\newcommand resulting in Illegal unit of measure (pt inserted)

Question

I am defining macros for width and height to be used by several plots in my document, as follows:

\newcommand{\tikzwidth}{.4\columnwidth}
\newcommand{\tikzheight}{.8\tikzwidth}

I think pdflatex just replaces the whole sequence of commands for \tikzheight, resulting in .8.4\columnwidth.

Is there a way to tell pdflatex to process \tikzwidth before "shrinking" it again?

Answer 1

If you use length or dimen registers, then you can use real numbers as factors:

\newdimen\tikzwidth
\setlength{\tikzwidth}{.4\columnwidth}
\newdimen\tikzheight
\setlength{\tikzheight}{.8\tikzwidth}

Or e-TeX's \dimexpr can be used:

\newcommand*{\tikzheight}{.8\dimexpr\tikzwidth\relax}