Pass a list of arguments to a matrix: issue with last step in iteration

Question

I wish to define a command like \myrowvector{values={1,2,3}} and have spacing details set in the preamble. I have adapted code by user31729 to great effect, except... How should I deal with the last step in the iteration? I would like to prevent the insertion of a separator & (row vector) or \\ (column vector).

In order to increase spacing around the values, I have borrowed code by CarLaTex to create an alternative to the environment bmatrix (named bbmatrix). This generally works, but fails inside my \myrowvector command, with a "Missing $ inserted" message. I don't quite follow the part of the code with {+b} and I would appreciate a hint on how to use a matrix with greater spacing around values.

\documentclass[a4paper]{article}
\usepackage[english]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{expl3}
\usepackage{xparse}
\usepackage{array}
\usepackage{tabularray}
\UseTblrLibrary{amsmath}
% https://tex.stackexchange.com/questions/570455/
% CarLaTex
\NewDocumentEnvironment{bbmatrix}{+b}{
    \begin{+bmatrix}[columns={1.0em, c, colsep=2pt}]
    #1
    \end{+bmatrix}
    }{}
% https://tex.stackexchange.com/questions/423115/
% user31729
\ExplSyntaxOn
\keys_define:nn { myrowvector }
{
  values .clist_set:N = \l_myrowvector_values_clist,
}
\keys_define:nn { mycolvector }
{
  values .clist_set:N = \l_mycolvector_values_clist,
}
\NewDocumentCommand\myrowvector{m}{
  \keys_set:nn { myrowvector } { #1 }
    \ensuremath{
      \begin{bmatrix}
        \clist_map_inline:Nn \l_myrowvector_values_clist {
        ##1 &
      }
      \end{bmatrix}
  }
}
\NewDocumentCommand\mycolvector{m}{
  \keys_set:nn { mycolvector } { #1 }
    \ensuremath{
      \begin{bmatrix}
        \clist_map_inline:Nn \l_mycolvector_values_clist {
        ##1 \
      }
      \end{bmatrix}
  }
}
\ExplSyntaxOff
\begin{document}
A row vector:
\begin{align}
\myrowvector{values={1,2,3}}
\end{align}
A column vector:
\begin{align}
\mycolvector{values={1,2,3}}
\end{align}
A less tightly spaced row vector:
\begin{align}
\begin{bbmatrix}
1 & 2 & 3
\end{bbmatrix}
\end{align}
A less tightly spaced column vector:
\begin{align}
\begin{bbmatrix}
1 \ 2 \ 3
\end{bbmatrix}
\end{align}
\end{document}

EDIT

I have accepted egreg's answer, but for posterity I'd like to copy skillmon's comment, which solved the spacing problem:

Instead of \clist_map_inline:Nn \l_mycolvector_values_clist { ##1 \\ } try to use \clist_use:Nn \l_mycolvector_values_clist { \\ } (or \clist_use:Nn \l_myrowvector_values_clist { & } respectively). See for instance tex.stackexchange.com/q/678399/117050.

Also, I have since found this related question: tex.stackexchange.com/questions/270223

Answer 1

As tabularray environments don't like that & is hidden in macros, you want to use +bmatrix as a command rather than an environment.

And you want to exploit \clist_use:Nn rather than mapping.

