Alpha counter in a foreach loop

Question

I need an alpha counter in a foreach loop :

\documentclass{article}
\usepackage{tikz}

\makeatletter

\newcommand{\TRAA}[1]{%
    % node/angle list
    \foreach \x/\y [count=\i from 1] in {#1} {%
        \expandafter\xdef\csname Pt@\Alph{\i}\endcsname{\x}
        \expandafter\xdef\csname Ang@\Alph{\i}\endcsname{\y}

    }
\draw (\Pt@A) -- (\Pt@B) ;
}

\makeatother

\begin{document}

essai

\begin{tikzpicture}
\coordinate (C) at (0,0) ;
\coordinate (B) at (109:7) ;

\TRAA{B/71,C/45}

\end{tikzpicture}
\end{document}

Answer 1

The \Alph macro requires a counter (as stated by Sergei already), but internally it uses \@Alph, which does not need a counter but the literal number only.

So

\makeatletter\@Alph{5}\makeatother

would print E, since E is the 5th letter in the Latin alphabet.

Replacing \Alph by \@Alph works, the necessary \makeatletter...\makeatother pair is in O.P. code already.

\documentclass{article}
\usepackage{tikz}

\makeatletter

\newcommand{\TRAA}[1]{%
    % node/angle list
  \foreach \x/\y [count=\i from 1] in {#1} {%
      \expandafter\xdef\csname Pt@\@Alph{\i}\endcsname{\x}
      \expandafter\xdef\csname Ang@\@Alph{\i}\endcsname{\y}
    }
    \draw (\Pt@A) -- (\Pt@B) ;
  }

\makeatother

\begin{document}

essai

Omitting a counter \makeatletter\@Alph{5}\makeatother

\begin{tikzpicture}
\coordinate (C) at (0,0) ;
\coordinate (B) at (109:7) ;

\TRAA{B/71,C/45}

\end{tikzpicture}
\end{document}