I have created a command that uses 3 arguments, i.e,
\newcommand{\mycommand}[3]{% \begin{tabular}{c} #1 \\ #2 \\ #3 \end{tabular} } so, when I use this command I use \mycommand{arg1}{arg2}{arg3}, but instead I want somethink like \mycommand{arg1,arg2,arg3}.
Thanks in advance.
As many rows as you want:
\documentclass{article} \usepackage{xparse} \ExplSyntaxOn \NewDocumentCommand{\mycommand}{m} { \begin{tabular}{c} \seq_set_from_clist:Nn \l_tmpa_seq { #1 } \seq_use:Nn \l_tmpa_seq { \\ } \end{tabular} } \ExplSyntaxOff \begin{document} \mycommand{a} \mycommand{a,b} \mycommand{a,b,c,d,e,f} \end{document} 
Alternative approach:
\documentclass{article} \usepackage{xparse} \ExplSyntaxOn \NewDocumentCommand{\mycommand}{m} { \begin{tabular}{c} \clist_map_function:nN { #1 } \joan_makerow:n \end{tabular} } \cs_new_protected:Nn \joan_makerow:n { #1 \\ } \ExplSyntaxOff \begin{document} \mycommand{a} \mycommand{a,b} \mycommand{a,b,c,d,e,f} \end{document} \documentclass{article} \makeatletter \newtoks\joan@rows \newcommand{\mycommand}[1]{% \begin{tabular}{c} \@for\next:=#1\do{\joan@rows=\expandafter{\the\expandafter\joan@rows\next\\}}% \the\joan@rows \end{tabular}% } \makeatother \begin{document} \mycommand{a} \mycommand{a,b} \mycommand{a,b,c,d,e,f} \end{document} Suppose you want to uppercase the items. Then the “no packages” approach is
\documentclass{article} \makeatletter \newtoks\joan@rows \newcommand{\mycommand}[1]{% \begin{tabular}{c} \@for\next:=#1\do{\joan@rows=\expandafter{\the\expandafter\joan@rows\expandafter\MakeUppercase\expandafter{\next}\\}}% \the\joan@rows \end{tabular}% } \makeatother \begin{document} \mycommand{a} \mycommand{a,b} \mycommand{a,b,c,d,e,f} \end{document} The xparse approach is much easier:
\documentclass{article} \usepackage{xparse} \ExplSyntaxOn \NewDocumentCommand{\mycommand}{m} { \begin{tabular}{c} \clist_map_function:nN { #1 } \joan_makerow:n \end{tabular} } \cs_new_protected:Nn \joan_makerow:n { \tl_upper_case:n { #1 } \\ } \ExplSyntaxOff \begin{document} \mycommand{a} \mycommand{a,b} \mycommand{a,b,c,d,e,f} \end{document} 
;-) \MakeUppercase, i.e, on the line \@for\next:=\MakeUppercase{#1}\do{\joan@rows=\expandafter{\the\expandafter\joan@rows\next\\}}%, but it does not work :( \@for\next:=#1\do{\joan@rows=\expandafter{\the\expandafter\joan@rows\expandafter\MakeUppercase\expandafter{\next}\\} \MakeUppercase ? Here's very crude approach
\documentclass{article} \makeatletter \newcommand\mycmd[1]{\ae@my@cmd#1;} \def\ae@my@cmd#1,#2,#3;{%% \begin{tabular}{c} #1 \\ #2\\ #3 \end{tabular}} \makeatother \begin{document} \noindent\mycmd{hello, there,folks} \end{document} Here's a non-xparse approach that allows you as few and as many arguments as you would like:
\documentclass{article} \usepackage{etoolbox} \makeatletter \newcommand\mycmd[1]{%% \def\my@table{\begin{tabular}{c}}%% \ae@my@cmd#1,\relax,\relax,\relax;} \def\ae@my@cmd#1,#2,#3;{%% \ifx\relax#1 \xdef\my@table{\expandonce\my@table\noexpand\end{tabular}}%% \let\my@execute\my@table \else \def\ae@tmp{#1 \\}%% \xdef\my@table{\expandonce\my@table\expandonce\ae@tmp}%% \def\my@execute{\ae@my@cmd#2,#3;}%% \fi \my@execute } \makeatother \begin{document} \noindent \mycmd{hello, there,folks}%% \hspace{0.5em}%%a \mycmd{a,b,c,d,e,f,g}%% \hspace{0.5em}%%a \mycmd{ONLY THIS ENTRY}%% \end{document} 
I didn't realize you didn't want to load any additional packages. Here I do something similar to egreg (using \newtoks), but instead of a for-loop I stick with the basic syntax I used above:
\documentclass{article} \makeatletter \newtoks\my@table \newcommand\mycmd[1]{%% \my@table={\begin{tabular}{c}}%% \ae@my@cmd#1,\relax,\relax,\relax;} \def\ae@my@cmd#1,#2,#3;{%% \ifx\relax#1 \my@table=\expandafter{\the\my@table\end{tabular}}%% \def\my@execute{\the\my@table}%% \else \my@table=\expandafter{\the\my@table#1 \\}%% \def\my@execute{\ae@my@cmd#2,#3;}%% \fi \my@execute } \makeatother \begin{document} \noindent \mycmd{hello, there,folks}%% \hspace{0.5em}%%a \mycmd{a,b,c,d,e,f,g}%% \hspace{0.5em}%%a \mycmd{ONLY THIS ENTRY}%% \end{document} 