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Viewing the last subsection and this one together gives you another example illustrating that control flow and data flow are equally important in computer programming. Data-flow testing is based on the idea that data usage is at least as error-prone as control flow. Boris Beizer claims that at least half of all code consists of data declarations and initializations (Beizer 1990). Data can exist in one of three states:
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Defined The data has been initialized, but it hasn t been used yet.
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22. Developer Testing
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Used The data has been used for computation, as an argument to a routine, or for something else. Killed The data was once defined, but it has been undefined in some way. For example, if the data is a pointer, perhaps the pointer has been freed. If it s a for-loop index, perhaps the program is out of the loop and the programming language doesn t define the value of a for-loop index once it s outside the loop. If it s a pointer to a record in a file, maybe the file has been closed and the record pointer is no longer valid.
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In addition to having the terms defined, used, and killed, it s convenient to have terms that describe entering or exiting a routine immediately before or after doing something to a variable:
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Entered The control flow enters the routine immediately before the variable is acted upon. A working variable is initialized at the top of a routine, for example. Exited The control flow leaves the routine immediately after the variable is acted upon. A return value is assigned to a status variable at the end of a routine, for example.
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Combinations of Data States
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The normal combination of data states is that a variable is defined, used one or more times, and perhaps killed. View the following patterns suspiciously:
Defined-Defined If you have to define a variable twice before the value sticks, you don t need a better program, you need a better computer! It s wasteful and error-prone, even if not actually wrong. Defined-Exited If the variable is a local variable, it doesn t make sense to define it and exit without using it. If it s a routine parameter or a global variable, it might be all right. Defined-Killed Defining a variable and then killing it suggests either that the variable is extraneous or that the code that was supposed to use the variable is missing.
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22. Developer Testing
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Entered-Killed This is a problem if the variable is a local variable. It wouldn t need to be killed if it hasn t been defined or used. If, on the other hand, it s a routine parameter or a global variable, this pattern is all right as long as the variable is defined somewhere else before it s killed. Entered-Used Again, this is a problem if the variable is a local variable. The variable needs to be defined before it s used. If, on the other hand, it s a routine parameter or a global variable, the pattern is all right if the variable is defined somewhere else before it s used. Killed-Killed A variable shouldn t need to be killed twice. Variables don t come back to life. A resurrected variable indicates sloppy programming. Double kills are also fatal for pointers one of the best ways to hang your machine is to kill (free) a pointer twice. Killed-Used Using a variable after it has been killed is a logical error. If the code seems to work anyway (for example, a pointer that still points to memory that s been freed), that s an accident, and Murphy s Law says that the code will stop working at the time when it will cause the most mayhem. Used-Defined Using and then defining a variable might or might not be a problem, depending on whether the variable was also defined before it was used. Certainly if you see a used-defined pattern, it s worthwhile to check for a previous definition.
Check for these anomalous sequences of data states before testing begins. After you ve checked for the anomalous sequences, the key to writing data-flow test cases is to exercise all possible defined-used paths. You can do this to various degrees of thoroughness, including All definitions. Test every definition of every variable (that is, every place at which any variable receives a value). This is a weak strategy because if you try to exercise every line of code you ll do this by default. All defined-used combinations. Test every combination of defining a variable in one place and using it in another. This is a stronger strategy than testing all definitions because merely executing every line of code does not guarantee that every defined-used combination will be tested.
Here s an example:
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