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    <p>I would model the state machine explicitly:</p> <pre><code>def StateMachine(): state = 'A' while True: print(state) input = (yield) if state == 'A': if input == 'a': state = 'B' elif input == 'b': state = 'C' else: break elif state == 'B': if input == 'a': state = 'C' elif input == 'b': state = 'A' else: break elif state == 'C': if input == 'a': state = 'A' elif input == 'b': state = 'B' else: break else: break </code></pre> <p>This should translate very neatly to C++ using nested <code>switch</code> statements or a state transition table.</p> <p>If you prefer an implicit model, you need a way to handle the cycles in your state machine. In C or C++, this would probably end up being <code>goto</code>. I wouldn't recommend this approach, but if you're more comfortable with it than explicit state, here's what it might look like:</p> <pre><code>#include &lt;stdio.h&gt; #define start(state) switch(state) { case 0:; #define finish default:;} #define yield(state, value) do { state = __LINE__; return (value); case __LINE__:; } while(0) struct coroutine { int state; }; int run(struct coroutine *c, char input) { start(c-&gt;state) { A: printf("Entered state A\n"); yield(c-&gt;state, 1); if(input == 'a') goto B; if(input == 'b') goto C; B: printf("Entered state B\n"); yield(c-&gt;state, 1); if(input == 'a') goto C; if(input == 'b') goto A; C: printf("Entered state C\n"); yield(c-&gt;state, 1); if(input == 'a') goto A; if(input == 'b') goto B; } finish; return 0; } int main(void) { int a; struct coroutine c = {0}; while((a = getchar()) != EOF &amp;&amp; run(&amp;c, a)); return 0; } </code></pre>
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    1. POImplementing a finite state machine with a single coroutine
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    1. POImplementing a finite state machine with a single coroutine
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    1. COHaving explicit states is always an option, but is something I'm trying to avoid.
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    2. COI think the problem you're going to have is that your trying to represent a cyclic graph as a tree. That's only going to be possible if you allow a way to re-enter the tree; that's going to take either something like a `goto` into the middle of the tree or a variable to save the state so that the tree can be re-entered at the top.
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