Variable

Facts

It is important that the variable be defined before it is used. <bib>vincenzi-etal:slides:2007</bib>

Variables that contain data values have a defined life cycle: they are created, used, and then killed (destroyed). <bib>vincenzi-maldonado:slides:2007</bib>

Variables defined within a code block are created when their definitions are executed. <bib>vincenzi-maldonado:slides:2007</bib>

Variables defined within a code block are automatically destroyed at the end of a block. <bib>vincenzi-maldonado:slides:2007</bib>

The scope of a variable is the period between its creation and destruction. <bib>vincenzi-maldonado:slides:2007</bib>

An occurrence of a variable in a program can be classified as a variable definition or a variable use. <bib>vincenzi-maldonado:slides:2007</bib>

Consider the following notation:

Three possibilities exist for the first occurrence of a variable through a program path:

d the variable does not exist, then it is defined (correct)
u the variable does not exist, then it is used (incorrect - depends on the language)
k the variable does not exist, then it is killed or destroyed (probably incorrect)

Now consider the following time-sequenced pairs of defined (d), used (u), and killed (k):

ku	Killed and used	A serious defect
dd	Defined and defined again	Not invalid, but suspicious
du	Defined and used	Perfectly correct
dk	Defined and then killed	Not invalid, but suspicious
ud	Used and defined	Acceptable
uu	Used and used again	Acceptable
uk	Used and killed	Acceptable
kd	Killed and defined	Acceptable
kk	Killed and killed	Probably a programming error