Drawing a quarter closed contour in tikz

I am trying to draw the below:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}

\begin{document}

\begin{tikzpicture}[decoration={markings,
mark=at position 0.5cm with {\arrow[line width=1pt]{>}},
mark=at position 2cm with {\arrow[line width=1pt]{>}},
mark=at position 7.85cm with {\arrow[line width=1pt]{>}},
mark=at position 9cm with {\arrow[line width=1pt]{>}}
}
]
% The axes
\draw[help lines,->] (-3,0) -- (3,0) coordinate (xaxis);
\draw[help lines,->] (0,-1) -- (0,3) coordinate (yaxis);

% The path
\path[draw,line width=0.8pt,postaction=decorate] (1,0) node[below] {$\varepsilon$} -- (2,0) node[below] {$r$} arc (0:180:2) -- (-1,0) arc (180:0:1);

% The labels
\node[below] at (xaxis) {$x$};
\node[left] at (yaxis) {$y$};
\node[below left] {$O$};
\node at (0.5,1.2) {$C_{\varepsilon}$};
\node at (1.5,1.8) {$C_{r}$};
\end{tikzpicture}

\end{document}


Except with only half of the half disc, in other words exactly this path but with half the area, slicing the half after the imaginary axis. I can't seem to get the arguments of \path to change without destroying the shape. Sorry if this is very basic, I am new to TiKz.

• try changing the path to \path[draw,line width=0.8pt,postaction=decorate] (1,0) node[below] {$\varepsilon$} -- (2,0) node[below] {$r$} arc (0:90:2) -- (0,1) arc (90:0:1); – AJN Apr 27 '16 at 6:57
• what do you mean by after the imaginary axis? To delete the section to the left or to the right? – AJN Apr 27 '16 at 7:01
• Delete the section to the left-sorry – qbert Apr 27 '16 at 13:31

The arc is a full semi circle since you are drawing from 0deg to 180deg (0:180:2 i.e., 0deg to 180deg with radius 2). To make it a quarter circle, draw it from 0 deg to 90 degree (0:90:2 i.e., 0deg to 90deg with radius 2).

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}

\begin{document}

\begin{tikzpicture}[decoration={markings,
mark=at position 0.5cm with {\arrow[line width=1pt]{>}},
mark=at position 2cm with {\arrow[line width=1pt]{>}},
mark=at position 7.85cm with {\arrow[line width=1pt]{>}},
mark=at position 9cm with {\arrow[line width=1pt]{>}}
}
]
% The axes
\draw[help lines,->] (-3,0) -- (3,0) coordinate (xaxis);
\draw[help lines,->] (0,-1) -- (0,3) coordinate (yaxis);


changed lines

    % The path
\path[draw,line width=0.8pt,postaction=decorate]
(1,0) node[below] {$\varepsilon$} --
(2,0) node[below] {$r$}
arc (0:90:2) --
(0,1) arc (90:0:1);


rest is unchanged

    % The labels
\node[below] at (xaxis) {$x$};
\node[left] at (yaxis) {$y$};
\node[below left] {$O$};
\node at (0.5,1.2) {$C_{\varepsilon}$};
\node at (1.5,1.8) {$C_{r}$};
\end{tikzpicture}

\end{document}


or alternatively

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}

\begin{document}

\begin{tikzpicture}[decoration={markings,
mark=at position 0.5cm with {\arrow[line width=1pt]{>}},
mark=at position 2.8cm with {\arrow[line width=1pt]{>}},
mark=at position 4.75cm with {\arrow[line width=1pt]{>}},
mark=at position 5.8cm with {\arrow[line width=1pt]{>}}
}
]
% The axes
\draw[help lines,->] (-3,0) -- (1,0) coordinate (xaxis);
\draw[help lines,->] (0,-1) -- (0,3) coordinate (yaxis);

% The path
\path[draw,line width=0.8pt,postaction=decorate]
(0,1) node[right] {$\varepsilon$} --
(0,2) node[right] {$r$}
arc (90:180:2) --
(-1,0)
arc (180:90:1);
\end{tikzpicture}

\end{document}


• ahhhhh the first argument in parentheses is also degrees, this is what was tripping me up. – qbert Apr 27 '16 at 19:45

Or maybe you want to get

Code:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}

\begin{document}

\begin{tikzpicture}[decoration={markings,
mark=at position 0.5cm with {\arrow[line width=1pt]{>}},
mark=at position 2cm with {\arrow[line width=1pt]{>}},
mark=at position 4.75cm with {\arrow[line width=1pt]{>}},% <- changed
mark=at position 6cm with {\arrow[line width=1pt]{>}}% <-changed
}
]
% The axes
\draw[help lines,->] (-3,0) -- (3,0) coordinate (xaxis);
\draw[help lines,->] (0,-1) -- (0,3) coordinate (yaxis);

% The path
\newcommand\qangle{45}
\path[draw,line width=0.8pt,postaction=decorate]
(\qangle:1)coordinate(e)
-- ([turn]0:1)coordinate(r)
arc (\qangle:{\qangle+90}:2)coordinate[midway](Cr)
-- ([turn]90:1)
arc ({\qangle+90}:\qangle:1)coordinate[midway](Ce)
;

% The labels
\node[below] at (xaxis) {$x$};
\node[left] at (yaxis) {$y$};
\node[below left] {$O$};
\node[below right]at(e){$\varepsilon$};
\node[below right]at(r) {$r$};
\node[above right]at(Cr){$C_{r}$};
\node[above right]at(Ce){$C_{\varepsilon}$};
\end{tikzpicture}
\end{document}


It is also possible to use another value for \qangle like 0 or 90.

With \newcommand\qangle{0} and \node[below]at(e){$\varepsilon$};\node[below]at(r) {$r$}; you get