BASIC IDEA

4.1 BASIC IDEA

Two kinds of basic statements in a program unit are assignment statements and conditional statements. An assignment statement is explicitly represented by using an assignment symbol, “ = ”, such as x = 2*y;, where x and y are variables. Program conditions are at the core of conditional statements, such as if(), for() loop, while() loop, and goto. As an example, in if(x! = y), we are testing for the inequality of x and y. In the absence of conditional statements, program instructions are executed in the sequence they appear. The idea of successive execution of instructions gives rise to the concept of control flow in a program unit. Conditional statements alter the default, sequential control flow in a program unit. In fact, even a small number of conditional statements can lead to a complex control flow structure in a program.

Function calls are a mechanism to provide abstraction in program design.

A call to a program function leads to control entering the called function. Similarly, when the called function executes its return statement, we say that control exits from the function. Though a function can have many return statements, for simplic- ity, one can restructure the function to have exactly one return. A program unit can

be viewed as having a well-defined entry point and a well-defined exit point. The execution of a sequence of instructions from the entry point to the exit point of a program unit is called a program path. There can be a large, even infinite, number of paths in a program unit. Each program path can be characterized by an input and an expected output. A specific input value causes a specific program path to be executed; it is expected that the program path performs the desired computation, thereby producing the expected output value. Therefore, it may seem natural to execute as many program paths as possible. Mere execution of a large number of

Software Testing and Quality Assurance: Theory and Practice , Edited by Kshirasagar Naik and Priyadarshi Tripathy Copyright © 2008 John Wiley & Sons, Inc.

89 paths, at a higher cost, may not be effective in revealing defects. Ideally, one must

4.2 OUTLINE OF CONTROL FLOW TESTING

strive to execute fewer paths for better effectiveness. The concepts of control flow in computer programs [1], program paths [2], and control flow testing [2–8] have been studied for many decades. Tools are being developed to support control flow testing [9]. Such tools identify paths from

a program unit based on a user-defined criterion, generate the corresponding input to execute a selected path, and generate program stubs and drivers to execute the test. Control flow testing is a kind of structural testing, which is performed by programmers to test code written by them. The concept is applied to small units of code, such as a function. Test cases for control flow testing are derived from the source code, such as a program unit (e.g., a function or method), rather than from the entire program.

Structurally, a path is a sequence of statements in a program unit, whereas, semantically, it is an execution instance of the unit. For a given set of input data, the program unit executes a certain path. For another set of input data, the unit may execute a different path. The main idea in control flow testing is to appropriately select a few paths in a program unit and observe whether or not the selected paths produce the expected outcome. By executing a few paths in a program unit, the programmer tries to assess the behavior of the entire program unit.