Special treatment for last element in @for

Question

I'm trying to make a macro such that I give it arguments like this
sqrtnorm{x,y,z}

such that it expands to,
sqrt{x^2+y^2+z^2}

I'm trying (for no special reason) doing this with @for command, this is what I've attempted
   makeatletter
    newcommandsqrtnorm[1]{
        sqrt{
    @forlevar:=#1do{
        {levar}^2+}
        }
    }
    makeatother

It's perfect in every way, except that there is a little + hanging around at the end. Is there a way to solve this?

campa · Answer

Instead of appending a + I prepend a macro @tempa, which defines itself as +. In this way at the first term the definition is executed, and only from the second term the + appears.
documentclass{article}

makeatletter
newcommand*{sqrtnorm}[1]{%
   def@tempa{def@tempa{+}}%        
   sqrt{%
      @forlevar:=#1do{@tempa{levar}^2}%
    }%
}
makeatother

begin{document}

[
sqrtnorm{x,y,z}
]

end{document}

I've added a couple of % to hide the end of line. That's not crucial here because the macro appears in math mode, but it's not a bad idea either (and saves token memory).

egreg · Answer

With expl3:
documentclass{article}
usepackage{amsmath}

ExplSyntaxOn
NewDocumentCommand{sqrtnorm}{m}
 {
  % make a sequence from the argument
  seq_set_from_clist:Nn l_tmpa_seq { #1 }
  % add ^2 to every item
  seq_set_map:NNn l_tmpb_seq l_tmpa_seq { ##1^2 }
  % deliver the sequence with + between items
  sqrt{seq_use:Nn l_tmpb_seq { + }}
 }
ExplSyntaxOff

begin{document}

$sqrtnorm{x,y,z}$

end{document}

Special treatment for last element in @for

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