Mfcttrf

I am currently writing a document, that sometimes requires me to alternate the colors of the letters in a sentence. I thought it would be easy to find a way to create a macro that does that, but I've run into some problems.

MWE:

% !TeX program = xelatex
% !TeX spellcheck = en_US

documentclass[11pt,a4paper,english,twoside,notitlepage]book

usepackagefontspec
usepackagelmodern
usepackage[english,main=english]babel
usepackagexcolor
usepackageetoolbox


definecolorpurpleHTML2B0057
definecolorcandyHTMLFF0000

newcounteralternate 

%the macro in question
newcommandcolset[1]%
 ifnumvaluealternate=0 colorcandy#1setcounteralternate1%
 elsecolorpurple#1setcounteralternate0%
 fi


%snippet for the loop taken from https://tex.stackexchange.com/questions/359189/looping-over-strings
%iterates over the supplied string and replaces every letter with colset<letter>
defgobblecharletxchar= 
defassignthencheckafterassignmentxloopgobblechar
defxloop%
 ifxrelaxxchar
 letnext=relax
 else
 colsetxcharletnext=assignthencheck
 fi
 next
defmarkletters#1setcounteralternate0assignthencheck#1relax

begindocument
marklettersHello World
enddocument

Output:

The issue is that I have found no way to skip the spaces (i.e. the "W" in "World" should be purple, not red). I thought it would be easy to simply check if the argument is a space and just skip the rest of the macro, but after hours of searching and just trial-and-error (approaches included etoolbox's ifblank#1, constructs like ifx#1 or even ifnum`#1=32, creating temporary macros [e.g. edeftemp#1ifblanktemp]) I am still no step closer, and I'm definitely out of my depth on this one.

Is there a way to check if the argument of a macro is only a space? If no, are there any other ways to construct this macro? (I'm using XeLaTeX via MiKTeX)

Thanks!

You are not iterating using macro arguments, so you do not need to test a macro argument, just test the token you have already:

documentclass[11pt,a4paper,english,twoside,notitlepage]book

usepackagefontspec
usepackagelmodern
usepackage[english,main=english]babel
usepackagexcolor
usepackageetoolbox


definecolorpurpleHTML2B0057
definecolorcandyHTMLFF0000

newcounteralternate 

%the macro in question
newcommandcolset[1]%
 ifnumvaluealternate=0 colorcandy#1setcounteralternate1%
 elsecolorpurple#1setcounteralternate0%
 fi

makeatletter
%snippet for the loop taken from https://tex.stackexchange.com/questions/359189/looping-over-strings
%iterates over the supplied string and replaces every letter with colset<letter>
defgobblecharletxchar= 
defassignthencheckafterassignmentxloopgobblechar
defxloop%
 ifxrelaxxchar
 letnext=relax
 else
 ifx@sptokenxcharsetcounteralternatenumexpr1-valuealternatefi
 colsetxcharletnext=assignthencheck
 fi
 next
makeatother
defmarkletters#1setcounteralternate0assignthencheck#1relax

begindocument
marklettersHello World
enddocument

You can substitute spaces with something that expands to a space.

documentclass[11pt,a4paper,english,twoside,notitlepage]book

usepackagefontspec
usepackagelmodern
usepackage[english,main=english]babel
usepackagexcolor

definecolorpurpleHTML2B0057
definecolorcandyHTMLFF0000

ExplSyntaxOn
NewDocumentCommandmarklettersm
 
 int_zero:N l_tmpa_int
 tl_set:Nn l_tmpa_tl #1 
 % replace spaces with something different
 tl_replace_all:Nnn l_tmpa_tl ~ c_space_tl 
 tl_map_inline:Nn l_tmpa_tl
 
 tl_if_blank:eTF ##1 
 ~ % don't advance the counter and issue a space
 
 textcolor int_if_odd:nTF l_tmpa_int purple candy ##1 
 int_incr:N l_tmpa_int
 
 
 
prg_generate_conditional_variant:Nnn tl_if_blank:n e T,F,TF,p 
ExplSyntaxOff

begindocument

marklettersHello World

enddocument