Essential complexity: Difference between revisions

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{{About-distinguish2|the numerical measure of a program's "structuredness" defined by McCabe|the notion antonymic to accidental complexity used by Brooks in [[No Silver Bullet]] and subsequent works}}

'''Essential complexity''' is a [[numerical measure]] defined by Thomas J. McCabe, Sr., in his highly cited, 1976 paper better known for introducing [[cyclomatic complexity]]. McCabe defined essential complexity as the cyclomatic complexity of the reduced CFG ([[control -flow graph]]) after iteratively replacing (reducing) all [[structured programming]] [[control structure]]s, i.e. those having a single entry point and a single exit point (for example if-then-else and while loops) with placeholder single statements.<ref name="mccabe76">{{cite journal| last=McCabe| date=December 1976| journal=IEEE Transactions on Software Engineering| pages=308–320| title=A Complexity Measure| issue=4| doi=10.1109/tse.1976.233837| s2cid=9116234}}</ref>{{rp|317}}<ref>http://www.mccabe.com/pdf/mccabe-nist235r.pdf</ref>{{rp|80}}<!-- note that the original paper has an error in the final formula for ev, but this is corrected in the technical report-->

McCabe's reduction process is intended to simulate the conceptual replacement of control structures (and actual statements they contain) with subroutine calls, hence the requirement for the control structures to have a single entry and a single exit point.<ref name="mccabe76"/>{{rp|317}} (Nowadays a process like this would fall under the umbrella term of [[refactoring]].) All structured programs evidently have an essential complexity of 1 as defined by McCabe because they can all be iteratively reduced to a single call to a top-level subroutine.<ref name="mccabe76"/>{{rp|318}} As McCabe explains in his paper, his essential complexity metric was designed to provide a measure of how far off this ideal (of being completely structured) a given program was.<ref name="mccabe76"/>{{rp|317}} Thus greater than 1 essential complexity numbers, which can only be obtained for non-structured programs, indicate that they are further away from the structured programming ideal.<ref name="mccabe76"/>{{rp|317}}

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In their paper, Böhm and Jacopini conjectured, but did not prove that it was necessary to introduce such additional variables for certain kinds of non-structured programs in order to transform them into structured programs.<ref name="K">{{cite journal|title=Analysis of structured programs|author=S. Rao Kosaraju|journal=Journal of Computer and System Sciences|volume=9|number=3|date=December 1974|doi=10.1016/S0022-0000(74)80043-7|pages=232–255}}</ref>{{rp|236}} An example of program (that we now know) does require such additional variables is a loop with two conditional exits inside it. In order to address the conjecture of Böhm and Jacopini, Kosaraju defined a more restrictive notion of program reduction than the Turing equivalence used by Böhm and Jacopini. Essentially, Kosaraju's notion of reduction imposes, besides the obvious requirement that the two programs must compute the same value (or not finish) given the same inputs, that the two programs must use the same primitive actions and predicates, the latter understood as expressions used in the conditionals. Because of these restrictions, Kosaraju's reduction does not allow the introduction of additional variables; assigning to these variables would create new primitive actions and testing their values would change the predicates used in the conditionals. Using this more restrictive notion of reduction, Kosaraju proved Böhm and Jacopini's conjecture, namely that a loop with two exits cannot be transformed into a structured program ''without introducing additional variables'', but went further and proved that programs containing multi-level breaks (from loops) form a hierarchy, such that one can always find a program with multi-level breaks of depth ''n'' that cannot be reduced to a program of multi-level breaks with depth less than ''n'', again without introducing additional variables.<ref name="K"/><ref>For more modern treatment of the same results see: Kozen, [http://www.cs.cornell.edu/~kozen/papers/bohmjacopini.pdf The Böhm–Jacopini Theorem is False, Propositionally]</ref>

McCabe notes in his paper that in view of Kosaraju's results, he intended to find a way to capture the essential properties of non-structured programs in terms of their control -flow graphs.<ref name="mccabe76"/>{{rp|315}} He proceeds by first identifying the control -flow graphs corresponding to the smallest non-structured programs (these include branching into a loop, branching out of a loop, and their if-then-else counterparts) which he uses to formulate a theorem analogous to [[Kuratowski's theorem]], and thereafter he introduces his notion of essential complexity in order to give a scale answer ("measure of the structuredness of a program" in his words) rather than a yes/no answer to the question of whether a program's control -flow graph is structured or not.<ref name="mccabe76"/>{{rp|315}} Finally, the notion of reduction used by McCabe to shrink the CFG is not the same as Kosaraju's notion of reducing flowcharts. The reduction defined on the CFG does not know or care about the program's inputs, it is simply a [[graph transformation]].<ref>McCabe footnotes the two definitions of on pages 315 and 317.</ref>

For example, the following C program fragment has an essential complexity of 1, because the inner '''if''' statement and the '''for''' can be reduced, i.e. it is a structured program.