How to draw finite state automata in TikZ?

Question

I am trying to draw the following diagram in tikz.

The code I am working on is:

\documentclass {article}
% example taken from 
% http://www.guitex.org/home/images/doc/GuideGuIT/introingtikz.pdf
\usepackage {tikz}
\usetikzlibrary {positioning}
%\usepackage {xcolor}
\definecolor {processblue}{cmyk}{0.96,0,0,0}
\begin {document}
\begin {center}
\begin {tikzpicture}[-latex ,auto ,node distance =4 cm and 5cm ,on grid ,
semithick ,
state/.style ={ very thick, circle ,top color =white ,  ,
draw , text=blue , minimum width =2 cm}]
%\node[state] (C)
%{$1$};
\draw[very thick] (0,0) circle (.8cm);
\node[state] (A)  {$q_0$};
\draw[very thick] (0,0) circle (.8cm);
\node[state] (B) [ right =of A] {$q_1$};
\path (A) edge [loop above] node[above] {$b$} (A);
\path (B) edge [loop above] node[above] {$b$} (B);
%\path (C) edge [bend left =25] node[below =0.15 cm] {$1/2$} (A);
%\path (A) edge [bend right = -15] node[below =0.15 cm] {$1/2$} (C);
\path (A) edge [bend left =25] node[above] {$a$} (B);
\path (B) edge [bend left =15] node[below =0.15 cm] {$a$} (A);
%\path (C) edge [bend left =15] node[below =0.15 cm] {$1/2$} (B);
%\path (B) edge [bend right = -25] node[below =0.15 cm] {$1/2$} (C);
\end{tikzpicture}
\end{center}
\end{document}

How to make the curved arrow head on the left and draw the rightarrow to q_0?

Answer 1

Here is my attempt:

\documentclass[border=0.2cm]{standalone}
% required packages and libraries
\usepackage{tikz}
\usetikzlibrary{automata, positioning}
\begin{document}
\begin{tikzpicture} [draw=cyan!70!black,
    node distance = 3cm, 
    on grid, 
    auto,
    every initial by arrow/.style = {thick}]
% State q0 
\node (q0) [state, 
    initial, 
    accepting, 
    initial text = {}] {$q_0$};
% State q1

\node (q1) [state,
    right = of q0] {$q_1$};
% Arrows
\path [-stealth, thick]
     (q0) edge[bend left] node {$a$}   (q1)
     (q1) edge[bend left] node {$b$}   (q0)
     (q0) edge [loop above]  node {b}()
     (q1) edge [loop above]  node {b}();
\end{tikzpicture}
\end{document}

which yields the following result: For more details, you can check this step by step tutorial: Automata diagrams in LaTeX

Answer 2

With use of the arrows.meta, automata, positioning and quotes library:

\documentclass[border=3mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{arrows.meta, automata,
                positioning,
                quotes}
\begin{document}
    \begin{tikzpicture}[auto,
            > = Stealth,
every edge quotes/.style = {font=\footnotesize}, % if you like to have smaller edge labels 
every edge/.append style = {->, draw=cyan, thick},
every loop/.append style = {<-, looseness = 12},
node distance = 22mm,
 state/.style = {circle, semithick, draw=cyan, text=cyan, minimum size=1.2em},
 initial text = ,
                    ]
\node (A) [state, initial, accepting]   {$q_0$};
\node (B) [state, right=of A]           {$q_1$};
\path   (A) edge [loop above, "$b$"]   ()
        (A) edge [bend left,"$a$"]     (B)
        (B) edge [bend left,"$a$"]     (A)
        (B) edge [loop above, "$b$"]   ();
    \end{tikzpicture}
\end{document}

Answer 3

Since you tagged your question tikz-cd, here's a solution via the TikZ-CD package.

Further more, it also uses the automata library for various styles and keys it provides as well as the bending library so that curved arrows look better.

I define Loop … styles with a capital L that have the angles swapped from the normal loop … styles. The initial by arrow key is replaced with the initial by arrow wo text key inside a TikZ-CD with the automata option as that sets everything up.

Code

\documentclass[tikz]{standalone}
\usetikzlibrary{cd, automata, bending}
\definecolor{processblue}{cmyk}{0.96,0,0,0}
\makeatletter
\tikzset{
  Loop/.style={anchor={180+#1}, out={#1-15}, in={#1+15}, loop},
  /utils/temp/.style args={#1=#2}{Loop #1/.style={Loop={#2}}, commutative diagrams/Loop #1/.style={/tikz/Loop #1, labels={auto=false}}},
  /utils/temp/.list={above=90, right=0, left=180, below=270, above right=45, above left=135, below left=235, below right=315},
  initial by arrow wo text/.code={\arrow[double=none, every initial by arrow, to path={([shift=(\tikz@initial@angle:\tikz@initial@distance)]\tikztostart.\tikz@initial@angle) -- (\tikztostart)}]}}
\makeatother
\tikzcdset{
  automata/.style={
    /tikz/initial/.style={initial by arrow wo text},
    /tikz/accepting by double/.style={double,outer sep=\pgflinewidth+.3pt},
    cells={nodes={state, draw=processblue}},
    labels={math mode=false, node font=\footnotesize\ttfamily},
    arrows={thick, color=processblue, text=black, /tikz/>={Latex[angle'=25]}, ->}}}
\begin{document}
\begin{tikzcd}[automata]
|[initial, accepting]|
   q_0 \ar[Loop above, "b"]
       \ar[r, bend left,  "a"]
 & q_1 \ar[Loop above, "b"]
       \ar[l, bend left, "a"]
\end{tikzcd}
\begin{tikzcd}[automata, bend angle=10, text depth=+0pt, /utils/exec=\def\c{{\normalfont, }}]
 & |[accepting, initial]| 1, 2 \ar[r,   "a"]   \ar[dr,             "b"]
 &                        2    \ar[d,   "b"]   \ar[    Loop right, "a"] \\
 &                        4    \ar[u,   "a\c b"']
 & |[accepting]|          3    \ar[l,   "a"]   \ar[d,              "b"] \\
2, 4                           \ar[uur, "a"]   \ar[dr, bend left,  "b"]
 & & |[accepting]|        2, 3 \ar[ll,  "a"]   \ar[    Loop right, "b"] \\
 & |[accepting]|       1, 2, 3 \ar[ul, bend left, "a"] \ar[ur,     "b"']
\end{tikzcd}
\end{document}

Output