How do I draw a DFA diagram when I only care about how the string ends?

Asked Mar 26 '18 at 15:43

Active Mar 26 '18 at 15:43

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Hi so this is my first week learning about automata and I have an exercise I'm going through. I'm supposed to draw a diagram for the DFA that accepts all strings of a and b where the third last symbol is a b. So it's like : $\{a,b\}^*b\{a,b\}^2$ right?

And if I would draw it I would do this to check the end:

I believe this to be correct but I'm not sure how to draw the possibilites of a's and b's beforehand

asked Mar 26 '18 at 15:43

JohnDoe

It is incorrect. Consider $bbbb$. If you are looking for the correct DFA, does this answer solve your problem? – xskxzr Mar 26 '18 at 16:41
@xskxzr ah that's right... No I don't really understand the answer at that link and it doesn't look familiar to anything I find in the chapters of the course book. – JohnDoe Mar 26 '18 at 16:58
There are some hints for this problem in Designing a DFA that accepts strings such that nth character from last satisfies condition. – roctothorpe Mar 26 '18 at 19:23
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Possible duplicate of Designing a DFA that accepts strings such that nth character from last satisfies condition – xskxzr Mar 27 '18 at 15:59

How do I draw a DFA diagram when I only care about how the string ends?

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