Question

I start by defining a command to store the string “Hello”:

\newcommand{\textstring}{Hello}

I would like to append the string ” world” but unfortunately this code causes an error:

\renewcommand{\textstring}{\textstring world}

Answer 1

You can accomplish this by using \expandafter. For example:

% redefine \textstring by appending " world" to it
\expandafter\def\expandafter\textstring\expandafter{\textstring { }world}

If you don’t use \expandafter then you end up with a recursion problem. You can read more about it here.