Building Your First Finite Automaton


The Editor Window
Creating States
Defining Initial and Final States
Creating Transitions
Deleting States and Transitions
Running the Finite Automaton on Multiple Strings
Building a Nondeterministic Finite Automaton
Highlighting Nondeterministic States
Running Input on a Nondeterminisitc Finite Automaton



JFLAP defines a finite automaton (FA) M as the quintuple M = (Q, Σ, δ, qs, F) where

Q is a finite set of states {qi | i is a nonnegative integer}
Σ is the finite input alphabet
δ is the transition function, δ : D 2Q where D is a finite subset of Q × Σ*
qs (is member of Q) is the initial state
F (is a subset of Q) is the set of final states

Note that this definition includes both deterministic finite automata (DFAs), which we will be discussing shortly, and nondeterministic finite automata (NFAs), which we will touch on later.

Building the different types of automata in JFLAP is fairly similar, so let's start by building a DFA for the language L = {ambn : m 0, n > 0, n is odd}. That is, we will build a DFA that recognizes that language of any number of a's followed by any odd number of b's. (Examples taken from JFLAP: An Interactive Formal Languages and Automata Package by Susan Rodger and Thomas Finley.)

The Editor Window

To start a new FA, start JFLAP and click the Finite Automaton option from the menu.

Starting a new FA

This should bring up a new window that allows you to create and edit an FA. The editor is divided into two basic areas: the canvas, which you can construct your automaton on, and the toolbar, which holds the tools you need to construct your automaton.

The editor window

Let's take a closer look at the toolbar.

The FA toolbar

As you can see, the toolbar holds four tools:

To select a tool, click on the corresponding icon with your mouse. When a tool is selected, it is shaded, as the Attribute Editor tool is above. Selecting the tool puts you in the corresponding mode. For instance, with the toolbar above, we are now in the Attribute Editor mode.

The different modes dictate the way mouse clicks affect the machine. For example, if we are in the State Creator mode, clicking on the canvas will create new states. These modes will be described in more detail shortly.

Now let's start creating our FA.

Creating States

First, let's create several states. To do so we need to activate that State Creator tool by clicking the button on the toolbar. Next, click on the canvas in different locations to create states. We are not very sure how many states we will need, so we created four states. Your editor window should look something like this:

States created

Now that we have created our states, let's define initial and final state.

Defining Initial and Final States

Arbitrarily, we decide that q0 will be our initial state. To define it to be our initial state, first select the Attribute Editor tool on the toolbar. Now that we are in Attribute Editor mode, right-click on q0. This should give us a pop-up menu that looks like this:

The state menu

From the pop-up menu, select the checkbox Initial. A white arrowhead appreas to the left of q0 to indicate that it is the inital state.

q0 defined as initial state

Next, let's create a final state. Arbitrarily, we select q1 as our final state. To define it as the final state, right-click on the state and click the checkbox Final. It will have a double outline, indicating that it is the final state.

q1 defined as final state

Now that we have defined initial and final states, let's move on to creating transtitions.

Creating Transitions

We know strings in our language can start with a's, so, the initial state must have an outgoing transition on a. We also know that it can start with any number of a's, which means that the FA should be in the same state after processing input of any number of a's. Thus, the outgoing transition on a from q0 loops back to itself.

To create such a transition, first select the Transition Creator tool from the toolbar. Next, click on q0 on the canvas. A text box should appear over the state:

Creating a transition

Note that λ, reprsenting the empty string, is initially filled in for you. If you prefer ε representing the empty string, select Preferences : Preferences in the main menu to change the symbol representing the empty string.

Type "a" in the text box and press Enter. If the text box isn't selected, press Tab to select it, then enter "a". When you are done, it should look like this:

Transition created

Next, we know that strings in our language must end with a odd number of b's. Thus, we know that the outgoing transtion on b from q0 must be to a final state, as a string ending with one b should be accepted. To create a transition from our initial state q0 to our final state q1, first ensure that the Transition Creator tool is selected on the toolbar. Next, click and hold on q0, and drag the mouse to q1 and release the mouse button. Enter "b" in the textbox the same way you entered "a" for the previous transition. The transition between two states should look like this:

Second transition created

Lastly, we know that only strings that end with an odd number of b's should be accepted. Thus, we know that q1 has an outgoing transition on b, that it cannot loop back to q1. There are two options for the transtion: it can either go to the initial state q0, or to a brand new state, say, q2.

