LaTeX programming: how to implement conditionals

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Entry available at: http://handyfloss.net/2007.08/latex-programming-how-to-implement-conditionals/

I have recently come across a problem while creating a LaTeX style (for making A0-size posters). Maybe it could be avoided or solved more elegantly, but I wanted to solve it with conditionals.

Basically, what I wanted to do was define a command (actually, an environment) that accepted one argument, and make it return different output, depending on the argument:

if (argument equals something) then
  do something
else
  do somethingelse
end if

It gave me some headaches to get it, but I also learned some interesting things on the way. There are at least two ways of playing with conditionals: defining boolean variables or directly using logical comparisons.

Defining logical valiables

We can define a logical variable logvar as follows:

\newif\iflogvar

By default, it is set to false. We can set it to true by:

\logvartrue

and back to false by:

\logvarfalse

The variable can be used in a conditional as follows:

\iflogvar
  aaaa
\else
  bbbb
\fi

You can think of the above code as a single object, the output value of which will be “aaaa” if logvar is true, and “bbbb” if false. Basically, the following code will, thus, output “Today is great“:

Today

\newif\ifismonday

\ismondayfalse

\ifismonday
  sucks!
\else
  is \textbf{great}
\fi

Direct logic comparison

The example I provide works for numbers, but check this page for more info. Recall that LaTeX works with integers (counters) and text strings. As far as I know, floating point operations are impossible in LaTeX (nothing is actually impossible in LaTeX, just veeery difficult).

For example, defining the following command in the preamble:

\newcommand{\isitthree}[1]
{
  \ifnum#1=3
    number #1 is 3
  \else
    number #1 is not 3
  \fi
}

allows us to call it in the document, so the following outputs “We know that number 33 is not 3″:

We know that \isitthree{33}

Nesting

Obviously the conditionals can be nested (put one inside another), when more than one condition needs to be tested. For example:

Today

\newif\ifismonday
\newif\ifistuesday

\ismondayfalse
\istuesdaytrue

\ifismonday
  sucks!
\else
  \ifistuesday
    almost sucks.
  \else
    is \textbf{great}
  \fi
\fi