Custom math environment

Question

Recently I've seen this post and I felt that I'd like to have something similar, but with math environment, so that if I write a single line of equation, it will put it at the centre, like equation does. And if I add line break \\, it should be left-aligned.

Here is how it should look like:

---

And if it is possible I would also really like to be able to use & in order to make columns that are centered i.e. & is only responsible for the columns, not the alignment.

Answer 1

Not really a good markup, in my opinion. But the customer is always right. ;-)

The environment has an optional argument (default 2em) to set the spacing between columns.

\documentclass{article}
\usepackage{amsmath,array}
\usepackage{xparse}

\ExplSyntaxOn

\NewDocumentEnvironment{mymath}{O{2em}b}
 {
  % #1 = spacing between columns, #2 = body
  \tl_if_in:nnT { #2 } { & }
   {
    \bool_set_true:N \l__antshar_mymath_ampersand_bool
   }
  \seq_set_split:Nnn \l__antshar_mymath_body_seq { \\ } { #2 }
  \int_compare:nTF { \seq_count:N \l__antshar_mymath_body_seq = 1 }
   {
    \__antshar_mymath_single:nn { #1 } { #2 }
   }
   {
    \__antshar_mymath_multiple:n { #1 }
   }
 }
 {\ignorespacesafterend}

\seq_new:N \l__antshar_mymath_body_seq
\seq_new:N \l__antshar_mymath_row_seq
\bool_new:N \l__antshar_mymath_ampersand_bool
\int_new:N \l__antshar_mymath_cols_int

\cs_new_protected:Nn \__antshar_mymath_single:nn
 {
  \bool_if:NTF \l__antshar_mymath_ampersand_bool
   {
    \seq_set_split:Nnn \l__antshar_mymath_row_seq { & } { #2 }
    \[
    \begin{array}
     {
      @{}
      >{\displaystyle}c
      *{\int_eval:n { \seq_count:N \l__antshar_mymath_row_seq - 1 }}
       {@{\hspace{#1}}>{\displaystyle}c}
      @{}
     }
    #2
    \end{array}
    \]
   }
   {
    \[ #2 \]
   }
 }

\cs_new_protected:Nn \__antshar_mymath_multiple:n
 {
  \bool_if:NTF \l__antshar_mymath_ampersand_bool
   {
    % count the maximum number of columns
    \int_zero:N \l__antshar_mymath_cols_int
    \seq_map_inline:Nn \l__antshar_mymath_body_seq
     {
      \seq_set_split:Nnn \l__antshar_mymath_row_seq { & } { ##1 }
      \int_set:Nn \l__antshar_mymath_cols_int
       {
        \int_max:nn { \l__antshar_mymath_cols_int } { \seq_count:N \l__antshar_mymath_row_seq }
       }
     }
    \[
    \begin{array}
     {
      @{}
      >{\displaystyle}c
      *{\int_eval:n { \l__antshar_mymath_cols_int - 1 } }
       {@{\hspace{#1}}>{\displaystyle}c}
      @{}
     }
    \seq_use:Nn \l__antshar_mymath_body_seq { \\\noalign{\vspace{\jot}} }
    \end{array}
    \]
   }
   {
    \seq_set_map:NNn \l__antshar_mymath_row_seq \l__antshar_mymath_body_seq
     {
      & \exp_not:n { {##1} } &&
     }
    \begin{flalign*}
    \seq_use:Nn \l__antshar_mymath_row_seq { \\ }
    \end{flalign*}
   }
 }

\ExplSyntaxOff

\begin{document}

\noindent Some text in front of the display, just for separation
\begin{mymath}
a=b
\end{mymath}
Some text in front of the display, just for separation
\begin{mymath}
a=b & c=d & e=ffffffff
\end{mymath}
Some text in front of the display, just for separation
\begin{mymath}
a=b & c=d & e=ffffffff \\
1=1 & 120\ne1000 & u
\end{mymath}
Some text in front of the display, just for separation
\begin{mymath}[4em]
a=b & c=d & e=ffffffff
\end{mymath}
Some text in front of the display, just for separation
\begin{mymath}
abc+def\\
=x+y
\end{mymath}

\end{document}

The image has been produced adding showframe.

Addition

The mymath* environment puts everything to the left.