If the transition on b was to the initial state q0, strings would not have to be of the form ambn; strings such as ababab would also be accepted. Thus, the transition cannot be to q0, and it must be to q2.

Create a transition on b from q1 to q2. As the FA should accept strings that end with an odd number of b's, create another transition on b from q2 to q1. Your FA is now a full, working FA! It should look something like this:

The working FA

You might notice that the q3 is not used and can be deleted. Next, we will describe how to delete states and transitions.

Deleting States and Transitions

To delete q3, first select the Deletor tool on the toolbar. Next, click on the state q3. Your editor window should now look something like this:

q3 deleted

Similarly, to delete a transition, simply click on the input symbol of the transition when in Deletor mode.

Your FA, faex.jff, is now complete.

Running the FA on Multiple Strings

Now that you've completed your FA, you might want to test it to see if it really accepts strings from the language. To do so, select Input : Multiple Run from the menu bar.

Starting a multiple run tab

A new tab will appear displaying the automaton on the left pane, and an input table on the right:

A new multiple run tab

To enter the input strings, click on the first row in the Input column and type in the string. Press Enter to continue to the next input string. When you are done, click Run Inputs to test your FA on all the input strings. The results, Accept or Reject are displayed in the Result column. You can also load the inputs from file delimited by white space. Simply click on Load Inputs and load the file to add additional input strings into multi-run pane.

Running multiple inputs

Clicking Clear deletes all the input strings, while Enter Lambda enters the empty string at the cursor. View Trace brings up a separate window that shows the trace of the slected input. To return to the Editor window, select File : Dismiss Tab from the menu bar.

Building a Nondeterministic Finite Automaton

Building a nondeterministic finite automaton (NFA) is very much like building a DFA. However, an NFA is different from a DFA in that it satisfies one of two conditions. Firstly, if the FA has two transitions from the same state that read the same symbol, the FA is considered an NFA. Secondly, if the FA has any transitions that read the empty string for input, it is also considered an NFA.

Let's take a look at this NFA, which can be accessed through ex1.3a.jff: (Note: This example taken from JFLAP: An Interactive Formal Languages and Automata Package by Susan Rodger and Thomas Finley.)


We can immediately tell that this is an NFA because of the four λ-transitions coming from q3 and q9, but we might not be sure if we have spotted all the nondeterministic states. JFLAP can help with that.

Highlighting Nondeterministic States

To see all the nondeterministic states in the NFA, select Test : Highlight Nondeterminism from the menu bar:

Highlighting nondeterministic states

A new tab will appear with the nondeterministic states shaded a darker color:

Nondeterministic states highlighted

As we can see, q3 and q9 are indeed nondeterministic because of their outgoing λ-transitions. Note that they would both be nondeterministic even if they each had one λ-transition instead of two: only one λ-transition is needed to make a state nondeterministic. We also see that q1 is nondeterministic because two of its outgoing transitions are on the same symbol, a. Next, we will go through JFLAP's tools for running input on an NFA.

Running Input on an NFA

To step through input on an NFA, select Input : Step with Closure... from the menu bar. A dialog box prompting you for input will appear. Ordinarily, you would enter the input you wish to step through here. For now, type "aaaabb" in the dialog box and press Enter. You can also load the input file instead of typing the string.

NOTE : When loading input from the file, JFLAP determines end of input string by the white space. Thus if there is string “ab cd” in a file, only “ab” will be considered as an input (“cd” will be ignored since there is a white space before them).

Entering input

A new tab will appear displaying the automaton at the top of the window, and configurations at the bottom. The current state is shaded.

A new input tab

First, let's take a closer look at a configuration:

A configuration icon

The configuration icon shows the current state of the configuration in the top left hand corner, and input on the white tape below. The processed input is displayed in gray, and the unprocessed input is black.

