Why \textbf\& and \textbf\"o behave differently

Question

I know it's not a good habit to write latex this way, But just wonder why \textbf\& is the same as \textbf{\&}, while \textbf\"o is not the same as \textbf{\"o}?

Answer 1

Others have answered most of your question so I will restrict to why you don't get an error with

  \textbf\"o

It's instructive to go

\textbf{hmmmm}

{\tracingmacros2 aaa \textbf\"o bbb}

The first one to make sure the bold fonts are set up before you turn on tracing, and the second one with tracing set, then look in the log file and you will see macros expand:

After checking to see what font setup is going to be used to get the accent (OT1 in the default case) TeX finally gets to:

\OT1\" #1->\@text@composite \OT1\" #1\@empty \@text@composite {\add@accent {127}{#1}}
#1<-\check@icr 

So the argument to \" is \check@icr which is the token that LaTeX adds to the end of all the \text__ commands to check on the italic slope of the following text to see if it needs italic correction.

So the answer to why this doesn't generate an error is basically just luck (or lack of luck, depending on your point of view) \" doesn't get an error as it has an argument, even if not the one you intended, and \check@icr, because it is designed to look ahead to whatever comes after \textbf is written fairly defensively and generally avoids making errors and if it doesn't find something it just doesn't insert italic correction, so even though it is being expanded in entirely the wrong place, inside the accent command it just decides not to insert an italic correction and things move on....

Answer 2

\& is one token while \"o are two tokens. If you dont use braces to delimit the argument, \textbf will affect only the next token it founds.

Answer 3

Expanding on Gonzalo's correct answer.

TeX operates on tokens scanning them one by one (in normal operations). In \textbf\"o there are three tokens

\textbf  \"  o

The syntax rules of TeX specify that when a command has an argument (which is the case for \textbf), this argument is determined as follows:

• if the next token is not an open brace {, that token is the argument
• if the next token is an open brace { the argument is everything that goes from the { up to the } that balances it.

Therefore with \textbf\& the argument is \&; with \textbf\"o the argument is \". The two calls are equivalent to \textbf{\&} and \textbf{\"}o, respectively.

On the other hand, in \textbf{\"{o}} the argument is \"{o} (because the inner braces balance each other).

Always use braces for commands with arguments (unless you belong to the category of ninja programmers who are supposed to know what they're doing) and you'll be OK.

Answer 4

\& is special purposes within LaTeX. if you use \textbf it will tag 1 character only. so Don't use like this. Use \textbf{text}