\documentclass{article}
\usepackage{amsmath,array}
\usepackage{xparse}
\usepackage{showframe}

\ExplSyntaxOn

\NewDocumentEnvironment{mymath}{O{2em}b}
 {
  \bool_set_false:N \l__antshar_mymath_left_bool
  \antshar_mymath_main:nn { #1 } { #2 }
 }
 {\ignorespacesafterend}

\NewDocumentEnvironment{mymath*}{O{2em}b}
 {
  \bool_set_true:N \l__antshar_mymath_left_bool
  \antshar_mymath_main:nn { #1 } { #2 }
 }
 {\ignorespacesafterend}

\seq_new:N \l__antshar_mymath_body_seq
\seq_new:N \l__antshar_mymath_row_seq
\bool_new:N \l__antshar_mymath_ampersand_bool
\bool_new:N \l__antshar_mymath_left_bool
\int_new:N \l__antshar_mymath_cols_int

\cs_new_protected:Nn \antshar_mymath_main:nn
 {
  % #1 = spacing between columns, #2 = body
  \tl_if_in:nnT { #2 } { & }
   {
    \bool_set_true:N \l__antshar_mymath_ampersand_bool
   }
  \seq_set_split:Nnn \l__antshar_mymath_body_seq { \\ } { #2 }
  \int_compare:nTF { \seq_count:N \l__antshar_mymath_body_seq = 1 }
   {
    \__antshar_mymath_single:nn { #1 } { #2 }
   }
   {
    \__antshar_mymath_multiple:n { #1 }
   }
 }

\cs_new_protected:Nn \__antshar_mymath_single:nn
 {
  \bool_if:NTF \l__antshar_mymath_ampersand_bool
   {
    \seq_set_split:Nnn \l__antshar_mymath_row_seq { & } { #2 }
    \[
    \bool_if:NT \l__antshar_mymath_left_bool { \hspace{0pt} }
    \begin{array}
     {
      @{}
      >{\displaystyle}c
      *{\int_eval:n { \seq_count:N \l__antshar_mymath_row_seq - 1 }}
       {@{\hspace{#1}}>{\displaystyle}c}
      @{}
     }
    #2
    \end{array}
    \bool_if:NT \l__antshar_mymath_left_bool { \hspace{1000pt minus 1fill} }
    \]
   }
   {
    \[
    \bool_if:NT \l__antshar_mymath_left_bool { \hspace{0pt} }
    #2
    \bool_if:NT \l__antshar_mymath_left_bool { \hspace{1000pt minus 1fill} }
    \]
   }
 }

\cs_new_protected:Nn \__antshar_mymath_multiple:n
 {
  \bool_if:NTF \l__antshar_mymath_ampersand_bool
   {
    % count the maximum number of columns
    \int_zero:N \l__antshar_mymath_cols_int
    \seq_map_inline:Nn \l__antshar_mymath_body_seq
     {
      \seq_set_split:Nnn \l__antshar_mymath_row_seq { & } { ##1 }
      \int_set:Nn \l__antshar_mymath_cols_int
       {
        \int_max:nn { \l__antshar_mymath_cols_int } { \seq_count:N \l__antshar_mymath_row_seq }
       }
     }
    \[
    \bool_if:NT \l__antshar_mymath_left_bool { \hspace{0pt} }
    \begin{array}
     {
      @{}
      >{\displaystyle}c
      *{\int_eval:n { \l__antshar_mymath_cols_int - 1 } }
       {@{\hspace{#1}}>{\displaystyle}c}
      @{}
     }
    \seq_use:Nn \l__antshar_mymath_body_seq { \\\noalign{\vspace{\jot}} }
    \end{array}
    \bool_if:NT \l__antshar_mymath_left_bool { \hspace{1000pt minus 1fill} }
    \]
   }
   {
    \[
    \hspace{0pt}
    \begin{array}{@{}>{\displaystyle}l@{}}
    \seq_use:Nn \l__antshar_mymath_body_seq { \\\noalign{\vspace{\jot}} }
    \end{array}
    \hspace{1000pt minus 1fill}
    \]
   }
 }

\ExplSyntaxOff

\begin{document}

\noindent Some text in front of the display, just for separation
\begin{mymath}
a=b
\end{mymath}
Some text in front of the display, just for separation
\begin{mymath}
a=b & c=d & e=ffffffff
\end{mymath}
Some text in front of the display, just for separation
\begin{mymath}
a=b & c=d & e=ffffffff \\
1=1 & 120\ne1000 & u
\end{mymath}
Some text in front of the display, just for separation
\begin{mymath}[4em]
a=b & c=d & e=ffffffff
\end{mymath}
Some text in front of the display, just for separation
\begin{mymath}
abc+def\\
=x+y
\end{mymath}
Some text in front of the display, just for separation
\begin{mymath*}
a=b
\end{mymath*}
Some text in front of the display, just for separation
\begin{mymath*}
a=b & c=d & e=ffffffff
\end{mymath*}
Some text in front of the display, just for separation
\begin{mymath*}
a=b & c=d & e=ffffffff \\
1=1 & 120\ne1000 & u
\end{mymath*}
Some text in front of the display, just for separation
\begin{mymath*}[4em]
a=b & c=d & e=ffffffff
\end{mymath*}
Some text in front of the display, just for separation
\begin{mymath*}
abc+def\\
=x+y
\end{mymath*}

\end{document}