Implementing switch cases

The following is from catoptions package. The following ifcasse is cptifcasse in catoptions package. I think Count Zero should simply load an existing package for his task.

documentclass{article}
usepackage{pdftexcmds}
makeatletter
longdefam@alltoendif#1endif{unexpanded{#1}}
longdefam@domatchcode#1#2endif{unexpanded{#1}}
longdefamifcond#1fi{csname @#1firstelse secondfi oftwoendcsname}
longdefamifstrcmp#1#2{%
  amifcondif0pdf@strcmp{detokenize{#1}}{detokenize{#2}}fi
}
defifcasse#1#2{%
  amifstrcmp{#1}ifnone{%
    am@alltoendif
  }{%
    amifstrcmp{#1}endif{}{%
      amifstrcmp{#2}ifnone{%
        am@alltoendif
      }{%
        amifstrcmp{#2}endif{}{am@ifcasse{#1}{#2}}%
      }%
    }%
  }%
}
defam@ifcasse#1#2#3{%
  amifstrcmp{#3}ifnone{%
    am@alltoendif
  }{%
    amifstrcmp{#3}endif{}{%
      #1{#2}{#3}am@domatchcode{am@ifcasse@i{#1}{#2}}%
    }%
  }%
}
defam@ifcasse@i#1#2#3{am@ifcasse{#1}{#2}}

% The user can also define his own logical test command and pass it as the first 
% argument of ifcasse. For example, we define ifnumtest:

defifnumtest#1#2{amifcondifnum#1#2fi}

% We rewrite Count Zero's macro:
newcommand*{dothis}[1]{%
  par
  ifcasseamifstrcmp{#1}%
    {a}{so you typed a}
    {b}{now this is b}
    {c}{you want me to do c?}
  ifnone
    [no match]%
  endif
}
makeatother

begin{document}
dothis{a}
dothis{b}
dothis{c}
dothis{e}

% Weird test that correctly gives x as 'empty':
edefx{%
  ifcasseamifstrcmp{x}
  endif
}
%showx

% Weird test that gives x as 'no match':
edefx{%
  ifcasseamifstrcmp{x}
  ifnone
    [no match]%
  endif
}
par``x''.

% Number test:
edefx{%
  ifcasseifnumtest{2}
    {=1}{equal to 2}
    {<3}{less than 3}
    {>4}{greater than 4}
  ifnone
    no match%
  endif
}
par``x''.
end{document}

Though it's most likely too late for the OP, I just worked out my own switch and thought I'd share it here for future readers. My solution uses solely the package xifthen (ifthen suffices too, but I already had xifthen installed...).

% Switch implementation
usepackage{xifthen}
newcommand{ifequals}[3]{ifthenelse{equal{#1}{#2}}{#3}{}}
newcommand{case}[2]{#1 #2} % Dummy, so renewcommand has something to overwrite...
newenvironment{switch}[1]{renewcommand{case}{ifequals{#1}}}{}

% Example: Pick color by ID
newcommand{incolor}[2]{
    begin{switch}{#1}
        case{1}{color{red}}
        case{2}{color{blue}}
        case{3}{color{green}}
        case{4}{color{black}}
        #2
    end{switch}
}

This code compiles perfectly fine in my TeXMaker (ofc. you'll need the color-package for this example, but it's not part of the switch). The example colors a given input in a color defined by an ID I chose (Usage: incolor{ID}{Content}). I used it for shorthand notations of lots of things (e.g. lp1, lp2, ... for parentheses in different colors using newcommand{lp}[1]{incolor{#1}{langle}}). Feel free to experiment ;)

I could imagine this to be expanded to a solution using only built-in control structures, but I've been too lazy for now to do so, yet ifx and else should do the trick.

Here is a solution via macro definitions (a single test makes the choice):

documentclass{article}
makeatletter
newcommandaddcase[3]{expandafterdefcsnamestring#1@case@#2endcsname{#3}}
newcommandmakeswitch[2][]{%
  newcommand#2[1]{%
    ifcsnamestring#2@case@##1endcsnamecsnamestring#2@case@##1endcsnameelse#1fi%
  }%
}
makeatother

makeswitch[nada]dothis
addcasedothis{a}{so you typed a}
addcasedothis{b}{so you typed b}
addcasedothis{c}{you want me to do c?}

begin{document}
dothis{a}

dothis{b}

dothis{c}

dothis{e}

end{document}

Based on the great answer by Thev and updated by MaxD I have implemented a default command for switch. I used ifthen package:

newboolean{default}
newcommand{case}{}
newcommand{default}{}

newenvironment{switch}[1]{%
    setboolean{default}{true}
    renewcommand{case}[2]{ifthenelse{equal{#1}{##1}}{%
        setboolean{default}{false}##2}{}}%
    renewcommand{default}[1]{ifthenelse{boolean{default}}{##1}{}}
}{}

Then implement it as such:

  begin{switch}{program}  
    case{2}{twodayprogram}  
    case{4}{fourdayprogram}  
    default{first last missing programpagebreak}  
  end{switch}

in case anyone is curious, this was a program for an off-season sports conditioning program and was being looped with datatool, the commands twodayprogram and fourdayprogram implement the program for a particular athlete

In LuaTeX you can use Lua tables to emulate a switch statement.

% arara: lualatex
documentclass{article}

newcommand*dothis[1]{%
  directlua{
    local cases = {
        a = "so you typed a",
        b = "now this is b",
        c = "you want to do c?"
    }

    if cases["luaescapestring{#1}"] string~= nil then
        tex.sprint(cases["luaescapestring{#1}"])
    else
        tex.sprint("[nada]")
    end
  }
}

begin{document}

dothis{a}

dothis{b}

dothis{c}

dothis{d}

end{document}

This is a simple extension to Thev's answer, adding support for a default case.

% Switch implementation
usepackage{xifthen}
newcommand{ifequals}[4]{ifthenelse{equal{#1}{#2}}{#3}{#4}}
newcommand{case}[2]{#1 #2} % Dummy, so renewcommand has something to overwrite...
newenvironment{switch}[1]{renewcommand{case}{ifequals{#1}}}{}

% Example: Pick color by ID
newcommand{incolor}[2]{
    begin{switch}{#1}
        case{1}{color{red}}{
        case{2}{color{blue}}{
        case{3}{color{green}}{
        case{4}{color{black}}{
        color{cyan}
        }}}}
    end{switch}
    #2
}

There is the built-in ifcase which works like a switch-case that interpret numbers. Because of this, I translated the strings 'a', 'b', 'c' to numbers 0, 1, 2. Note that the first case is 0. It don't need packages and is very simple.

documentclass{article}
newcommand{dothis}[1]{%
ifcase#1relax so you typed a % Case 0.

or now this is b % Case 1.

or you want me to do c? % Case 2.

else Default case.
fi
}
begin{document}
dothis{0}

dothis{1}

dothis{2}

dothis{3}
end{document}