equationsmath-mode
When I try this:
\begin{equation}
\overline{bel}(x_{t}) = p ( x_{t} \textbar z_{1:t-1} , u_{1:t} )
\end{equation}
I get following warning:
Command \textbar invalid in math mode on input line 12.
I aim to produce following output:
What should I do ?
At the start of an alignment cell TeX expands macros looking for \omit (\multicolumn) the math start in the cell template has not been added at this point so \ifmmode is always false.
You need to put \relax in front of it to kill the pre-scan for \omit or make the command containing this construct be robust so it doesn't expand in this context (using LaTeX \DeclareRobustCommand or eTeX \protected)
To show the various approaches, this plain eTeX file
\def\foo{\ifmmode \phi\else fi\fi}
\protected\def\pfoo{\ifmmode \phi\else fi\fi}
\def\rfoo{\relax\ifmmode \phi\else fi\fi}
\halign{#)&$#$\hfill&\quad dif#cult\cr
0&\omit $\phi$\hfill&\omit \quad dif{f}icult\cr
1&\omit $\phi$\hfill&\omit \quad difficult\cr
2&\foo&\foo\cr
3&\pfoo&\pfoo\cr
4&\rfoo&\rfoo\cr
}
\bye
produces
(1) is what you want.
(0) loses the ffi ligature.
(2) the non robust unprotected command gets the ffi ligature but messes up the if math test.
(3) the etex \protected macro does the same as (1).
(4) using \relax (which would be the same as using \DeclareRobustCommand in LaTeX in this context) gets the math test correct but loses the ffi ligature.
There is no need to use \textit (actually it's wrong):
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
m(A) &= a, \\
\bar{A} &= 1-a,\\
m(B) &= 0,\quad\text{for $B \neq A$ or $B \neq \bar{A}$}
\end{align*}
\end{document}
Notice how math mode is used inside \text to get spacing right without doing strange things. I don't think it's good to move the textual condition much to the right as you seem to want.
Best Answer
Simply typing
|should be completely sufficient:If you show us what the result is supposed to be, we can maybe help a little more.
Please, also provide a MWE (minimum working example, that is a complete file with preamble and only what is actually needed).