\hfill in math mode

Question

Is there any easy way of declaring a new command which emulates \hfill in math mode. I don't want flalign or something similar. I want to use the command in the same way (and with the same freedom) I use \hfill.

Is that possible?

i.e. this is an example:

\begin{align*}
    a + b + c &= a + b + c + d + e + f + g + h + i \\
    & \mathhfill \text{(foo)} 
\end{align*}

where I want (foo) to be right aligned (under + h + i and here

\begin{align*}
    a + b + c + d + e + f + g + h + i &= a + b + c \\
    \text{(foo)} \mathhfill &
\end{align*}

I want the text aligned on the left (just under the a + b +. But I don't want flalign or other solutions. Because there are lots of situations (this is a minimal example). And I would like to see a solution which work just as \hfill.

EDIT: well, as Carlisle said, it works in $a \hfill b$, but I would like to see it working in align, i.e.

EDIT2: The answers solved my needs. But it would be great if somebody would give a command which works exactly like \hfill or i.e. \dotfill in align.

Answer 1

You can do it, but not with \hfill, because of how align works. Here's a way:

\documentclass{article}
\usepackage{amsmath}

\makeatletter
\newcommand{\pushright}[1]{\ifmeasuring@#1\else\omit\hfill$\displaystyle#1$\fi\ignorespaces}
\newcommand{\pushleft}[1]{\ifmeasuring@#1\else\omit$\displaystyle#1$\hfill\fi\ignorespaces}
\makeatother

\begin{document}
\begin{align*}
    a + b + c &= a + b + c + d + e + f + g + h + i \\
    & \pushright{\text{(foo)}}
\end{align*}


\begin{align*}
    a + b + c + d + e + f + g + h + i &= a + b + c \\
    \pushleft{\text{(foo)}} &
\end{align*}

\end{document}

A slightly different implementation allows you to use \hfill where you want:

\documentclass{article}
\usepackage{amsmath}

\makeatletter
\newcommand{\specialcell}[1]{\ifmeasuring@#1\else\omit$\displaystyle#1$\ignorespaces\fi}
\makeatother

\begin{document}
\begin{align*}
    a + b + c &= a + b + c + d + e + f + g + h + i \\
    & \specialcell{\hfill\text{(foo)}}
\end{align*}


\begin{align*}
    a + b + c + d + e + f + g + h + i &= a + b + c \\
    \specialcell{\text{(foo)}\hfill} \\
    \specialcell{\hfill\text{(foo)}\hfill}
\end{align*}

\end{document}

I'd recommend against using the simple \omit.

Answer 2

In align you can use \omit at the start of cells:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
    a + b + c &= a + b + c + d + e + f + g + h + i \\        
              &\omit\hfill foo\\        
             a&=b\\        
   \omit text & =c \\
   \omit\hfill text&=d
\end{align*}
\end{document}

Answer 3

Another way (the only one that worked for me) is to use \hskip. You will have to adjust it manually, though.

\begin{align}
     & x + y, &  \text{(Foo)} \hskip0.7\textwidth 
\end{align}

Answer 4

The most obvious answer is \hfill as demonstrated by this plain TeX file

$a\hfill b$

\bye

Answer 5

I couldn't for the life of me get @egreg's solution to work for me: always got misplaced \omit, no matter what I tried. I ended up finding a little bit of a hacky solution, but it works for me. It uses some code found here. Here's the MWE:

\documentclass{article}

\usepackage{amsmath}

\usepackage{tikz}
\usetikzlibrary{calc}

\newcommand{\currentsidemargin}{%
  \ifodd\value{page}%
    \oddsidemargin%
  \else%
    \evensidemargin%
  \fi%
}

\newlength{\whatsleft}

\newcommand{\measureremainder}[1]{%
\begin{tikzpicture}[overlay,remember picture]
    % Helper nodes
    \path (current page.north west) ++(\hoffset, -\voffset)
        node[anchor=north west, shape=rectangle, inner sep=0, minimum width=\paperwidth, minimum height=\paperheight]
        (pagearea) {};


    \path (pagearea.north west) ++(1in+\currentsidemargin,-1in-\topmargin-\headheight-\headsep)
        node[anchor=north west, shape=rectangle, inner sep=0, minimum width=\textwidth, minimum height=\textheight]
        (textarea) {};

    % Measure distance to right text border
    \path let \p0 = (0,0), \p1 = (textarea.east) in
        [/utils/exec={\pgfmathsetlength#1{\x1-\x0}\global#1=#1}];

\end{tikzpicture}%
}

\newcommand\rightalignedtext[1]{\measureremainder{\whatsleft}\begin{minipage}[t]{\whatsleft}\hfill#1\hspace*{1em}\end{minipage}}

\begin{document}

\begin{equation*}
    \begin{aligned}
        a + b + c &= a + b + c + d + e + f + g + h + i \rightalignedtext{(foo)}
    \end{aligned}
\end{equation*}

\end{document}