I am a LaTeX guy, slowly working into the miracles of TeX.
Can anybody tell me the differences between \def, \edef, \gdef and \xdef?
Where shall I use which, what are the pros and cons? Or is it suspected to be unclean code, if I mix TeX commands in LaTeX?
Please give a short example, of how to use the command.
There are no pros and cons: \def and \edef perform different tasks. With
\def<cs><parameter text>{<replacement text>} you define <cs> to look for its arguments (if any) and to be replaced by <replacement text>, which is not interpreted in any way at definition time. With
\edef<cs><parameter text>{<replacement text>} the replacement text is fully expanded at definition time.
For instance, if we have
\def\aaa{aaa} \def\bbb{x\aaa} \edef\ccc{y\aaa} \def\aaa{AAA} a call like
\bbb \ccc would produce
xAAAyaaa
because the replacement text of \ccc is what remains after full expansion, so \edef\ccc{y\aaa} is the same as \def\ccc{yaaa}.
Note that the expansion in \edef is done at definition time, so parameter tokens like #1 and so on will be untouched.
A less silly example: if you want that \thissection expands to the value of the section counter at the time the command is defined, you have to say
\edef\thissection{\thesection} because this “freezes” the value by doing the expansion at definition time. To the contrary, with \def\thissection{\thesection} the macro \thissection would print the current section number.
LaTeX has the variant \protected@edef that avoids some quirks with “robust macros”, so something like \protected@edef\cs{\textbf{a}} works whereas \edef\cs{\textbf{a}} wouldn't (there's plenty of examples on the site).
About \gdef and \xdef there's not much to say: the former is completely equivalent to \global\def and the latter to \global\edef (assuming primitive meaning of \global, of course). LaTeX has \protected@xdef.
\global. :-) \global simply means that the definition persists when the current group closes, instead of the pre-group definition being restored. \?defs and did not find an answer. Now it should be found in either way. I am personally glad, I finally learned about that TeX stuff. Next target: what is \long good for? \long could be useful; i couldn't find any, although it's not easy to look for. i did find this: Is there a simple way to retroactively add the \long prefix to a macro's definition?, but it presupposes that someone already knows the meaning. actually, the meaning is simple: a \long macro can include a paragraph break, whereas a macro that is not \long will throw an error if it encounters \par or a blank line.
\gdefand\xdef.texdoc, so it's a simpletexdoc programmingfor me.\gdef(\xdef) is merely a shorthand for\global\def(\global\edef), so the question about these can be reduced to\defand\edef.