Custom environment problems using environ

Question

As shown by my MWE, I have two small issues that I'd like to get assistance with in fixing.

First off: When I start two problem environments one after another without a blank space between the \end{problem} and \begin{problem}, my second problem goes in the right margin instead of beneath the previous one.

Secondly, I'd like it, if possible, to not have a blank space (as shown in the 4th problem) if I choose to omit a problem description. I'd like to do as in problem 5, but as it is now, the first letter shows up incorrectly as the problem title.

\documentclass{article}

\usepackage{environ}
\usepackage{array}

\newcounter{problem}
\setcounter{problem}{0}
\newlength{\mylen}
\settowidth{\mylen}{100}

\NewEnviron{problem}[1]{%
  \stepcounter{problem}\noindent
  \begin{tabular}{m{\mylen}p{\dimexpr\linewidth-\mylen-3\tabcolsep\relax}@{}}
    \arabic{problem}. & \textbf{#1}\\[1mm]
    & \BODY
  \end{tabular}}%

\begin{document}
  \begin{problem}{foobar}
    test    
  \end{problem}
  \begin{problem}
    test
  \end{problem}

\bigskip\bigskip\bigskip

  \begin{problem}{foobar}
    test    
  \end{problem}

  \begin{problem}{}
    test2
  \end{problem}

  \begin{problem}
    test
  \end{problem}
\end{document}

Answer 1

You need to add \par to the definition avoid the need to force a new paragraph in the document. Alsoe use \refstepcounter so that you can \label your problems.

There is no need here to grab the environment body as a macro, so I reverted to a standard environment definition, and as you want the title to be optional, I declared it so defaulting to empty, which means that it takes [] not {} syntax.

\documentclass{article}

%\usepackage{environ}
\usepackage{array}

\newcounter{problem}
\setcounter{problem}{0}
\newlength{\mylen}
\settowidth{\mylen}{100}

\newenvironment{problem}[1][]{%
  \refstepcounter{problem}
  \par\noindent
  \begin{tabular}{m{\mylen}p{\dimexpr\linewidth-\mylen-3\tabcolsep\relax}@{}}
  \arabic{problem}. & \ifx\relax#1\relax \else\textbf{#1}\\[1mm]&\fi
  %  \arabic{problem}. & \ifx\relax#1\relax \\ \else\textbf{#1}\\[1mm]\fi  &
  }
  {\end{tabular}\ignorespacesafterend}%

\begin{document}
  \begin{problem}[foobar]
    test    
  \end{problem}
  \begin{problem}
    test
  \end{problem}

\bigskip\bigskip\bigskip

  \begin{problem}[foobar]
    test    
  \end{problem}

  \begin{problem}[]
    test2
  \end{problem}

  \begin{problem}
    test
  \end{problem}
\end{document}