This chapter focuses on two different techniques used by software engineers to model algorithms, namely, activity diagrams and state diagrams. Both of these techniques utilize abstract pictures that depict selected aspects of algorithms. Both are part of the common diagramming standard notation known as the Unified Modeling Language (UML). Use case diagrams and class diagrams (see Chapter 4) are also a part of the UML standard notations. You should recall that algorithms consist of two main ingredients:

1. Instructions
2. Data

These ingredients explain the primary difference between activity diagrams and state diagrams: one is focused on instructions (activity) and the other begins with data (and ist state).