Click Step to process the next symbol of input. You will notice q1 becomes the shaded state in the NFA, and that the configuration icon changes, reflecting the fact that the first a has been processed. Click Step again to process the next a. You will find that four states are shaded instead of one, and there are four configurations instead of one.

aa processed

This is because the machine is nondeterministic. From q1, the NFA took both a transitions to q2 and q9. As q9 has two λ-transitions (which do not need input), the NFA further produced two more configurations by taking those transitions. Thus, the simulator now has four configurations. Click Step again to process the next input symbol.

aaa processed

Notice that two of the configurations are highlighted red, indicating they were rejected. Looking at their input, we also know that only aa was processed. What happened?

Producing a Trace

To select a configuration, click on it. It will become a solid color when selected, instead of the slightly graded color. Click on the icon for the rejected configuration with state q11, and click Trace. A new widow will appear showing the traceback of that configuration:

A configuration's traceback

The traceback shows the configuration after processing each input symbol. From the traceback, we can tell that that configuration started at q0 and took the transition to q1 after processing the first a. After processing the second a, it was in q11. Although q11 is not adjacent to q1, it can be reached by taking a λ-transition from q9. As the simulator tried to process the next a on this configuration, it realized that there are no outgoing a transitions from q11 and thus rejected the configuration.

Although rejected configurations will remove themselves in the next step, we can also remove configurations that have not been rejected.

Removing Configurations

Looking at the tracebacks of the rejected configurations, we can tell that any configurations that are in q11 or q6 and whose next input symbol is a will be rejected.

As the next input symbol is a, we can tell that the configurations that are currently in q6 and q11 will be rejected. Click once on each of the four configurations to select them, then click Remove. The simulator will no longer step these configurations. (Although we are only removing configurations that are about to be rejected, we can remove any configurations for any purpose, and the simulator will stop stepping through input on those configurations.)

Your simulator should now look something like this:

Rejected configurations removed

Now when we step the simulator, the two configurations will be stepped through.

Looking at the two configurations above, we might realize that the configuration on q3 will not lead to an accepting configuration. We can test our idea out by freezing the other configuration.

Freezing and Thawing configurations

To freeze the configuration on q10, click on q10 once, then click the Freeze button. When a configuration is frozen, it will be tinted a darker shade of purple:

Configuration on q10 frozen

With that configuration frozen, as you click Step to step through the configuration on q3, the frozen configuration remains the same. Clicking Step two more times will reveal that the configuration on q3 is not accepted either. Your simulator will now look like this:

Other configurations rejected

To proceed with the frozen configuration, select it and click Thaw. The simulator will now step through input as usual. Click Step another three times to find an accepting configuration. An accepting configuration is colored green:

Accepting configuration found

If we click Step again, we will see that the last configuration is rejected. Thus, there is only one accepting configuration. However, we might be unsure that this is really the case, as we had removed some configurations. We can double-check by resetting the simulator.

Resetting the simulator

At any point in the simulation, we can restart the entire simulation process by clicking Reset. This will clear all the current configurations and restart the simulation. If we click Reset and step all the configurations, we will find that there is, indeed, only one accepting configuration.

This concludes the walkthrough, although there is an appendix noting a few more features that JFLAP supports.



To add a note to a JFLAP file, choose the Attribute Editor, right click and select “Add Note”. The note will start with the message "insert_text". To change the text simply click in the note, select where you want to start typing, and type your note. Click outside the note to get rid of the cursor. Click and drag the note to move it.



To select more than one state or block at once, choose the attribute editor, click on empty space, and drag the mouse. A bounding box appears and all states and blocks within the box are selected, their color now blue. To move the selected states as a group, click and drag any of them. To deselect them, click anywhere else.

Layout Commands (as of JFLAP version 6.2)

JFLAP will now let you apply predefined graph layout commands to your graph, which can help with a more aesthetically pleasing graph. There is a new menu in the automaton editor window, the “View” menu, which will allow one to both save the current graph layout and to apply different graph layout commands and algorithms. For a full tutorial on how to use these features, and to see a description of the built-in layout commands, feel free to read the layout command tutorial.

The following are pictures of the finite automaton used earlier, ex1.3a.jff, with new graph layouts. The first picture demonstrates the automaton under a “GEM” layout algorithm, the second under a “Tree (Degree, Vertical)” layout algorithm, and the last under a “Two Circle” layout algorithm.

This concludes our brief tutorial on building finite automata. Thanks for reading!