1 Complexity metrics  measure certain aspects of the software (lines of code, # of if-statements, depth of nesting, …)  use these numbers as a criterion to assess a design, or to guide the design  interpretation: higher value  higher complexity  more effort required (= worse design)  two kinds:  intra-modular: inside one module  inter-modular: between modules

2 Sized-based complexity measures  counting lines of code (intra-modular)  differences in scale for different programming languages  Halstead’s metrics: counting operators and operands

3 Halstead’s metrics:  n 1 : number of unique operators  n 2 : number of unique operands  N 1 : total number of operators  N 2 : total number of operands

4 Example program public static void sort(int x []) { for (int i=0; i < x.length-1; i++) { for (int j=i+1; j < x.length; j++) { if (x[i] > x[j]) { int save=x[i]; x[i]=x[j]; x[j]=save } operator, 1 occurrence operator, 2 occurrences

5 operator # of occurrences public sort() int [] {} for {;;} if () = < … n 1 = … N 1 = 39

6 Example program public static void sort(int x []) { for (int i=0; i < x.length-1; i++) { for (int j=i+1; j < x.length; j++) { if (x[i] > x[j]) { int save=x[i]; x[i]=x[j]; x[j]=save } operand, 2 occurrences

7 operand # of occurrences x length i j save 0 1 n 2 = N 2 = 29

8 Metrics:  size of vocabulary: n = n 1 + n 2  program length: N = N 1 + N 2  volume: V = N log 2 n  level of abstraction: L = V*/ V, V*=volume of fct prototype. For sort(x), V* = 2 log 2. For main(), L is high. approximation: L’ = (2/n 1 )(n 2 /N 2 )  programming effort: E = V/L  estimated programming time: T ’ = E/18  estimate of N: N ’ = n 1 log 2 n 1 + n 2 log 2 n 2  for this example: N = 68, N ’ = 89, L =.015, L’ =.028

9 Remember…  Metrics are only estimates, give some idea on complexity  critique:  different definitions of “operand” and “operator”  explanations are not convincing (empirical, may not apply to other projects)  Source code must exist

10 Other metrics (structure-based)  use  control structures  data structures  or both  example complexity measure based on data structures: average number of instructions between successive references to a variable  best known measure is based on the control structure: McCabe’s cyclomatic complexity

11 Example program public static void sort(int x []) { for (int i=0; i < x.length-1; i++) { for (int j=i+1; j < x.length; j++) { if (x[i] > x[j]) { int save=x[i]; x[i]=x[j]; x[j]=save }

12 Cyclomatic complexity e = number of edges (13) n = number of nodes (11) p = number of connected components (1) CV = e - n + p + 1 (4)

13 Intra-modular complexity measures, summary  for small programs, the various measures correlate well with programming time  however, a simple length measure such as LOC does equally well  complexity measures are not very context sensitive  complexity measures take into account few aspects  it might help to look at the complexity density instead

14 System structure: inter-module complexity  measures dependencies between modules: draw graph:  modules =nodes  edges connecting modules may denote several relations, most often: A uses B (ex: procedure calls)

15 The uses relation  the call-graph  chaos (general directed graph)  hierarchy (acyclic graph)  strict hierarchy (layers)  tree

16 In a picture: chaos strict hierarchy tree

17 Measurements } size # nodes # edges width height

18 Deviation from a tree strict hierarchy tree

19 Tree impurity metric  complete graph with n nodes has e = n(n-1)/2 edges  a tree with n nodes has e = (n-1) edges  tree impurity for a graph with n nodes and e edges: m(G) = 2(e-n+1)/(n-1)(n-2) (0 for tree, 1 for complete graph)

20 Any tree impurity metric:  m(G) = 0 if and only if G is a tree  m(G 1 ) > m(G 2 ) if G 1 = G 2 + an extra edge  if G 1 and G 2 have the same # of “extra” edges wrt their spanning tree, and G 1 has more nodes than G 2, then m(G 1 ) < m(G 2 )  m(G)  m(K n ) = 1, where G has n nodes, and K n is the (undirected) complete graph with n nodes

21 Information flow metric  tree impurity metrics only consider the number of edges, not their “thickness”  Shepperd’s metric:  there is a local flow from A to B if:  A invokes B and passes it a parameter  B invokes A and A returns a value  there is a global flow from A to B if A updates some global structure and B reads that structure

22 Shepperd’s metric  fan-in(M) = # (local and global) flows whose sink is M  fan-out(M) = # (local and global) flows whose source is M  complexity(M) = (fan-in(M) * fan-out(M)) 2

23 Point to ponder: What does this program do? procedure X(A: array [1..n] of int); var i, k, small: int; begin for i:= 1 to n do small:= A[i]; for k:= i to n-1 do if small <= A[k] then swap (A[k], A[k+1]) end

24 Object-oriented metrics  WMC: Weighted Methods per Class  DIT: Depth of Inheritance Tree  NOC: Number Of Children  CBO: Coupling Between Object Classes  RFC: Response For a Class  LCOM: Lack of COhesion of a Method

25 Weighted Methods per Class  measure for size of class  WMC =  c(i), i = 1, …, n (number of methods)  c(i) = complexity of method i  mostly, c(i) = 1

26 Depth of Class in Inheritance Tree  DIT = distance of class to root of its inheritance tree  Good: forest of classes of medium height

27 Number Of Children  NOC: counts immediate descendants in inheritance tree  higher values NOC are considered bad:  possibly improper abstraction of the parent class  also suggests that class is to be used in a variety of settings

28 Coupling Between Object Classes  two classes are coupled if a method of one class uses a method or state variable of another class  CBO = count of all classes a given class is coupled with  high values: something is wrong  all couplings are counted alike; refinements are possible

29 Response For a Class  RFC = size of the “response set” of a class  response set = {set of methods: M}  {set methods called by M1: R 1 }  {R2}  … M1 M3 M2 R1

30 Lack of Cohesion of a Method  cohesion = glue that keeps the module (class) together, eg. all methods use the same set of state variables  if some methods use a subset of the state variables, and others use another subset, the class lacks cohesion  LCOM = number of disjoint sets of methods in a class  two methods in the same set share at least one state variable

31 OO metrics  WMC, CBO, RFC, LCOM most useful  Predict fault proneness during design  Strong relationship to maintenance effort  Many OO metrics correlate strongly with size