Line numbering in algorithmic

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,float}

\renewcommand\thealgorithm{}
\newcommand{\setalglineno}[1]{%
  \setcounter{ALC@line}{\numexpr#1-1}}

\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 equivalent) environments offered by algorithmicx. For these you have to use ALG@line instead of ALC@line though.