Programming complexity is a term that encompasses numerous properties of a piece of software, all of which affect internal interactions. According to several commentators, there is a distinction between the terms complex and complicated. Complicated implies being difficult to understand but with time and effort, ultimately knowable. Complex, on the other hand, describes the interactions between a number of entities. As the number of entities increases, the number of interactions between them would increase exponentially, and it would get to a point where it would be impossible to know and understand all of them. Similarly, higher levels of complexity in software increase the risk of unintentionally interfering with interactions and so increases the chance of introducing defects when making changes. In more extreme cases, it can make modifying the software virtually impossible. The idea of linking software complexity to the maintainability of the software has been explored extensively by Professor Manny Lehman, who developed his Laws of Software Evolution

Lehman's laws of software evolution

In Software engineering, the Laws of Software Evolution refer to a series of laws that Lehman and Belady formulated starting in 1974 with respect to Software evolution....

 from his research. He and his co-Author Les Belady explored numerous possible Software Metrics in their oft cited book, that could be used to measure the state of the software, eventually reaching the conclusion that the only practical solution would be to use one that uses deterministic complexity models.

Measures

Many measures of software complexity have been proposed. Many of these, although yielding a good representation of complexity, do not lend themselves to easy measurement. Some of the more commonly used metrics are

• McCabes cyclomatic complexity metric
  Cyclomatic complexity
  Cyclomatic complexity is a software metric . It was developed by Thomas J. McCabe, Sr. in 1976 and is used to indicate the complexity of a program. It directly measures the number of linearly independent paths through a program's source code...
  
  
• Halsteads software science metrics
  Halstead complexity measures
  Halstead complexity measures are software metrics introduced by Maurice Howard Halstead in 1977 as part of his treatise on establishing an empirical science of software development....
  
  
• Henry and Kafura introduced Software Structure Metrics Based on Information Flow in 1981 which measures complexity as a function of fan in and fan out. They define fan-in of a procedure as the number of local flows into that procedure plus the number of data structures from which that procedure retrieves information. Fan-out is defined as the number of local flows out of that procedure plus the number of data structures that the procedure updates. Local flows relate to data passed to and from procedures that call or are called by, the procedure in question. Henry and Kafura's complexity value is defined as "the square of procedure length multiplied by fan-in multiplied by fan-out."

• A Metrics Suite for Object Oriented Design was introduced by Chidamber and Kemerer in 1994 focusing, as the title suggests on metrics specifically for object oriented code. They introduce six OO complexity metrics; weighted methods per class, coupling between object classes, response for a class, number of children, depth of inheritance tree and lack of cohesion of methods

There are several other metrics that can be used to measure programming complexity:

• Branching complexity (Sneed Metric)
• Data access complexity (Card Metric)
• Data complexity (Chapin Metric)
• Data flow complexity (Elshof Metric)
• Decisional complexity (McClure Metric)

Types

Associated with, and dependent on the complexity of an existing program, is the complexity associated with changing the program. The complexity of a problem can be divided into two parts:

1. Accidental complexity: Relates to difficulties a programmer faces due to the chosen software engineering tools. A better fitting set of tools or a more high-level programming language may reduce it.
2. Essential complexity: Is caused by the characteristics of the problem to be solved and cannot be reduced.

See also

• Software crisis
  Software crisis
  Software crisis is the term used in early days of computing science. The term was used to describe the impact of rapid increases in computer power and the complexity of the problems that could be tackled. In essence, it refers to the difficulty of writing correct, understandable, and verifiable...
  
   (and subsequent programming paradigm
  Programming paradigm
  A programming paradigm is a fundamental style of computer programming. Paradigms differ in the concepts and abstractions used to represent the elements of a program and the steps that compose a computation A programming paradigm is a fundamental style of computer programming. (Compare with a...
  
   solutions)
• Software metric
  Software metric
  A software metric is a measure of some property of a piece of software or its specifications. Since quantitative measurements are essential in all sciences, there is a continuous effort by computer science practitioners and theoreticians to bring similar approaches to software development...
  
  s - quantitative measure of some property of a program.