Intentando cambiar un cuadrado a un círculo en el diagrama

Question

Estoy intentando hacer un diagrama de compartimentos 1D con círculos, pero cambiar el estilo tikz con círculo no lo cambia de cuadrado a círculo.

\documentclass[12pt]{report}
\usepackage{caption}
\usepackage{tikz}
\usetikzlibrary{arrows}

\begin{document}

\tikzstyle{int}=[draw, line width = 0.5mm, minimum size=5em]
\tikzstyle{make circle}=[circle, line width = 0.5 mm, minimum size=5em]

\begin{figure} 
\centering

\begin{tikzpicture}[node distance=3cm,auto,>=latex']

\node [int] (a) {};

%%%%%%%%%%%%%
\node [makecircle] (c) [] {$M$}; %% This is the problem (still makes a square)
%%%%%%%%%%%

\node [int] (d) [right of=c, node distance=4cm] {$E$};

\draw[->, ultra thick, blue] (c) edge node {$r(1-p)$} (d);

\draw [green, ultra thick, ->] 
    ([xshift=-0.25cm]a.north) to[out=120, in=60, distance=1.5cm] ([xshift=0.25cm]a.north);
\node [above=1.0cm,text=green] at (a.north) {$pr$};
\draw [red, ultra thick, ->] (0,-1) -- (0,-2);
\node [right=0.5cm,below=0.15cm,text=red] at (a.south) {$\approx 0$};

\end{tikzpicture}
\caption{This is a two compartment model}

\end{figure}

\end{document}

Answer 1

Dibujas el círculo con \node [int] (a) {};(aquí no hay ningún círculo definido) pero escribes el texto M con \node [makecircle] (c) [] {$M$};(aquí no hay draw) (escribiste makecircleen lugar de make circle).

Como make circlesolo agregaría el circle, eliminé el tikzstyle y reutilicé el intestilo.

\documentclass[12pt]{report}
\usepackage{caption}
\usepackage{tikz}
\usetikzlibrary{arrows}

\begin{document}

\tikzstyle{int}=[draw, line width = 0.5mm, minimum size=5em]

\begin{figure} 
\centering

\begin{tikzpicture}[node distance=3cm,auto,>=latex']
    \node[int, circle] (c) [] {$M$};

    \node [int] (d) [right of=c, node distance=4cm] {$E$};

    \draw[->, ultra thick, blue] (c) edge node {$r(1-p)$} (d);

    \draw [green, ultra thick, ->] 
        ([xshift=-0.25cm]c.north) to[out=120, in=60, distance=1.5cm] ([xshift=0.25cm]c.north);
    \node [above=1.0cm,text=green] at (c.north) {$pr$};
    \draw [red, ultra thick, ->] (0,-1) -- (0,-2);
    \node [right=0.5cm,below=0.15cm,text=red] at (c.south) {$\approx 0$};
\end{tikzpicture}
\caption{This is a two compartment model}

\end{figure}

\end{document}