MC CASE : A COMPLEXITY MEASURE 309 CYCtoMATIC COMPLEXITY

This paper describes a graph-theoretic complexity measure and illustrates how it can be used to manage and control program complexity .The paper first explains how the graph-theory concepts apply and gives an intuitive explanation of the graph concepts in programming terms. The control graphs of several actual Fortran programs are then presented to illustrate the conelation between intuitive complexity and the graph-theoretic complexity .Several properties of the graphtheoretic complexity are then proved which show, for example, that complexity is independent of physical size (adding or subtracting functional statements leaves complexity unchanged) and complexity depends only on the decision structure of a program. The issue of using non structured control flow )s also discussed. A characterization of nonstructured control graphs is given and a method of measuring the "structuredness" of a program is developed. The relationship between structure and reducibility is illustrated with several examples. The last section of this paper deals with a testing methodology used in conjunction with the complexity measure; a testing strategy is defined that dictates that a program can either admit of a certain minimal testing level or the program can be structurally reduced. Index Temls-Basis, complexity measure, control flow, decomposition, graph theory , independence, linear, modularization, programming, reduction, software, testing. II. A COMPLEXITY MEASURE In this sl~ction a mathematical technique for program modularization will be developed. A few defmitions and theorems from graph theory will be needed, but several examples will be presented in order to illustrate the applications of the technique. The complexity measure approach we will take is to measure and control the number of paths through a program. This approach, however, immediately raises the following nasty problem: "Any program with a backward branch potentially has an infinite number of paths." Although it is possible to defme a set of algebraic expressions that give the total number of possible paths through a (structured) program,l using the total number of paths has been found to be impractical. Because of this the complexity measure developed here is defmed in terms of basic paths-that when taken in combinatio~ will generate every possible path. The following mathematical preliminaries will be needed, all of which can be found in Berge [I] . Definition 1: The cyclomatic number V(G) of a graph G with n vertices, e edges, and p connected components is