Conditional cases expression

Question

How can I create a command \if with 1 argument, namely #1,

\if{#1=1}{symbol1}...{#1=n}{symboln}

that returns symbol1, when #1 is 1, ..., and returns symboln, when #1 is n, and returns nothing, when #1 isn't 1,...,n?

For example, I'd like to use the command

\if{#1=1}{\mapsto}{#1=2}{\mapsfrom}{#1=3}{\rightarrow}

when I'm defining a larger command.

Answer 1

For simple integer comparisons, TeX provides \ifcase:

\ifcase<integer>
  % Case 0
\or
  % Case 1
\or
...
\else
  % Optional
\fi

So you can define a wrapper such as

\newcommand\myswitch[1]{%
  \ifcase#1\relax
    \ValueWasZero
  \or
    \ValueWasOne
  \or
    \ValueWasTwo
  ...
  \else
     \OtherCases
  \fi
}

Here, you start from the 0 case and work upward: often you'll see \ifcase#1\relax\or with no 0 case at all. You can use as many \or statements as you like, depending on how many numbers you need. The \else is optional, and is used if you want 'otherwise do this' functionality.

By the way, don't define \if: this is a TeX primitive which you should not change!

Answer 2

Using \ifcase as Joseph Wright suggested is the best way most the time.

An alternative which might have benefits for some applications would be to define a set of macros which names dependent on the number. Using \csname ...\endcsname or \@namedef macro names can contain numbers or these can be converted to a Roman number using \romannumeral. Then \@nameuse can be used to use these macros. The etoolbox package also provides the identical user-level macros \csdef and \csuse.

\documentclass{article}

\makeatletter
\newcommand\defcase[1]{\@namedef{mycase@\the\numexpr#1\relax}}
\newcommand\myswitch[1]{\@nameuse{mycase@\the\numexpr#1\relax}}
\makeatother

% or alternatively:
\makeatletter
\newcommand\defcase[1]{\@namedef{mycase@\romannumeral#1}}
\newcommand\myswitch[1]{\@nameuse{mycase@\romannumeral#1}}
\makeatother

% Define cases:
\defcase{0}{code for case 0}
\defcase{1}{code for case 1}
\defcase{2}{code for case 2}
\defcase{3}{...}

\begin{document}
% Usage
\myswitch{1}
% \myswitch{some integer expression}

\end{document}

The use of the eTeX primitive \numexpr makes the whole thing a little more robust to fancy user input.