Typesetting a Row Vector

You have to collect the row beforehand, as doing it inside an array environment is quite difficult, as TeX ends a cell as soon as it sees an &. I prefer to do in with a token register and the \count255 scratch counter.

We collect two arguments: the first is the number of entries, the second the first entry, in order to initialize the token register with it. We set the counter to the number of requested element (since we have already collected one, we step it down immediately).

Then we start the recursion; the macro \rowvectnexta examines the value of the counter; if it's still greater than 0, it calls \rowvecnextb that gathers another argument, stores it into the token register (adding an &) and steps down the counter; otherwise it builds the pmatrix by releasing the contents of the register.

\newtoks\rowvectoks
\newcommand{\rowvec}[2]{%
  \rowvectoks={#2}\count255=#1\relax
  \advance\count255 by -1
  \rowvecnexta}
\newcommand{\rowvecnexta}{%
  \ifnum\count255>0
    \expandafter\rowvecnextb
  \else
    \begin{pmatrix}\the\rowvectoks\end{pmatrix}
  \fi}
\newcommand\rowvecnextb[1]{%
    \rowvectoks=\expandafter{\the\rowvectoks&#1}%
    \advance\count255 by -1
    \rowvecnexta}

It shouldn't be a big limitation the fact that you need at least one element:

\rowvec{1}{a}
\rowvec{2}{a}{b}
\rowvec{3}{a}{b}{c}

Notice: \usepackage{amsmath} is required. So a minimal example might be the following.

\documentclass{article}
\usepackage{amsmath}

\newtoks\rowvectoks
\newcommand{\rowvec}[2]{%
  \rowvectoks={#2}\count255=#1\relax
  \advance\count255 by -1
  \rowvecnexta}
\newcommand{\rowvecnexta}{%
  \ifnum\count255>0
    \expandafter\rowvecnextb
  \else
    \begin{pmatrix}\the\rowvectoks\end{pmatrix}
  \fi}
\newcommand\rowvecnextb[1]{%
    \rowvectoks=\expandafter{\the\rowvectoks&#1}%
    \advance\count255 by -1
    \rowvecnexta}

\begin{document}

$\rowvec{1}{a}$
$\rowvec{2}{a}{b}$
$\rowvec{3}{a}{b}{c}$

\end{document}

---

A completely different solution that can be generalized to column vectors uses xparse and LaTeX3.

\documentclass{article}
\usepackage{amsmath}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand{\Rowvec}{ O{,} m }
 {
  \vector_main:nnnn { p } { & } { #1 } { #2 }
 }
\NewDocumentCommand{\Colvec}{ O{,} m }
 {
  \vector_main:nnnn { p } { \\ } { #1 } { #2 }
 }

\seq_new:N \l__vector_arg_seq
\cs_new_protected:Npn \vector_main:nnnn #1 #2 #3 #4
 {
  \seq_set_split:Nnn \l__vector_arg_seq { #3 } { #4 }
  \begin{#1matrix}
  \seq_use:Nnnn \l__vector_arg_seq { #2 } { #2 } { #2 }
  \end{#1matrix}
 }
\ExplSyntaxOff

\begin{document}

$\Rowvec{a}\Rowvec{a,b}\Rowvec[;]{a;b;c}$

$\Colvec{a}\Colvec{a,b}\Colvec[;]{a;b;c}$

\end{document}

The optional argument is the delimiter to be used to separate entries in the list given as argument.

It's sufficient to change { p } into { b } in the two main definitions to get brackets instead of parentheses.

Note: this requires expl3 dated September 2012 or later.

I would like to suggest a dirty solution, which I use:

\newcommand*{\rowvec}[1]{\left( #1\right)}

The advantage is that its usage is simple: \rowvec{a,b,c,d,e}. I can easily imagine that it is not robust... For me it works.

This is an old question... but as it surfaced again let me point out that one can do these things without token register or macro storage or count registers.

\documentclass{article}
\usepackage{amsmath}
\makeatletter
% USAGE \Matrix { a,..,z; A,.., Z ; ... ; aA, ..., zZ}
% NO semi-colon for the last row.
\newcommand{\Matrix}[1]
    {\begin{pmatrix}
      \Matrix@r #1;\@bye;\Matrix@r
     \end{pmatrix}}

\def\Matrix@r #1;{\@bye #1\Matrix@z\@bye\Matrix@s #1,\@bye, }%
\def\Matrix@s #1,{#1\Matrix@t }%
\def\Matrix@t #1,{\@bye #1\Matrix@y\@bye\@firstofone {&#1}\Matrix@t}%
\def\Matrix@y #1\Matrix@t{\\ \Matrix@r }%
\def\Matrix@z #1\Matrix@r {}
\def\@bye  #1\@bye   {}% (the idea of \@bye is from xint code)

\makeatother

\begin{document}\thispagestyle{empty}


\[ \Matrix { a, b  , c , d ; e ,f , 3, 5; 8, -1, x, y }\]

\[ \Matrix { abc , \mathrm{XYZ}; 7, 913; \alpha, \beta }\] 

\end{document}