“For” loop without “do”; “if” statement without “then” [closed]

Question

How can I get rid of the do in a for loop and the then in an if statement?

Answer 1

You can redefine \algorithmicthen and \algorithmicdo:

\renewcommand\algorithmicthen{}
\renewcommand\algorithmicdo{}

Adding the previous lines in the preamble (respectively, before a particular algorithmic environment enclosing the construct inside a group) will remove the "then" and "do" from all the algorithmic environments (for the particular environment, respectively). If the change has to be applied only to some structures of a particular algorithm while others still will have the "then" and "do", then you can define commands to "switch on/off" the expressions:

\documentclass{article}
\usepackage{algorithm}
\usepackage{algorithmic}

\newcommand\NoDo{\renewcommand\algorithmicdo{}}
\newcommand\ReDo{\renewcommand\algorithmicdo{\textbf{do}}}
\newcommand\NoThen{\renewcommand\algorithmicthen{}}
\newcommand\ReThen{\renewcommand\algorithmicthen{\textbf{then}}}

\begin{document}

\begin{algorithm}
\caption{Calculate $y = x^n$}
\label{alg1}
\begin{algorithmic}
\STATE $y \leftarrow 1$
\NoThen
\IF{$n < 0$}
\STATE $X \leftarrow 1 / x$
\ENDIF
\ReThen
\IF{$n > 0$}
\STATE $X \leftarrow 1$
\ENDIF
\NoDo
\FOR{$N \neq 0$}
\STATE $X \leftarrow X \times X$
\ENDFOR
\ReDo
\WHILE{$N \neq 0$}
\STATE $X \leftarrow X \times X$
\ENDWHILE
\end{algorithmic}
\end{algorithm}

\end{document}

Answer 2

Although this may be up to user preference, the algorithmicx package provides a similar interface to that of algorithmic but with some more (or easy) customization.

Here is an implementation of @Gonzalo's answer that defines \NoThenIf ... \EndIf and \NoDoFor ... \EndFor pairs using "the ONE defining macro \algdef:

\documentclass{article}
\usepackage{algorithm}% http://ctan.org/pkg/algorithm
\usepackage{algpseudocode}% http://ctan.org/pkg/algorithmicx
\begin{document}

\algdef{SE}[FOR]{NoDoFor}{EndFor}[1]{\algorithmicfor\ #1}{\algorithmicend\ \algorithmicfor}%
\algdef{SE}[IF]{NoThenIf}{EndIf}[1]{\algorithmicif\ #1}{\algorithmicend\ \algorithmicif}%
\begin{algorithm}
  \caption{Calculate $y = x^n$}\label{alg1}
  \begin{algorithmic}[1]
    \State $y \leftarrow 1$
    \NoThenIf{$n < 0$}
      \State $X \leftarrow 1 / x$
    \EndIf
    \If{$n > 0$}
      \State $X \leftarrow 1$
    \EndIf
    \NoDoFor{$N \neq 0$}
      \State $X \leftarrow X \times X$
    \EndFor
    \While{$N \neq 0$}
      \State $X \leftarrow X \times X$
    \EndWhile
  \end{algorithmic}
\end{algorithm}
\end{document}

The flags SE in both definitions refer to "Starting command with text" and "Continuing command, with default Ending text". See the algorithmicx documentation for more information on these command definitions (section 4.7 The ONE defining macro, p 20 onward).