Changes between Initial Version and Version 1 of ControlSystems/SoftwareTeam/Training/GettingStarted/StateMachines1

Timestamp:

Nov 6, 2019, 11:49:33 AM (6 years ago)

Author:

David Albert

Comment:

--

ControlSystems/SoftwareTeam/Training/GettingStarted/StateMachines1

@@ +1 @@
+Consider a program that must drive a robot forward some distance and then turn the robot 90-degrees to face a target.  As the program runs, it spends time in at least two states:
+* Driving Forward
+* Turning
+It may remain in each state for a considerable amount of time.  A useful way to model this in software is using a '''State Machine'''.
+
+As the name implies, a State Machine is a program that has stateful behavior, meaning that what it does at any given moment depends on its current state.  We could model the above sequence of states with this program:
+{{{
+public class StateMachine {
+    private enum State { START, FORWARD, TURN, STOP };
+
+    public static void main(String args[]) {
+        State state = State.START;
+        while (true) {
+            switch (state) {
+                case START:
+                    System.out.println("START->FORWARD...");
+                    state = State.FORWARD;
+                    break;
+                case FORWARD:
+                    System.out.println("FORWARD->TURN...");
+                    state = State.TURN;
+                    break;
+                case TURN:
+                    System.out.println("TURN->STOP...");
+                    state = State.STOP;
+                    break;
+                case STOP:
+                default:
+                    // finished..nothing to do                
+            }
+        }
+    }
+}
+}}}
+
+This program introduces a new type of variable called an enumeration ('''enum''').  An enum is a variable that can hold one of the values you specify when you declare it.  For example: {{{ enum Color { RED, GREEN, BLUE }; }}}  In this example, we've enumerated the states the robot can be in: starting, driving forward, turning, and stopped.
+
+The program also introduces the '''switch''' statement which is another type of conditional like ''if ,else if, else''.  With a switch statement, the condition is the value of a variable; execution jumps to the ''case'' that matches the variable's value and proceeds forward from there until a ''break'' statement is encountered which exits the switch block.  If there is no case statement matching the variable's value, the '''default''' case is executed.
+
+Run the program in the debugger and observe how the state variable and switch statement interact.
+
+Notice that:
+* The current state is stored in a ''state variable'' named '''state''' in this program.
+* The case matching the current state is run each time we go through the loop
+* The program can stay in any state for as long as needed and explicitly advances to the next state when it's ready.