Error with \foreach in TikZ

Question

In the MWE below

\documentclass{article}
\usepackage{tikz}
\begin{document}
  \begin{tikzpicture}
  \foreach \i in {0, 1, 2, 3}
    \draw (\i, 0) rectangle +(0.5, 0.5);
%   \draw (\i, 1) rectangle +(0.5, 0.5);
%   Error if above line is uncommented:
%   ! Undefined control sequence.
%   \pgfmath@dimen@ ...men@@ #1=0.0pt\relax \pgfmath@
  \end{tikzpicture}
\end{document}

the file compiles as is with pdflatex but gives an error as noted in MWE if the commented line is uncommented.

I know I am doing something silly, but being an occasional user of TikZ, I am unsure what it is. Grateful for an explanation.

Answer 1

To have more than one \draw (or similar) within a loop, you have to enclose them in braces ({}):

\documentclass{article}
\usepackage{tikz}
\begin{document}
  \begin{tikzpicture}
  \foreach \i in {0, 1, 2, 3} {
    \draw (\i, 0) rectangle +(0.5, 0.5);
    \draw (\i, 1) rectangle +(0.5, 0.5);
   }
  \end{tikzpicture}
\end{document}

Answer 2

I agree with Torbjørn's answer but in your case, it was possible to write the next code. I use only one command \draw on one path. I think is important to understand what is a path to work correctly with TikZ. The end of the path is determined by ;

\documentclass{article}
\usepackage{tikz}
\begin{document}
  \begin{tikzpicture}
  \foreach \i in {0, 1, 2, 3} 
    \draw (\i, 0) rectangle +(0.5, 0.5)
          (\i, 1) rectangle +(0.5, 0.5);
  \end{tikzpicture}
\end{document}