In a previous post, I explained how to get a control sequence
\char to be
\let to a character in a "TeX string". I will now show how to iterate over the whole
Hello world!, using it for instance to count the number of characters.
The idea is to put a
marker at the end of the string, and having a function that continues eating characters until the marker is found. I will use the macro
\nil (which does not even need to be defined) as a marker. As an example, look at the following code:
\let\char=a
\ifx\char\nil yes\else no\fi.
\let\char=\nil
\ifx\char\nil yes\else no\fi.
This produces "
no. yes.", which is exactly what we need.
Now the problem is that TeX processes token on-line, so we need a macro, e.g. that would behave like this: "if the next token is not
\nil, execute me again, i.e., put me again before the remaining of the tokens.
The magic TeX construct that we need is called
\afterassignment. It saves the next token and insert it back in the stream after the next assignment, which will be a
\let in our case. As an example, consider the following code:
\def\showchar{Char is [\char].}
\afterassignment\showchar\gobblechar Hello world!
It produces "
Char is [H].ello world!" since
\showchar is put just after `H' has been assigned to
\char.
I will now use
\ifx and
\afterassignemnt in two macros that will call each other. The first one handles the assignment of one token (character) to
\char and calls the second; The second check if the token is
\nil and if not, calls the first.
\def\assignthencheck{\afterassignment\checknil\gobblechar}
\def\checknil{%
\ifx\char\nil%
STOP%
\let\next=\relax%
\else%
(\char)\let\next=\assignthencheck%
\fi%
\next%
}
\assignthencheck Hello world!\nil
This produces "
(H)(e)(l)(l)(o)( )(w)(o)(r)(l)(d)(!)STOP"
The
\assignthencheck macro should be clear after the explanation above. So, after
\char has been
\let to the first character `H',
\checknil is called. This one assign the
\next depending on what is
\char: since it is not
\nil, I asked TeX to print
\char between parentheses (to see what happens), then
\next is
\let to
\assignthencheck. At the end of the macro,
\next is inserted back on the stream of tokens, i.e., before
ello world!\nil. Hence, all characters are processed and printed in parentheses until
\nil is found. In which case there is nothing left to do, so I print "STOP" and assign
\next is
\relax, which means "do nothing", to ensure the
\next at the end of the macro will not do anything.
This is now easy to combine the whole to count the number of characters, using a TeX counter. I put here the complete example from the beginning for completeness.
\def\gobblechar{\let\char= }
\newcount\charcount
\def\countunlessnil{%
\ifx\char\nil \let\next=\relax%
\else%
\let\next=\auxcountchar%
\advance\charcount by 1%
\fi\next
}%
\def\auxcountchar{%
\afterassignment\countunlessnil\gobblechar%
}
\def\countchar#1{\edef\xx{#1}\charcount=0 \expandafter\auxcountchar\xx\nil}
\def\shownumchar#1{%
\countchar{#1}%
There are \the\charcount\ characters in [#1].%
}
\shownumchar{Hello world!}
\shownumchar{ Hello world!}
\def\text{Hello world!}
\def\atext{ \text\ }
\shownumchar{\atext}
This produces
There are 12 characters in [Hello world!].
There are 13 characters in [ Hello world!].
There are 14 characters in [ Hello world! ].
The
\shownumchar macro is easy, it just calls
\countchar then uses the TeX counter
\charcount to display the number of characters in its argument.
The
\countchar macro uses another trick that allows it to be called on macros that themselves call macros (last example, with
\atext): the
\edef expands its argument as much as possible before assigning it to
\xx, then the
\expandafter ensures
\xx is expanded before
\auxcountchar (otherwise, the number of characters would be 1, since we are in fact counting
tokens and an unexpanded macro is a single token).
Final note: for completeness reasons, I must mention that in LaTeX there exists a
\@tfor macro that iterates over a list of tokens. However, it does not handle spaces: they are skipped over unless explicit like "
\ ".