\documentclass[a4paper]{article}
\usepackage[english]{babel}
%\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
%\usepackage{expl3}
%\usepackage{xparse}
\usepackage{array}
\usepackage{tabularray}
\UseTblrLibrary{amsmath}
\ExplSyntaxOn
% adapted from
% https://tex.stackexchange.com/questions/570455/
% CarLaTex
\cs_new_protected:Nn \patrickt_bbmatrix:n
 {
  \begin{+bmatrix}[columns={1.0em, c, colsep=2pt}]
  #1
  \end{+bmatrix}
 }
\cs_generate_variant:Nn \patrickt_bbmatrix:n { e }
% https://tex.stackexchange.com/questions/423115/
% user31729
\keys_define:nn { patrickt/vector }
 {
  values .clist_set:N = \l_patrickt_vector_values_clist,
 }
\NewDocumentCommand\myrowvector{m}
 {
  \keys_set:nn { patrickt/vector } { #1 }
  \patrickt_bbmatrix:e { \clist_use:Nn \l_patrickt_vector_values_clist { & } }
 }
\NewDocumentCommand\mycolvector{m}{
  \keys_set:nn { patrickt/vector } { #1 }
  \patrickt_bbmatrix:e { \clist_use:Nn \l_patrickt_vector_values_clist { \ } }
}
\ExplSyntaxOff
\begin{document}
A row vector:
\begin{equation}
\myrowvector{values={1,2,3}}
\end{equation}
A column vector:
\begin{equation}
\mycolvector{values={1,2,3}}
\end{equation}
\end{document}

No need to define two different sets of keys. I also removed \ensuremath that has no real purpose and changed align* to equation* (never use the former for single equations).

On the other hand, I see no real reason to use a syntax like

\myrowvector{values={1,2,3}}

over

\myrowvector{1,2,3}

that you can accomplish with

\documentclass[a4paper]{article}
\usepackage[english]{babel}
\usepackage[T1]{fontenc}
\usepackage{array}
\usepackage{tabularray}
\UseTblrLibrary{amsmath}
\ExplSyntaxOn
% adapted from
% https://tex.stackexchange.com/questions/570455/
% CarLaTex
\cs_new_protected:Nn \patrickt_bbmatrix:n
 {
  \begin{+bmatrix}[columns={1.0em, c, colsep=2pt}]
  #1
  \end{+bmatrix}
 }
\cs_generate_variant:Nn \patrickt_bbmatrix:n { e }
\NewDocumentCommand\myrowvector{m}
 {
  \patrickt_bbmatrix:e { \clist_use:nn { #1 } { & } }
 }
\NewDocumentCommand\mycolvector{m}{
  \patrickt_bbmatrix:e { \clist_use:nn { #1 } { \ } }
}
\ExplSyntaxOff
\begin{document}
A row vector:
\begin{equation}
\myrowvector{1,2,3}
\end{equation}
A column vector:
\begin{equation}
\mycolvector{1,2,3}
\end{equation}
\end{document}

A different and “unified” approach:

\documentclass[a4paper]{article}
\usepackage[english]{babel}
\usepackage[T1]{fontenc}
\usepackage{array}
\usepackage{tabularray}
\UseTblrLibrary{amsmath}
\ExplSyntaxOn
% adapted from
% https://tex.stackexchange.com/questions/570455/
% CarLaTex
\cs_new_protected:Nn \patrickt_bbmatrix:n
 {
  \begin{+bmatrix}[columns={1.0em, c, colsep=2pt}]
  #1
  \end{+bmatrix}
 }
\cs_generate_variant:Nn \patrickt_bbmatrix:n { V }
\NewDocumentCommand\mymatrix{m}
 {
  __patrickt_bbmatrix_build:n { #1 }
  \patrickt_bbmatrix:V \l__patrickt_bbmatrix_body_tl
 }
\tl_new:N \l__patrickt_bbmatrix_body_tl
\seq_new:N \l__patrickt_bbmatrix_rows_seq
\cs_new_protected:Nn __patrickt_bbmatrix_build:n
 {
  \seq_set_split:Nnn \l__patrickt_bbmatrix_rows_seq { ; } { #1 }
  \tl_clear:N \l__patrickt_bbmatrix_body_tl
  \seq_map_inline:Nn \l__patrickt_bbmatrix_rows_seq
   {
    \tl_put_right:Nx \l__patrickt_bbmatrix_body_tl { \clist_use:nn { ##1 } { & } \exp_not:N \ }
   }
 }
\ExplSyntaxOff
\begin{document}
\begin{equation}
\mymatrix{1,2,3}\ne
\mymatrix{1;2;3}\ne
\mymatrix{1,2;3,4;5,6}
\end{equation}
\end{document}