How do I avoid eqnarray?

Question

We have been told to avoid eqnarray and use align instead.

The output of eqnarray here:

\newcommand{\Prob}{\operatorname{\mathbb{P}}}
\newcommand{\tp}{\operatorname{tp}}
\newcommand{\fp}{\operatorname{fp}}
\newcommand{\fn}{\operatorname{fn}}
\newcommand{\tn}{\operatorname{tn}}
\newcommand{\Act}{\operatorname{Actual}}
\newcommand{\Pre}{\operatorname{Predicted}}
\newcommand{\Ar}{\operatorname{Accuracy}}
\newcommand{\Ps}{\operatorname{Precision}}
\newcommand{\Rc}{\operatorname{Recall}}

\begin{eqnarray*}
  \Ar =& \Prob(\Act=\Pre)  &= \frac{\tp+\tn}{N} \\
  \Rc =& \Prob(\Pre=1|\Act=1)  &= \frac{\tp}{\tp+\fn} \\
  \Ps =& \Prob(\Act=1|\Pre=1)  &= \frac{\tp}{\tp+\fp}
\end{eqnarray*}

is imperfect:

but I cannot get even that with either align or alignat...

Answer 1

\documentclass{article}
\begin{document}

\begin{eqnarray*}
foo &=& bar &=& baz \\
foobar &=& barquux &=& bazzot
\end{eqnarray*}
\end{document}

produces

! Extra alignment tab has been changed to \cr.

the standard eqnarray only takes two &

Using

\documentclass{article}
\usepackage{amsmath}
\begin{document}

\begin{alignat*}{3}
foo &= bar &&= baz \\
foobar &= barquux &&= bazzot
\end{alignat*}
\end{document}

produces

With the updated example, made into a complete document, the results are

\documentclass{article}
\usepackage{amsmath,amsfonts}
\begin{document}

\newcommand{\Prob}{\operatorname{\mathbb{P}}}
\newcommand{\tp}{\operatorname{tp}}
\newcommand{\fp}{\operatorname{fp}}
\newcommand{\fn}{\operatorname{fn}}
\newcommand{\tn}{\operatorname{tn}}
\newcommand{\Act}{\operatorname{Actual}}
\newcommand{\Pre}{\operatorname{Predicted}}
\newcommand{\Ar}{\operatorname{Accuracy}}
\newcommand{\Ps}{\operatorname{Precision}}
\newcommand{\Rc}{\operatorname{Recall}}

\begin{eqnarray*}
  \Ar =& \Prob(\Act=\Pre)  &= \frac{\tp+\tn}{N} \\
  \Rc =& \Prob(\Pre=1|\Act=1)  &= \frac{\tp}{\tp+\fn} \\
  \Ps =& \Prob(\Act=1|\Pre=1)  &= \frac{\tp}{\tp+\fp}
\end{eqnarray*}

\begin{alignat*}{3}
  \Ar &= \Prob(\Act=\Pre)  &&= \frac{\tp+\tn}{N} \\
  \Rc &= \Prob(\Pre=1|\Act=1)  &&= \frac{\tp}{\tp+\fn} \\
  \Ps &= \Prob(\Act=1|\Pre=1)  &&= \frac{\tp}{\tp+\fp}
\end{alignat*}



\end{document}

Note how align gives the = on the longest entry just the space normally given to = and just adds space as necessary to align. eqalign always pads the central column with additional space causing asymmetric spacing around = on all rows.

Answer 2

For your updated example code, I actually suggest that you use an array environment, with the value of \arraystretch increased to mimic a displaymath setting.

Two additional suggestions: (i) for the "conditioning operator", use \mid instead of | -- you'll get much better spacing; (ii) most of the items you've declared as math operators are, as far as I can tell, simple text strings; I would use \textrm instead of \operatorname in the definitions of \tp, \fp, etc.

The following screenshot shows the outputs of the array and eqnarray* environments:

\documentclass{article}
\usepackage{amsmath,amsfonts}
\DeclareMathOperator{\Prob}{\mathbb{P}}
\newcommand{\tp}{\textnormal{tp}}
\newcommand{\fp}{\textnormal{fp}}
\newcommand{\fn}{\textnormal{fn}}
\newcommand{\tn}{\textnormal{tn}}
\newcommand{\Act}{\textnormal{Actual}}
\newcommand{\Pre}{\textnormal{Predicted}}
\newcommand{\Ar}{\textnormal{Accuracy}}
\newcommand{\Ps}{\textnormal{Precision}}
\newcommand{\Rc}{\textnormal{Recall}}
\begin{document}
Output of \texttt{array}
\[
\renewcommand\arraystretch{2}
\begin{array}{r @{{}={}} c @{{}={}} l} % "{}={}" is inserted in the intercolumn spaces
  \Ar & \Prob(\Act=\Pre)          & \dfrac{\tp+\tn}{N} \\
  \Rc & \Prob(\Pre=1\mid \Act=1)  & \dfrac{\tp}{\tp+\fn} \\
  \Ps & \Prob(\Act=1\mid \Pre=1)  & \dfrac{\tp}{\tp+\fp} \\
\end{array}
\]

