Line numbering in algorithmic

Question

I want to change the first number of the line numbering in my algorithm, but how is it done? This is my example:

\documentclass[12pt]{beamer}
\usepackage{amsmath, amsfonts, amssymb, amsthm}
\usepackage{algorithmic, algorithm}
\usepackage{float}
\renewcommand\thealgorithm{}

    \begin{document}
    \begin{frame}[t, fragile]
    \frametitle{...}
       \begin{algorithm}[H]
       \caption{...}
       \begin{algorithmic}[1]
          \STATE ...
       \end{algorithmic}
       \end{algorithm}
    \end{frame}
    \end{document}

The first state should be numbered with 2 for example.

Answer 1

The line number in algorithmic (from the algorithms bundle) is governed by the counter ALC@line. The following MWE defines \setalglineno{<num>} which allows you to modify the number of any line in your algorithmic:

\documentclass[12pt]{beamer}
\usepackage{algorithmic, algorithm}% http://ctan.org/pkg/algorithms
\usepackage{float}% http://ctan.org/pkg/float
\renewcommand\thealgorithm{}
\makeatletter
\newcommand{\setalglineno}[1]{%
  \setcounter{ALC@line}{\numexpr#1-1}}
\makeatother
\begin{document}
\begin{frame}[t, fragile]
  \frametitle{...}
  \begin{algorithm}[H]
    \caption{...}
    \begin{algorithmic}[1]
      \setalglineno{2}% Set following line number to 2
      \STATE ...
    \end{algorithmic}
  \end{algorithm}
\end{frame}
\end{document}

In fact, \setalglineno{<num>} sets ALC@line to <num>-1, since the algorithm is set in a list, where each line counter is incremented before it is set. So, technically, to obtain a line numbered 2, you need to set the counter value to 1.

A similar setup would be used within the algpseudocode (or similar) environments offered by algorithmicx.