¿Cómo realizo una comparación de cadenas expandibles?

Solución:

Puedes usar expl3; los strcompare macro es solo una versión de nivel de usuario de la ampliable función interna str_if_eq:nnTF.

documentclass{article}
usepackage{expl3}

ExplSyntaxOn
cs_new_eq:NN strcompare str_if_eq:nnTF
ExplSyntaxOff

begin{document}

startpar
strcompare{abc}{def}{OOPS}{OK}par
strcompare{abc}{abcd}{OOPS}{OK}par
strcompare{abcd}{abc}{OOPS}{OK}par
strcompare{a}{}{OOPS}{OK}par
strcompare{}{a}{OOPS}{OK}par
end

medskip

startpar
strcompare{abc}{abc}{OK}{OOPS}par
strcompare{}{}{OK}{OOPS}par
end

end{document}

Lo he leído con usepackage{pdftexcmds}, puedes usar [email protected] pero no he comprobado si funciona en este caso.

Alternativamente, el código siguiente define [email protected]@equal{str1}{str2}{if-case}{else-case}. Como @JavierBezos anotado en los comentarios, los espacios no se tienen en cuenta (por lo que "a b" = "ab"), y una versión que los tenga en cuenta sería más difícil de lograr (¿si es posible?). Se puede utilizar de la siguiente manera:

makeatletter
start%
[email protected]@equal{abc}{def}{OOPS}{OK}%
[email protected]@equal{abc}{abcd}{OOPS}{OK}%
[email protected]@equal{abcd}{abc}{OOPS}{OK}%
[email protected]@equal{a}{}{OOPS}{OK}%
[email protected]@equal{}{a}{OOPS}{OK}%
end

start%
[email protected]@equal{abc}{abc}{OK}{OOPS}%
[email protected]@equal{}{}{OK}{OOPS}%
end
makeatother

Qué impresiones (observe que no se inserta ningún espacio extraño):

startOKOKOKOKOKend
startOKOKend

Aquí está la definición de [email protected]@equal:

makeatletter

%         #1       #2          #3    #4                                  sepend
% Params: {if-case}{else-case} sepr sepr                               sepend
% Or:     {if-case}{else-case} B     right-…(may be *nothing*)seprsepr sepend
def@[email protected]@[email protected]#1#2#3#4sepend{%
  ifx#3sepr% #3right… = Empty string, a.k.a. "sepr"
    % First case in the signature above (#3=sepr, #4=sepr)
    #1% The strings are both empty, and therefore equal. Execute #1.
  else%
    % Second case in the signature above (#3=one char, #4=right…seprsepr)
    #2% The right string is longer (there's at least one more character), so they are different. Execute #2.
  fi%
}%
%         #1       #2         #3          #4       sepl #5    #6                        sepr #7        sepend
% Params: {if-case}{else-case}{left-first}{left-…} sepl sepr *nothing*                 sepr *nothing* sepend
% Or:     {if-case}{else-case}{left-first}{left-…} sepl B     right-…(may be *nothing*) sepr sepr     sepend
def@[email protected]@[email protected]#1#2#3#4sepl#5#6sepr#7sepend{%
  ifx#5sepr% #5#6 = Empty string, a.k.a. "sepr"
    % First case in the signature above (#5=sepr, #6=*nothing*, #7=*nothing*)
    #2% But the left string was not empty, so we run else-case
  else%
    % Second case in the signature above (#5=one char, #6=right…, #7=sepr)
    ifx#3#5% First character of left == first character of right
      @[email protected]@equal{#1}{#2}#4seplsepl#6seprseprsepend% Recursion on the rest
    else%
      #2% The strings are different at this character, execute #2.
    fi%
  fi%
}%
% Two possibilities:
%               #1       #2          #3    #4                       sepl #5          sepend
% @[email protected]@equal {if-case}{else-case} sepl *nothing*                sepl <rest>      sepend
% @[email protected]@equal {if-case}{else-case} A     left-…(may be *nothing*) sepl sepl<rest> sepend
def@[email protected]@equal#1#2#3#4sepl#5sepend{%
  ifx#3sepl% #3#4 = Empty string, a.k.a. "sepl"
    % First case in the signature above (#3=sepl, #4=*nothing*, #5=<rest>)
    @[email protected]@[email protected]{#1}{#2}#5sepend%
  else%
    % First case in the signature above (#3=one char, #4=left…, #5=sepl<rest>)
    @[email protected]@[email protected]{#1}{#2}{#3}{#4}#5sepend%
  fi%
}
def[email protected]@equal#1#2#3#4{%
  % @[email protected]@equal#1sepl#4%#2sepl#4
  %
  % @[email protected]@equal{#4}seplsepl%
  @[email protected]@equal{#3}{#4}#1seplsepl#2seprseprsepend%
}%

makeatother

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