Tikz foreach loop and coordinate computation including scaling

Question

I am trying to plot a number of lines from a point on the segment connecting points from a given list of points to the coordinate origin (0,0). Since I want to automate this procedure, I am using \foreach. When I am trying to use the calc features to calculate the coordinates of the points I am running into an error.

Here is my code

\documentclass{standalone}

\usepackage{tikz}
\usetikzlibrary{calc}

\def\points{(-2,0), (-1.2, 1), (0, 1.55), (0.8, 1.65), (1.3, 1.4), (2, 0.1), (1.8, -0.7), (1,-1.6), (-1, -1)}

\begin{document}
\begin{tikzpicture}
  \foreach \p in \points{
    \draw[gray!80!white] \p--(0,0);
    \draw[gray, ->] \p--($\p!0.1!(0,0)$);}
\end{tikzpicture}
\end{document}

And here the error:

File ended while scanning use of \tikz@cc@parse@factor.

Note that this question is different from this one since I do not use the axis environment.

Note that you're writing also ...!(0.0)$), it should be ...!(0,0)$). — Alenanno, Jan 16 '18 at 13:56
A quick and durty hack is to add single ( at the begining, like this $(\p!0.1!(0,0)$ — Kpym, Jan 16 '18 at 20:27

score 3 · Accepted Answer · answered Jan 16 '18 at 13:54

Here are two possible alternatives. In the first one I changed the definition of the \points macro to not include (), and added those in the code as well. You then need {} instead of () in \points to delimit the entries in the "array".

The other version uses your definition of \points, but leaves out the coordinate calculation altogether, using instead a decoration to add the arrow tips.

\documentclass[border=5mm]{standalone}

\usepackage{tikz}
\usetikzlibrary{calc,decorations.markings}

\def\points{{-2,0}, {-1.2, 1}, {0, 1.55}, {0.8, 1.65}, {1.3, 1.4}, {2, 0.1}, {1.8, -0.7}, {1,-1.6}, {-1, -1}}

\begin{document}
\begin{tikzpicture}
  \foreach \p in \points{
    \draw[gray!80!white] (\p)--(0,0);
    \draw[gray, ->] (\p)--($(\p)!0.1!(0,0)$);
}
\end{tikzpicture}

% second version
\def\points{(-2,0), (-1.2, 1), (0, 1.55), (0.8, 1.65), (1.3, 1.4), (2, 0.1), (1.8, -0.7), (1,-1.6), (-1, -1)}

\begin{tikzpicture}[decoration={markings,mark=at position 0.1 with \arrow{>}}]
  \foreach \p in \points
    \draw[gray!80!white, postaction={decorate}] \p--(0,0);
\end{tikzpicture}
\end{document}

Is there a simple explanation why the original code did not work? Is it a mistake on my side or is there a bug in one of the packages? — Christian, Jan 16 '18 at 13:57
@Christian I would guess an expansion issue, that the \p in ($\p!0.1!(0,0)$) isn't expanded so that the parser doesn't "see" that coordinate, but I don't know. Which I wouldn't necessarily call a bug. I don't know how to fix it either. — Torbjørn T., Jan 16 '18 at 14:00
@Christian it has to see a naked ( to understand that it is a coordinate expression. — percusse, Jan 16 '18 at 14:13

Tikz foreach loop and coordinate computation including scaling

1 Answers1