UK TeX FAQ -- question label spinmacro

Spaces in macros

It’s very easy to write macros that produce space in the typeset output where it’s neither desired nor expected. Spaces introduced by macros are particularly insidious because they don’t amalgamate with spaces around the macro (unlike consecutive spaces that you type), so your output can have a single bloated space that proves to be made up of two or even more spaces that haven’t amalgamated. And of course, your output can also have a space where none was wanted at all.

Spaces are produced, inside a macro as elsewhere, by space or tab characters, or by end-of-line characters. There are two basic rules to remember when writing a macro: first, the rules for ignoring spaces when you’re typing macros are just the same as the rules that apply when you’re typing ordinary text, and second, rules for ignoring spaces do not apply to spaces produced while a macro is being obeyed (“expanded”).

Spaces are ignored in vertical mode (between paragraphs), at the beginning of a line, and after a command name. Since sequences of spaces are collapsed into one, it ‘feels as if’ spaces are ignored if they follow another space. Space can have syntactic meaning after certain sorts of non-braced arguments (e.g., count and dimen variable assignments in Plain TeX) and after certain control words (e.g., in \hbox to, so again we have instances where it ‘feels as if’ spaces are being ignored when they’re merely working quietly for their living.

Consider the following macro, fairly faithfully adapted from one that appeared on comp.text.tex:

\newcommand{\stline}[1]{ \bigskip \makebox[2cm]{ \textbf{#1} } }

The macro definition contains five spaces:

after the opening { of the macro body; this space will be ignored, not because “because the macro appears at the start of a line”, but rather because the macro was designed to operate between paragraphs
after \bigskip; this space will be ignored (while the macro is being defined) because it follows a command name
after the { of the mandatory argument of \makebox; even though this space will inevitably appear at the start of an output line, it will not be ignored
after the } closing the argument of \textbf; this space will not be ignored, but may be overlooked if the argument is well within the 2cm allowed for it
after the } closing the mandatory argument of \makebox; this space will not be ignored

The original author of the macro had been concerned that the starts of his lines with this macro in them were not at the left margin, and that the text appearing after the macro wasn’t always properly aligned. These problems arose from the space at the start of the mandatory argument of \makebox and the space immediately after the same argument. He had written his macro in that way to emphasise the meaning of its various parts; unfortunately the meaning was rather lost in the problems the macro caused.

The principal technique for suppressing spaces is the use of % characters: everything after a % is ignored, even the end of line itself (so that not even the end of line can contribute an unwanted space). The secondary technique is to ensure that the end of line is preceded by a command name (since the end of line behaves like a space, it will be ignored following a command name). Thus the above command would be written (by an experienced programmer with a similar eye to emphasising the structure):

\newcommand{\stline}[1]{%
  \bigskip
  \makebox[2cm]{%
    \textbf{#1}\relax
  }%
}

Care has been taken to ensure that every space in the revised definition is ignored, so none appears in the output. The revised definition takes the “belt and braces” approach, explicitly dealing with every line ending (although, as noted above, a space introduced at the end of the first line of the macro would have been ignored in actual use of the macro. This is the best technique, in fact — it’s easier to blindly suppress spaces than to analyse at every point whether you actually need to. Three techniques were used to suppress spaces:

placing a % character at the end of a line (as in the 1st, 3rd and 5th lines),
ending a line ‘naturally’ with a control sequence, as in line 2, and
ending a line with an ‘artificial’ control sequence, as in line 4; the control sequence in this case (\relax) is a no-op in many circumstances (as here), but this usage is deprecated — a % character would have been better.

Beware of the (common) temptation to place a space before a % character: if you do this you might as well omit the % altogether.

In “real life”, of course, the spaces that appear in macros are far more cryptic than those in the example above. The most common spaces arise from unprotected line ends, and this is an error that occasionally appears even in macros written by the most accomplished programmers.

This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=spinmacro