\bigskip
Output of \texttt{eqnarray*}
\begin{eqnarray*}
  \Ar =& \Prob(\Act=\Pre)          &= \frac{\tp+\tn}{N} \\
  \Rc =& \Prob(\Pre=1\mid \Act=1)  &= \frac{\tp}{\tp+\fn} \\
  \Ps =& \Prob(\Act=1\mid \Pre=1)  &= \frac{\tp}{\tp+\fp}
\end{eqnarray*}

\end{document}

Answer 3

Here is an application of a macro I showed in Aligning equations with left and right comment (you find there explanations).

\documentclass{article}
\usepackage{amsmath,amssymb}

\newcommand{\Prob}{\operatorname{\mathbb{P}}}
\newcommand{\tp}{\mathrm{tp}}
\newcommand{\fp}{\mathrm{fp}}
\newcommand{\fn}{\mathrm{fn}}
\newcommand{\tn}{\mathrm{tn}}
\newcommand{\Act}{\mathrm{Actual}}
\newcommand{\Pre}{\mathrm{Predicted}}
\newcommand{\Ar}{\mathrm{Accuracy}}
\newcommand{\Ps}{\mathrm{Precision}}
\newcommand{\Rc}{\mathrm{Recall}}

% the magic trick, see https://tex.stackexchange.com/a/209732/4427
\makeatletter
\newcommand{\Cen}[2]{%
  \ifmeasuring@
    #2%
  \else
    \makebox[\ifcase\expandafter #1\maxcolumn@widths\fi]{$\displaystyle#2$}%
  \fi
}
\makeatother

\begin{document}

\begin{alignat}{2}
  \Ar ={}& \Cen{2}{\Prob(\Act=\Pre)}        &&= \frac{\tp+\tn}{N} \\
  \Rc ={}& \Cen{2}{\Prob(\Pre=1\mid\Act=1)} &&= \frac{\tp}{\tp+\fn} \\
  \Ps ={}& \Cen{2}{\Prob(\Act=1\mid\Pre=1)} &&= \frac{\tp}{\tp+\fp}
\end{alignat}

\end{document}

We need two groups of left-right columns, with no space between them, so I use alignat (the alignat* version will not print numbers). In the column we want centered, which is the second, I enclose the material as the second argument to \Cen (the first argument is the column number). We need &&= because the last column must be left aligned.

Note that the only real operator name is \Prob, the other symbols are just variables, so they should be defined with \mathrm. Note also \mid instead of |, which gives a better spacing.

Whether to center the second column is a judgment question; in my document I'd simply use gather or align to put the three equations flush left.

\begin{gather}
  \Ar = \Prob(\Act=\Pre) = \frac{\tp+\tn}{N} \\
  \Rc = \Prob(\Pre=1\mid\Act=1) = \frac{\tp}{\tp+\fn} \\
  \Ps = \Prob(\Act=1\mid\Pre=1) = \frac{\tp}{\tp+\fp}
\end{gather}

\begin{align}
&  \Ar = \Prob(\Act=\Pre) = \frac{\tp+\tn}{N} \\
&  \Rc = \Prob(\Pre=1\mid\Act=1) = \frac{\tp}{\tp+\fn} \\
&  \Ps = \Prob(\Act=1\mid\Pre=1) = \frac{\tp}{\tp+\fp}
\end{align}

Answer 4

You can centre the middle column with the makebox or the eqparbox packages. The eqparbox commands use a tag that makes all boxes that use that tag have width equal to the largest width of boxes; by default, their contents is centred in the box.

\documentclass{article}
\usepackage{amsmath}
\usepackage{makebox}

\usepackage{eqparbox}

\begin{document}

\begin{alignat*}{3}
foo &= \makebox*{$ barquux $} {$ bar $}&&= baz \\
foobar &= barquux &&= bazzot
\end{alignat*}

\begin{alignat*}{3}
foo &= \eqmakebox[Col]{$ bar$} &&= baz \\
foobar &= \eqmakebox[Col]{$ barquux $} &&= bazzot
\end{alignat*}

\end{document}