How to do length calculations with based on a `\foreach` array

Question

I'm trying to improve the readability of my \drawHoshis macro by creating variables for the x and y coordinates, besides also running a calculation with a minus inside parentheses (e.g. x * (y - 1)). I don't know why but I just can't seem to make TeX work with it. I think it's something related to the array from the TikZ/PGF \foreach:

\foreach \sloc in {{3, 3}, {3, 7}, {7, 3}, {7, 7}, {5, 5}}{
  % This is what I'm tryig to improve:
  \filldraw (#3 * {\sloc}[0] - #3, #3 * {\sloc}[1] - #3)
    circle [radius=#3 / 10];
}

I'm trying to make it look more like this:

\newlength{\hoshiCoordX}
\newlength{\hoshiCoordY}
% Parameters
%
% 1: dimension (in cm)
% 2: board size (square)
% 3: step
% 
% Example: A 19x19 board with size 10cm x 10cm: `\drawHoshis{10}{19}{\step}'
\newcommand{\drawHoshis}[3]{
  \ifthenelse{#2 = 9} { 
    \foreach \sloc in {{3, 3}, {3, 7}, {7, 3}, {7, 7}, {5, 5}}{
      \setlength{\hoshiCoordX}{\dimexpr #3 * ({\sloc}[0] - 1) \relax}
      \setlength{\hoshiCoordY}{\dimexpr #3 * ({\sloc}[1] - 1) \relax}
  \filldraw (\hoshiCoordX, \hoshiCoordY)
    circle [radius=#3 / 10];
}

}{}
}

Here's the complete working example:

\documentclass{article}
\newlength{\step}
% Parameters
%
% 1: dimension (in cm)
% 2: board size (square)
% 
% Example: A 19x19 board with size 10cm x 10cm: `\gogrid{10}{19}'
\newcommand{\goGrid}[2]{
  \setlength{\step}{\dimexpr #1cm / (#2 - 1) \relax} % chktex 1
\draw[step=\step] (0, 0) grid (#1, #1);
\drawHoshis{#1}{#2}{\step}
}
% Parameters
%
% 1: dimension (in cm)
% 2: board size (square)
% 3: step
% 
% Example: A 19x19 board with size 10cm x 10cm: `\drawHoshis{10}{19}{\step}'
\newcommand{\drawHoshis}[3]{
  \ifthenelse{#2 = 9} { 
    \foreach \sloc in {{3, 3}, {3, 7}, {7, 3}, {7, 7}, {5, 5}}{
      % This is what I'm tryig to improve:
      \filldraw (#3 * {\sloc}[0] - #3, #3 * {\sloc}[1] - #3)
        circle [radius=#3 / 10];
    }
  }{}
}
\usepackage{tikz}
\begin{document}
  \begin{figure}[ht]
    \begin{center}
      \begin{tikzpicture}
        \goGrid{10}{9}
      \end{tikzpicture}
      \caption{Goban 1}\label{my_goban_1}
    \end{center}
  \end{figure}
\end{document}

I've tried many variations of this but none have worked so far (I did try without \dimexpr as well). Is anyone able to spot what I'm missing?

Answer 1

I am unsure what you want the code to produce, but things will be much easier if you use \pgfmathsetmacro instead of \setlength as this will automatically do the calculations for you. There is no need to add a length unit, because the base unit in TikZ is 1 cm anyways. I'd also replace the \ifthenelse command that requires some package by \ifnum.

If you want to do calculations inside a coordinate that require the use of parenthesis, you need to wrap things in curly braces. The following two lines are both representing the same coordinate:

(#3 * {\sloc}[0] - #3, #3 * {\sloc}[1] - #3) 
({#3 * ({\sloc}[0] - 1}, {#3 * ({\sloc}[1] - 1)}) 

In any case, it seems that in your code the indices for selecting the items from the arrays are wrong, at least in one of the code snippets you show. Make sure that these are correct.

\documentclass[border=10pt]{standalone}
\usepackage{tikz}
% Parameters
%
% 1: dimension (in cm)
% 2: board size (square)
% 
% Example: A 19x19 board with size 10cm x 10cm: `\gogrid{10}{19}'
\newcommand{\goGrid}[2]{
  \pgfmathsetmacro{\step}{#1 / (#2 - 1)} % chktex 1
  \draw[step=\step] (0, 0) grid (#1, #1);
  \drawHoshis{#1}{#2}{\step}
}
% Parameters
%
% 1: dimension (in cm)
% 2: board size (square)
% 3: step
% 
% Example: A 19x19 board with size 10cm x 10cm: `\drawHoshis{10}{19}{\step}'
\newcommand{\drawHoshis}[3]{
  \ifnum#2=9\relax 
    \foreach \sloc in {{3, 3}, {3, 7}, {7, 3}, {7, 7}, {5, 5}}{
      % This is what I'm trying to improve:
      %\filldraw (#3 * {\sloc}[0] - #3, #3 * {\sloc}[1] - #3)
      \filldraw ({#3 * ({\sloc}[0] - 1}, {#3 * ({\sloc}[1] - 1)})
        circle[radius={#3 / 10}];
    }
  \fi
}
\begin{document}
  \begin{tikzpicture}
    \goGrid{10}{9}
  \end{tikzpicture}
\end{document}

Alternatively:

\documentclass[border=10pt]{standalone}
\usepackage{tikz}
% Parameters
%
% 1: dimension (in cm)
% 2: board size (square)
% 
% Example: A 19x19 board with size 10cm x 10cm: `\gogrid{10}{19}'
\newcommand{\goGrid}[2]{
  \pgfmathsetmacro{\step}{#1 / (#2 - 1)} % chktex 1
  \draw[step=\step] (0, 0) grid (#1, #1);
  \drawHoshis{#1}{#2}{\step}
}
% Parameters
%
% 1: dimension (in cm)
% 2: board size (square)
% 3: step
% 
% Example: A 19x19 board with size 10cm x 10cm: `\drawHoshis{10}{19}{\step}'
\newcommand{\drawHoshis}[3]{
  \ifnum#2=9\relax 
    \foreach \sloc in {{3, 3}, {3, 7}, {7, 3}, {7, 7}, {5, 5}}{
      \pgfmathsetmacro{\hoshiCoordX}{#3 * ({\sloc}[0] - 1)}
      \pgfmathsetmacro{\hoshiCoordY}{#3 * ({\sloc}[1] - 1)}
      \filldraw (\hoshiCoordX, \hoshiCoordY)
        circle[radius={#3 / 10}];
    }
  \fi
}
\begin{document}
  \begin{tikzpicture}
    \goGrid{10}{9}
  \end{tikzpicture}
\end{document}

Output of both code snippets: