flalign in algorithm

Question

This question is a bit related to the answer provided here: How can I use an align environment flush left?

I use algorithm2e and mathtools to write an algorithm with an equation inside. I want it to be on the left side. Why I have to add additional & (space after it added for clarity) at the beginning of a line when without algorithm environment I didn't have to?

MWE:

\documentclass{article}
\usepackage{mathtools}
\usepackage[ruled,vlined]{algorithm2e}
\begin{document}
\begin{algorithm}
    \begin{flalign}
    &     \mu_\beta \leftarrow \frac{1}{m} \sum_{i=1}^{m}{x_1}&
    \end{flalign}
  \end{algorithm}
\end{document}

Welcome to TEX.SE! Please provide a full minimal working example starting with \documentclass{...} and ending with \end{document} instead of a code snippet. This makes our lives easier and increases the chance of people helping you. — campa, Feb 25 '21 at 13:54

score 1 · Accepted Answer · answered Feb 25 '21 at 14:45

This has nothing to do with algorithm2e so I'll leave that out. Basically align and its siblings are in fact arrays of successive rl columns. flalign then stretches the intercolumn space between these rl pairs to fill the page width. This works of course only if you have at least two such pairs, meaning that you need at least two &: if you have only one & then you just jumped from the r to the l column, but you still have what amsmath calls "an equation column" (which as I said is realy a pair of columns), and this will be still centered.

\documentclass{article}
\usepackage{mathtools}
\usepackage[showframe,textwidth=8cm]{geometry}
\begin{document}
\begin{flalign}
R & L &            R & L \
right & left & right & left
\end{flalign}
%
\begin{flalign}
fooooo & \
baz    &
\end{flalign}
%
\begin{flalign}
& fooooo \
& baz
\end{flalign}
%
\begin{flalign}
& fooooo & \
& baz    & 
\end{flalign}
%
\begin{flalign}
fooooo &&\
baz    &&
\end{flalign}
\end{document}

Note that in the last two examples it is in principle enough to put the second & in only one row, as that is enough to trigger the spreading mechanism.

That explains a lot! So does this means that math adds after first rl "an equation column" and other rl for stretching/centering and that I end up with 3 columns:rl, rl, and rl? So in the 1st example, the last column has 0 length (or still some- I experimented a bit and couldn't find a difference?) — Wilk, Feb 25 '21 at 16:31
@Wilk I don't really understand the first part of your comment. In the first example there are four columns rlrl, and the stretching is between the first pair rl and the second one. — campa, Feb 25 '21 at 16:35
I thought, that by standard align environment have r and l column, and You have written that amsmath adds "an equation column" (l+r), so to make centering one would need other columns- maybe I misunderstood — Wilk, Feb 25 '21 at 17:53

flalign in algorithm

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