 is again added to get the cyclomatic complexity.

Once the internal working of the different procedure are tested, then the testing for the overall functionality of program structure is tested. For this black box testing techniques are used which are discussed in the following pages.


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