# What's the best way of typing the following 58 equations into LaTeX?

For an applied math setting, I need to write multiple inequality constraints into a LaTeX report which are all in the form

x_{i} + x_{j} \ge 1.

I've typed up what "i" and "j" are in an excel sheet as parallel columns. I was wondering if there would be a way to loop through these values to generate the 58 equations, which would save a lot of typing (and it would also really make this scalable if we needed 500 equations, for instance)?

• See the datatool package, you will find that you want ! (ctan.org/pkg/datatool)
– flav
Nov 25 '19 at 7:45
• @flav Thank you!! Could you please give a small example on how I might implement this package? Nov 25 '19 at 7:47
• Other people have answered the technical question of "how to do it". I will suggest a different idea: don't do it. Instead, say $x_{i} + x_{j} \geq 1$ for all pairs $(i,j)$ in... if I saw what you wanted in a book I'd think the author was trying to pad the page length. Nov 26 '19 at 0:29
• @nomen Thanks for the comment. Yes indeed, it would be a good idea to avoid it, but my purpose is to provide a self-contained report for a integer programming problem, and I'd prefer if the reader need not look elsewhere to get all the details required. I've managed to fit it on one page as a reference table of sorts, But thanks for the suggestion! I will keep something like that in mind for future reports. Nov 26 '19 at 1:34

Here's a possible implementation; the delimiters can be changed.

\documentclass{article}
\usepackage{amsmath,multicol}
\usepackage{xparse}

\ExplSyntaxOn

\NewDocumentCommand{\constraints}{omO{x}}
{
\IfValueTF{#1}{\begin{multicols}{#1}\centering}{\begin{center}}
\egreg_costraints:nn { #2 } { #3 }
\IfValueTF{#1}{\end{multicols}}{\end{center}}
}

\seq_new:N \l__egreg_constraints_seq

\cs_new_protected:Nn \egreg_costraints:nn
{
\seq_set_split:Nnn \l__egreg_constraints_seq { \\ } { #1 }
\seq_map_inline:Nn \l__egreg_constraints_seq
{
\__egreg_constraints_item:nn { ##1 } { #2 }
}
}

\cs_new_protected:Nn \__egreg_constraints_item:nn
{
\tl_if_blank:nF { #1 }
{
$#2\sb{\clist_item:nn { #1 } { 1 }} + #2\sb{\clist_item:nn { #1 } { 2 }}\ge1$ \\
}
}

\ExplSyntaxOff

\begin{document}

\constraints{
1,2 \\ 3,4 \\ 5,6
}

\constraints{
1,2 \\ 3,4 \\ 5,6
}[y]

\constraints{
1,2 \\ 3,4 \\ 5,6 \\
1,2 \\ 3,4 \\ 5,6 \\
1,2 \\ 3,4 \\ 5,6 \\
1,2 \\ 3,4 \\ 5,6 \\
1,2 \\ 3,4 \\ 5,6 \\
1,2 \\ 3,4 \\ 5,6 \\
1,2 \\ 3,4 \\ 5,6 \\
1,2 \\ 3,4 \\ 5,6 \\
1,2 \\ 3,4 \\ 5,6 \\
}

\end{document} You can automate the job by exporting the Excel spreadsheet to a comma delimited file (for example subscripts.csv) and reading your data with the readarray package.

### MWE

Assuming that the content of subscripts.csv is:

1,2
1,3
2,39
7,5
26,5


The following code:

\documentclass{article}

\usepackage{forloop}

\newcommand\constraint{$x_{#1} + x_{#2} \ge 1$}

\begin{document}

\newcounter{MyCounter}
\forloop{MyCounter}{1}{\value{MyCounter} < \MyDataROWS}
{
\constraint{\MyData[\theMyCounter,1]}{\MyData[\theMyCounter,2]}\par
}

\end{document}


renders: Use Excel's CONCAT function.

It's probably easiest to do this in Excel, especially if it's a one-time thing. Excel has several text-manipulation functions, and I have often found it a very convenient way to generate several similar lines of text or code. CONCAT(text1,text2,...) concatenates a bunch of text and data values into one text output.

If columns A and B contain your i and j values, enter the following in cell C1:

=CONCAT("x_{", A1, "} + x{", B1, "} \ge 1.")


and hit Enter. You can then copy-paste the function all the way down column C and get the result you want. You can then select all the output cells in column C and paste into your text editor. • concat can also be written using "&", so that the above is equivalent to ="x_{"&A1&"}+x_{"&B1&"}\ge 1." Whether this is simpler depends on the code involved. Nov 25 '19 at 19:22
• Effectively: Q: "How do I use a hammer to plant a screw?" A: "Use a screwdriver" Nov 27 '19 at 3:35
• Funny thing is I haven't used LaTeX in years, but I did know how to solve this problem. Nov 27 '19 at 13:21

I've always found the simplest way to do loops of this sort is the /foreach construct in PGF (\usepackage{pgffor}). The simplest way to use it would be something like:

\foreach \i/\j in {1/2, 1/3, 2/39} {$x_{\i} + x_{\j} \ge 1$ \quad}


which would just put everything on one line with \quads between them.

If you want to format this, you might want to line up the greater-thans, e.g. in an align environment. Unfortunately, just doing a \foreach loop in an align breaks as it can't handle the \\ line breaks properly. Instead, we can use a \foreach loop to build up a macro, and then afterwards wrap that macro in the align environment. Below (example 2) I use \xappto from the etoolbox package to build up the \constraintlist macro in this way.

By building things up in a macro, we can get quite creative. In example 3, I'm using the pgfmath library to only add \\ to the macro after every third constraint (otherwise & is added), which gives us the constraints in three columns, all nicely aligned on the greater-than sign.

Here's the code:

\documentclass{article}

\usepackage{pgffor}     % \foreach
\usepackage{amsmath}    % align* environment
\usepackage{etoolbox}   % \xappto and \gappto
\usepackage{pgfmath}    % \pgfmathparse and \pgfmathresult

\begin{document}

Example 1:
\begin{center}
\foreach \i/\j in {
1/2,
1/3,
2/39,
2019/2020
}{$x_{\i} + x_{\j} \ge 1$ \\}
\end{center}

Example 2:
\def\constraintlist{}
\foreach \i/\j in {1/2, 1/3, 2/39, 2019/2020}{
\xappto\constraintlist{x_{\i} + x_{\j} &\ge 1 \noexpand\\}
}
\begin{align*}
\constraintlist
\end{align*}

Example 3:
\def\constraintlist{}
\foreach \i/\j [count = \index] in {1/2, 1/3, 2/39, 2019/2020, a/b, foo/bar}{
\xappto\constraintlist{x_{\i} + x_{\j} &\ge 1}

\pgfmathparse{mod(\index, 3)==0}
\ifnum\pgfmathresult=1
\gappto\constraintlist{\\}
\else
\gappto\constraintlist{&}
\fi
}
\begin{align*}
\constraintlist
\end{align*}

\end{document}