Question

In what areas of programming would I use state machines ? Why ? How could I implement one ?

EDIT: please provide a practical example , if it’s not too much to ask .

Answer 1

In what areas of programming would I use a state machine?

Use a state machine to represent a (real or logical) object that can exist in a limited number of conditions (‘states‘) and progresses from one state to the next according to a fixed set of rules.

Why would I use a state machine?

A state machine is often a very compact way to represent a set of complex rules and conditions, and to process various inputs. You’ll see state machines in embedded devices that have limited memory. Implemented well, a state machine is self-documenting because each logical state represents a physical condition. A state machine can be embodied in a tiny amount of code in comparison to its procedural equivalent and runs extremely efficiently. Moreover, the rules that govern state changes can often be stored as data in a table, providing a compact representation that can be easily maintained.

How can I implement one?

Trivial example:

enum states {      // Define the states in the state machine.   NO_PIZZA,        // Exit state machine.   COUNT_PEOPLE,    // Ask user for # of people.   COUNT_SLICES,    // Ask user for # slices.   SERVE_PIZZA,     // Validate and serve.   EAT_PIZZA        // Task is complete. } STATE;  STATE state = COUNT_PEOPLE; int nPeople, nSlices, nSlicesPerPerson;  // Serve slices of pizza to people, so that each person gets /// the same number of slices.    while (state != NO_PIZZA)  {    switch (state)  {    case COUNT_PEOPLE:          if (promptForPeople(&nPeople))  // If input is valid..            state = COUNT_SLICES;       // .. go to next state..        break;                          // .. else remain in this state.    case COUNT_SLICES:          if (promptForSlices(&nSlices))           state = SERVE_PIZZA;         break;    case SERVE_PIZZA:        if (nSlices % nPeople != 0)    // Can't divide the pizza evenly.        {                                         getMorePizzaOrFriends();   // Do something about it.            state = COUNT_PEOPLE;      // Start over.        }        else        {            nSlicesPerPerson = nSlices/nPeople;            state = EAT_PIZZA;        }        break;    case EAT_PIZZA:        // etc...        state = NO_PIZZA;  // Exit the state machine.        break;    } // switch } // while 

Notes:

• The example uses a switch() with explicit case/break states for simplicity. In practice, a case will often ‘fall through’ to the next state.

• For ease of maintaining a large state machine, the work done in each case can be encapsulated in a ‘worker’ function. Get any input at the top of the while(), pass it to the worker function, and check the return value of the worker to compute the next state.

• For compactness, the entire switch() can be replaced with an array of function pointers. Each state is embodied by a function whose return value is a pointer to the next state. Warning: This can either simplify the state machine or render it totally unmaintainable, so consider the implementation carefully!

• An embedded device may be implemented as a state machine that exits only on a catastrophic error, after which it performs a hard reset and re-enters the state machine.