Global Coordinates

Question

How can I declare coordinates in a block and access them from outside of the block?

\begin{tikzpicture}
  \foreach \x / \y in {
    1 /  3,
    2 /  8,
    3 / 14,
  }{
    \coordinate (N\x) at (\x,\y);
    \path[draw] (N\x) circle (1em);      %% ok
  };
  \path[draw] (N1) -- (N3);              %% not ok
\end{tikzpicture}

Answer 1

Remove the spaces between your coordinates. i.e.

\foreach \x / \y in {1/3,2/8,3/14}

or

\foreach \x / \y in {
   1/3,
   2/8,
   3/14%
}

should work.

NOTE

As Andrew pointed out, the % is necessary in the second code block to avoid additional spaces etc. getting appended to the intended 14. This is not necessary in the first code block as the 14 is followed immediately by a }. The first code block is probably the best way to do this for short lists.

The original code from the question uses a comma where the % is in the second block above, which causes the foreach to do an additional unwanted loop iteration with no content. This results in the creation of a coordinate called N at (1,1), which is neither wanted nor desirable.

Answer 2

As it is written in others answers, it's not a problem of global coordinatesbut a problem with wrong names of coordinates because blank are dangerous in the arguments of \foreach

In this special case, would be more concise : ( but in the general case, the line \coordinate (N\x) at (\x,\y);can be avoided)

\documentclass{scrartcl}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
  \foreach \y [count=\x from 1] in {3,8,14}
      \draw   (\x,\y) coordinate (N\x) circle (1em);
  \draw   (N1) -- (N3); 
\end{tikzpicture} 

\end{document}

Answer 3

You can do this without modification. The problem is that your coordinates are not named N1, N2, and N3 due to spurious spaces. Change it to:

\begin{tikzpicture}
  \foreach \x / \y in {1/3,2/8,3/14%
  }{
    \coordinate (N\x) at (\x,\y);
    \path[draw] (N\x) circle (1em);      %% ok
  };
  \path[draw] (N1) -- (N3);
\end{tikzpicture}

and you are fine.

As Andrew already mentioned as a comment to your question. The last , has to go. It runs through the loop again with \x and \y empty. This does actually draw an extra circle at (1,1) and defines the coordinate N. I am not completely sure why it gets drawn at (1,1) to be honest. I would expect it to default to (0,0) instead. Does anybody have an explanation for this?

Edit: I think I know why it occurs at (1,1). The coordinate system has a value for x and y and when you say something like (3,7) this means take 3 steps of size x in the x direction and 7 steps of size y in the y direction. This allows to write coordinates without units and change the 'scale' afterwards. These values for x and y default to 1 cm and not 0 cm such that writing (3,7) becomes (3 cm, 7 cm) instead of (0 cm, 0 cm) which would be far less usefull. When you leave them empty, the defaults for x and y are used.