[Solved]: Why are decision problems commonly used in complexity theory?

Problem Detail: 

From Wikipedia:

The type of computational problem: The most commonly used problems are decision problems. However, complexity classes can be defined based on function problems, counting problems, optimization problems, promise problems, etc.

I also saw the definitions of NP-complete, NP-hard, NP, ..., are defined for decision problems only. I wonder why that is the case?

Is it because any other problem can be equivalently converted to a decision problem?

Asked By : Tim

Answered By : Christoph Wintersteiger

Oftentimes decision problems are used because they allow a precise and simple definition of the problem and, as stated, many other problems can be converted to an equivalent decision problem.

Other types of problems are also considered in complexity theory, for instance Function Problems and Search Problems.

Best Answer from StackOverflow

Question Source : http://cs.stackexchange.com/questions/4874