[OS X TeX] Puzzled by an accented i

Ross Moore ross.moore at mq.edu.au
Mon Mar 14 01:01:48 CET 2011

Hello Josep,

On 14/03/2011, at 9:48 AM, Josep Maria Font wrote:

> But I thought that to obtain the "right" way one had to use \i. When I learned TeX (around 25 years ago), \' was a command that puts an accent on top of the first character at its right, whatever it is, so that for instance \'n and \'s could be used in Polish. In those times, \'i really produced a dotted accented i, and that's why Knuth invented the \i (and \j). Later on, we learned that in LaTeX it was better to consistently write \'{\i}...

It is probably still that way in Plain TeX.
Here a macro is essentially a pointer to a coding subroutine.

But LaTeX is more of a "declarative" language.
You declare what it is that you want, and the system provides it
as best it can.

\' now means "give me the version of the following character,
having an accent over it, using the current font.

To achieve this, LaTeX's NFSS (font selection scheme) tests for 
the existence of several methods, in a specific order. These
are the coding blocks to be used, taking into account both
which accent and the letter being modified.
Thus when you know that it is an 'i' following, then you
don't have to worry about whether it is dotted or dotless,
because the method being called can take that into account.

To see this happening, use:

>>>  {\tracingall \'i }  % braces round the {i} are irrelevant

Depending upon what other packages are loaded, concerning font
encodings, you may find a line such as:

>>>    \\OT1\'-i ->\@tabacckludge '\i 

which is the point at which the 'i' loses its dot, so to speak.
After further macro expansion, we get to:

>>> \@secondoftwo #1#2->#2
>>> #1<-\\OT1\'-\i 
>>> #2<-\add at accent {19}{\i }

which is choosing the  \add at accent  macro with specific accent
to be placed over \i (not over i ).

However, a macro package, or author, could have made a definition
of  \\OT1\'-\i  to do whatever (s)he wants.
It such a macro existes, then it will be used instead.

Similarly, doing 

>>>  {\tracingall \'\i }

gets to the same point:

>>> \@secondoftwo #1#2->#2
>>> #1<-\\OT1\'-\i 
>>> #2<-\add at accent {19}{\i }

> Thus, probably with time the definition of the \' command has changed.

Yes. The declarative approach is much more powerful,
allowing the possibility of testing your macro arguments
and other stylistic parameters, to help decide exactly what
should be displayed in response to specific input.

On the other hand, it means much more processing may
need to be done, in many situations where a simple decision
could have more quickly lead to the same piece of coding.

Computers are much, much faster now, so that extra coding
doesn't result in any noticeable slow-down.

> Which makes all the more interesting to know that with the package tipa this is possible (in my case, precisely to write down a typos list showing that this should *not* be done; the original was written more than 20 years ago!).

> JMaF

Hope this helps,


Ross Moore                                       ross.moore at mq.edu.au 
Mathematics Department                           office: E7A-419      
Macquarie University                             tel: +61 (0)2 9850 8955
Sydney, Australia  2109                          fax: +61 (0)2 9850 8114

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