pgfmath: (어차피 tikz와 함께 쓸 거라면...)

\usepackage{multido}
\usepackage{tikz}

%%%

\pgfkeys{/pgf/number format/.cd,fixed,precision=4}
\newcommand*\pgflog[1]{%
    \pgfmathparse{ln(#1)/ln(10)}%
    \pgfmathroundtozerofill{\pgfmathresult}\pgfmathresult
}

\def\test{$\i=\pgflog{\i}$\par}

\multido{\i=1+1}{20}{\test}