How to assign values with foreach?

Question

I have trouble to assign values with \foreach. Can somebody help me? The following code is wrong. The purpose is to asing the values: \lx1=1, \ly1=2, \lz1=3, for example.

\foreach \x/\y in {\lx1/1,\ly1/2,\lz1/3}
{%
\def\x{\y}
\x{\y}
{

Answer 1

There are several reasons, why this does not work:

• \lx1 is not a command name, it consists of the command \lx, followed by digit 1. Either replace 1 by a letter or the usage is a little more complex via \csname.

• \x and \y contain the x and y values, the definition of \x defines \x, not the macro inside and \y whill change with every loop. \expandafter expands a macro one level:

  \expandafter\def\x{...}
  

  Doing the same with \y requires lots of more \expandafter, therefore using \let is easier here.

• \foreach puts the loop body inside groups, thus local definitions will be lost. This can be solved by global definitions.

Example:

\usepackage{pgffor}

\foreach \x/\y in {\lxA/1,\lyA/2,\lzA/3}
{%
  \global\expandafter\let\x\y
}

Or with \lx1 as macro:

\foreach \x/\y in {\lx1/1,\ly1/2,\lz1/3}
{%
  \begingroup
    \escapechar=-1 % suppresses backslash with \string
    \global\expandafter\let
    \csname \expandafter\string\x\endcsname\y
  \endgroup
}

\typeout{\expandafter\string\csname lx1\endcsname:
  \expandafter\meaning\csname lx1\endcsname}

Addition for the fans of \expandafter:

\gdef can be used instead of \global\let by using additional \expandafter:

\foreach \x/\y in {\lxA/1,\lyA/2,\lzA/3}
{%
  \expandafter\expandafter\expandafter
  \gdef\expandafter\x\expandafter{\y}%
}

\typeout{\string\lxA: \meaning\lxA}

\foreach \x/\y in {\lx1/1,\ly1/2,\lz1/3}
{%
  \begingroup
    \escapechar=-1 % suppresses backslash with \string
    \expandafter\gdef
    \csname \expandafter\string\x\expandafter\endcsname
    \expandafter{\y}%
  \endgroup
}

\typeout{\expandafter\string\csname lx1\endcsname:
  \expandafter\meaning\csname lx1\endcsname}

As can be seen, the version with \let is more efficient and elegant.