Cross-referencing a custom environment with two arguments

Question

Sorry if my question title is ambiguous.

I want to write an environment called "problem" that works as follows. I want to be able to write

    \begin{problem}{1}{2}
    this is a problem statement \label{myprob}
    \end{problem}

    Look at problem number \ref{myprob}

and get the following output:

    Problem 1.2: This is a problem statement.

    Look at problem number 1.2.

Any ideas as to how I would do this?

Answer 1

If that's all, then this will do:

\documentclass{article}
\makeatletter
\newenvironment{problem}[2]
  {\def\@currentlabel{#1.#2}%
  \noindent\textbf{Problem~\@currentlabel}:\space\ignorespaces}
  {\par}
\makeatother
\begin{document}
\begin{problem}{1}{2}
This is a problem statement. \label{myprob}
\end{problem}
\end{document}

This relies on the fact that referencing is done using \@currentlabel. Therefore, setting this appropriately. Passing macros as arguments to problem might require some expansion.

Answer 2

Here is another solution using theorems environment from amsthm package.

\documentclass{report}
\usepackage{amsthm}

\newcounter{problemH}
\setcounter{problemH}{1} % set up the first number
\newtheorem{problem}{Problem}[problemH]

\begin{document}    
\begin{problem} 
 this is a problem statement \label{myprob}
\end{problem}

Look at problem number \ref{myprob}

\begin{problem}
  this is a problem statement 
\end{problem}
\end{document}

To replace the period . by colon : use

%\newtheoremstyle{stylename}{spaceabove}{spacebelow}{bodyfont}{indentamt}{headfont}{headpunct}{headspace}{headspec}
\newtheoremstyle{problemH}{3pt}{3pt}{}{}{\bfseries}{:}{.5em}{}
\theoremstyle{problemH}
\newtheorem{problem}{Problem}[problemH]

to obtain