Software Metrics Software metrics help evaluate development and testing efforts needed,

March 2003 91.3913 R. McFadyen 1

Software Metrics

Software metrics help evaluate

•development and testing efforts needed,

•understandability, maintainability and reusability of software.


Metrics

Fan-in/fan-out

Lines of code

Cyclomatic complexity*

Comment percentage

Length of identifiers

Depth of conditional nesting

Fog index

Weighted methods per class

Response for a class

Lack of cohesion of methods

Coupling between objects*

Depth of inheritance tree

Number of children

Number of overriding operations

Traditional Object-Oriented

* March 2003

see http://satc.gsfc.nasa.gov/support/STC_APR98/apply_oo/apply_oo.doc

http://www.stsc.hill.af.mil/crosstalk/frames.asp?uri=1997/04/quality.asp

http://satc.gsfc.nasa.gov/support/STC_APR98/apply_oo/apply_oo.doc

http://www.stsc.hill.af.mil/crosstalk/frames.asp?uri=1997/04/quality.asp


Traditional Metrics

Fan-in/fan-out Fan-in is a measure of the number of functions that call some other function, say X

Fan-out is a measure of the number of functions which are called by a function, say X.

(See RFC)

Cyclomatic complexity

CC is a count of the number of test cases required to test comprehensively


Traditional Metrics

Lines of Code LOC can be counted as•lines (LOC)•non-comment non-blank lines (NCNB)•executable statements (EXEC)

Note that NASA uses executable lines of code to evaluate size

Comment percentage

Calculated as the total number of comments divided by the {total lines of code less the number of blank lines}


Traditional Metrics

Depth of conditional nesting

Measure of the depth of nesting of IF statements in programs

Fog index Measure of the average length of words and sentences in documents. The higher the value for the Fog index, the more difficult ...

Length of identifiers

Measures average length of distinct identifiers


OO Metrics


WMC is the number of methods included in a class weighted by the complexity of each method


RFC is a count of all methods that can be invoked in response to a message to an object of the class


OO Metrics


LOCM measures the dissimilarity of methods in a class. A highly cohesive module stands alone. Lack of cohesion increases complexity.

Coupling between objects

CBO is a count of the number of other classes to which a class is coupled. Measured by counting the number of distinct non-inheritance related class hierarchies on which a class depends


OO Metrics


DIT is the maximum number of steps from the class node to the root of the tree

Number of children

NOC is the number of immediate subclasses subordinate to a class in the hierarchy

Number of overriding operations

Number of operations in a superclass that are overridden in a subclass


WMC


WMC is the number of methods included in a class weighted by the complexity of each method

How does one count this?

a) simply count the number of methods, or,

b) sum the complexities of the methods (complexity might be rated: 1 Simple, 2 Moderately complex, 3 Complex)


WMC

Shows that most classes

have less than 20

methods, … but some

have over 200 methods.


RFC


RFC is a count of all methods that can be invoked in response to a message to an object of the class


For a class, we count the number of distinct methods that can be invoked in response to a message, including all methods within the class hierarchy

For a class, we count all messages that can be sent by all methods in the class


RFC

In this case, about half th

e

classes have an RFC <=20


LCOM


LOCM measures the dissimilarity of methods in a class. A highly cohesive module stands alone. Lack of cohesion increases complexity.


We need a way of measuring cohesiveness (or not).

First proposal:

a) compute the average over all attributes in a class

number of methods using the attribute

total number of methods

convert to a percentage and subtract from 100


LCOM

Second proposal:

b) for each method, create a set of the attributes it accesses. For each pair of methods, perform the intersection of attributes. If an intersection is not empty put the set in P; if it is empty put the set in Q.

The measure is the difference in cardinalities of P and Q:

LCOM = P - Q if P > Q

otherwise LCOM = 0


LCOM


CBO

Coupling between objects

CBO is a count of the number of other classes to which a class is coupled. Measured by counting the number of distinct non-inheritance related class hierarchies on which a class depends


• count the number of distinct noninheritance-related class hierarchies on which a class depends


CBO

A CBO value of zero

corresponds to a class

that is only coupled to

others in its

related

inheritance hierarchy


DIT


Most classes are at

level 0 and 1

DIT is the maximum number of steps from the class node to the root of the tree


NOC

Number of children

NOC is the number of immediate subclasses subordinate to a class in the hierarchy


• the number of immediate subclasses subordinate to a class in the hierarchy


NOC

One o

f the

se cl

asse

s, at

dept

h 3,

has

alm

ost 4

0

child

ren!


CC


CC is a count of the number of test cases required to test comprehensively

CC measures the amount of decision logic in a module.

A value of 10 or less is the general recommendation.

Formally, based on a program’s flowgraph,


= number of edges – number of nodes + 2


CC

Control flowgraphs describe the logic structure of software modules.

• A module corresponds to a single function or subroutine in typical languages, has a single entry and exit point, and is able to be used as a design component via a call/return mechanism.

Each flowgraph consists of nodes and edges.

• nodes represent computational statements or expressions,

• edges represent transfer of control between nodes.


Flowgraph Construction - Sequence

x=z

y=7

y=10

Sequences are reducible to a single node:

CC=?


Flowgraph Construction – If then

if x>4 then

x=z

y=7

q=q*2

end if

CC=


Flowgraph Construction – If Else

if x>4 then

x=z

y=7

else

y=10

end if

CC=


Flowgraph Construction - While

while x>0 do

x=x/2

…

end while

CC=


Flowgraph Construction – Do Until

Do

…

Read x

Until x=0

CC=


Program

FindMean (FILE ScoreFile){ float SumOfScores = 0.0;

int NumberOfScores = 0; float Mean=0.0; float Score;Read(ScoreFile, Score);while (! EOF(ScoreFile) {

if (Score > 0.0 ) {SumOfScores = SumOfScores + Score;NumberOfScores++;}

Read(ScoreFile, Score);}/* Compute the mean and print the result */if (NumberOfScores > 0) {

Mean = SumOfScores / NumberOfScores;printf(“ The mean score is %f\n”, Mean);

} elseprintf (“No scores found in file\n”);

}

1

2

3

4

5

7

6

89


Flowgraph

4

3

2

1

5

6

7 8

9


= CC

= 11-9+2 = 4

decision structures:

•one while

•one if then

•one if else


CC

Software Metrics Software metrics help evaluate development and testing efforts needed,

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