Difference between \newcommand and \pgfmathsetmacro used in TikZ \coordinate

Question

Consider the following code:

\documentclass{article}
\usepackage{tikz}
\begin{document}
 \begin{tikzpicture}
  \newcommand{\MA}{4}
  \pgfmathsetmacro{\MB}{4};
  \coordinate (V11) at (0,0);
  \coordinate (V44) at (4,4);
  \draw[black] (V11) rectangle (V\MA\MA);
 \end{tikzpicture}
\end{document}

This works. However changing the \draw command to:

   \draw[black] (V11) rectangle (V\MB\MB);

produces the following error:

! Package pgf Error: No shape named V4 is known .

Why is \newcommand working here, when \pgfmathsetmacro fails?

Answer 1

The reason is that \pgfmathsetmacro{\MB}{4} (there is no ; required) sets \MB to the arithmetic result of 4.0 not 4. Therefore you get V4.04.0 as node expression. Everything after a dot is taken as node anchor, i.e. TikZ looks for a node V4 with the anchor 04.0, which leads to the error you get. (Numeric anchors are also taken as angle (in degrees), so V\MB would be node V4 with the angle node at 0 degrees.)

To fix this use \pgfmathtruncatemacro{\MB}{4} which will give you 4.

See also the related questions How to draw lines between nodes that point at the node center, but stop at the nodes edge and Referring to tikz matrix coordinates using pgf mathematics operations, which where caused from the same behaviour of \pgfmathsetmacro.