Tikz obtain node coordinates

Question

In tikzpicture, how do I get a single coordinate of a node?

This is the circuit I'm making, and I need to know the x coordinate of the node (opamp.out) so that i can make R2 vertically aligned with that node, and link it with a vertical line. This is my code for R2:

\draw (opamp.-) to [short,*-] ++(0,1) node{} to [R,l=$R_2$] ++(2,0);

where, instead of ++(2,0), I would like to input ++(opamp.out.x-opamp.-.x,0);

Answer 1

An alternative: use of orthogonal coordinate

(p -| q) refers to the point horizontally aligned with p and vertically aligned with q. The -| was chosen in such a way that it's easy to remember which is which; the - side is horizontal, and the | side is vertical. The CircuiTikz manual has further information.

Complete Examples:

Code:

\documentclass[border=10pt,varwidth]{standalone}  
\usepackage{tikz}
\usepackage[american,siunitx]{circuitikz}
\usetikzlibrary{calc,positioning}
\begin{document}

An alternative solution

\begin{circuitikz}  
\draw
(0,0) node[op amp](opamp){}
(opamp.out) to[short,*-o] (2,0)node[]{} node[right]{$v_{out}$};
\draw (opamp.-) -- ++(-0.2,0) to[R,l_=$R_1$] ++ (-2,0)node[ground,rotate=-90]{};
\draw (opamp.+) to[short,-o] ++ (-0.5,0)node[left]{$V_{in}$};

\draw (opamp.-) to [short,*-] ++(0,1) node{} to [R,l=$R_2$] ([yshift=1cm]opamp.- -|opamp.out);
\end{circuitikz}

\medskip

@Astrinus' solution

\begin{circuitikz}
\draw
(0,0) node[op amp](opamp){}
(opamp.out) to[short,*-o] (2,0)node[]{} node[right]{$v_{out}$};

\draw (opamp.-) -- ++(-0.2,0) to[R,l_=$R_1$] ++ (-2,0)node[ground,rotate=-90]{};
\draw (opamp.+) to[short,-o] ++ (-0.5,0)node[left]{$V_{in}$};

\draw let \p1=(opamp.out),\p2=(opamp.-) in (opamp.-) to [short,*-] ++(0,1) 
node{} to [R,l=$R_2$] ++($(\x1,0)-(\x2,0)$);
\end{circuitikz}

\end{document}

Answer 2

\draw let \p1=(opamp.out),\p2=(opamp.-) in (opamp.-) to [short,*-] ++(0,1) node{} to [R,l=$R_2$] ++($(\x1,0)-(\x2,0)$);

Note that you have to \usetikzlibrary{calc}.

The let syntax let you alias the points with names \p1 ... \pN, where N is the number of points. Then, after in, you could access points' coordinates with the syntax \x1 .. \xN and \y1 .. \yN (I think it's possible to access also polar coordinates but honestly I don't remember the syntax :-8), matching the order of \p assignments.

The library calc let you make calculations with coordinates and get the resulting coordinates, with the syntax ($<calculations>$).

Look at pgfmanual.pdf or at "TikZ pour l'impatient" if you read french (I can't speak french at all, but since my mother language is neo latin I had absolutely no problem reading it).