66

I feel really stupid for asking this, but how do you form more complex if conditionals in TeX? I'm looking for something like:

\ifnum\x=1 OR \ifnum\x=14
    {do this}
\else
    {do that}
\fi

I don't want to have to resort to copy-pasting the entire condition just to change the expression when the body is the same.

3

6 Answers 6

50

There are a number of approaches. Assuming you are looking for a purely primitive-based on, then something like

\ifnum\ifnum\x=1 1\else\ifnum\x=14 1\else0\fi\fi
   =1 %
   <do this>
 \else
   <do that>
 \fi

Thus you use 'secondary' conditionals to convert the original problem into a simple TRUE/FALSE situation, where the 'outer' \ifnum is simply testing for 0 or 1. (This works as TeX keeps expanding until it finds a number when considering the outer \ifnum.)

It's important to get the number-termination correct when using this approach. In the example, the spaces after \x=1 and \x=14 are required to get the correct outcome. With a bit more imagination, you can make more complex constructs using the same approach (for example, you can having combined OR and AND conditions in this way.)

An alternative method if the logic gets complex would be to include the 'payload' as separate macros:

\ifnum\x=1 %
  \expandafter\myfirstcase
\else
  \ifnum\x=14 %
    \expandafter\expandafter\expandafter\myfirstcase
  \else
    \expandafter\expandafter\expandafter\mysecondcase
  \fi
\fi
\def\myfirstcase{do this}
\def\mysecondcase{do that}

This is what you often see with larger 'to do' blocks. The \expandafter use is 'good practice' but may not be needed depending on the exact nature of the code to insert.

2
  • 8
    Yes, TeX is Turing-complete, but...
    – raphink
    Sep 29, 2011 at 12:27
  • 6
    @Raphink There are alternative, logic-based, approaches. However, the question says tex-core, so I went with a pure primitive approach. (I'd personally use LaTeX3's \bool_if:nTF.)
    – Joseph Wright
    Sep 29, 2011 at 12:28
24
\usepackage{etoolbox}

\newcommand{\mytest}[1]{%
  \ifboolexpr{ test {\ifnumcomp{#1}{=}{1}} or test {\ifnumcomp{#1}{=}{14}} }
    {do this}
    {do that}}

\mytest{1} \mytest{14} \mytest{0}

There is also the xifthen package that provides for "composite" tests.

1
  • I like this answer, because the longer a document gets, the more likely it's going to use the etoolbox package (either directly or via some other package). So chances are this is available without extra packages. Apr 27, 2018 at 16:40
8

The package xintexpr implements boolean logic on arithmetic expressions. We can use therein the \pdfstrcmp utility (if the engine makes it available) to compare strings.

\documentclass{article}

\usepackage{xintexpr}

\begin{document}

\def\x{14}

\xintifboolexpr { \x = 1 || \x = 14 }
  {True}
  {False}

\xintifboolexpr { even(\x) && \x < 24 }
  {True}
  {False}

\xintifboolexpr { \x = floor(sqrt(\x))^2 }
  {True}
  {False}

% \pdfstrcmp {text1}{text2} evaluates to 0 if text1 and text2 are equal
% to -1 if text1 comes first in lexicographic order, to +1 else
% To test if the strings are equal we thus use not(\pdfstrcmp {text1}{text2})
% (or we can use the ! as synonym of the "not" function)

\xintifboolexpr { 1=1 && (2=3 || 4<= 4 || not(\pdfstrcmp {abc}{def})) && !(2=4)}
  {True}
  {False}

\xintifboolexpr {\pdfstrcmp {abc}{def} = -1}
  {True}
  {False}

\end{document}

Compiled with PDFLaTeX:

enter image description here

The package can also be used with Plain TeX.

As seen above && is AND and || is OR. It is also possible to write 'and' and respectively 'or' (quotes mandatory).

5

LaTeX3 provides testing of boolean expressions that allows for using infix notation && (for AND), || (for OR) and ! (for negation). In addition to this, parentheses can be used to isolate sub-expressions.

\documentclass{article}

\usepackage{xparse}

\begin{document}

\def\x{14}

\ExplSyntaxOn
\bool_if:nTF { \int_compare_p:n {\x = 1} || \int_compare_p:n {\x = 14} }
  { True }
  { False }
\ExplSyntaxOff

\end{document}

The following (more complex) example was taken from section VI.3 Boolean expressions of the LaTeX3 interfaces documentation:

\int_compare_p:n { 1 = 1 } &&
  (
    \int_compare_p:n { 2 = 3 } ||
    \int_compare_p:n { 4 <= 4 } ||
    \str_if_eq_p:nn { abc } { def }
  ) &&
! \int_compare_p:n { 2 = 4 }
0

With functional package (which provides functional LaTeX2 interface for expl3) you can write the following code:

\documentclass{article}

\usepackage{functional}

\begin{document}

\IgnoreSpacesOn

\TlSet \lTmpaTl {2}
\BoolVarOrTF { \IntCompare {\lTmpaTl} = {1} } { \IntCompare {\lTmpaTl} = {14} }
  { \Result {True} }
  { \Result {False} }

\TlSet \lTmpaTl {14}
\BoolVarOrTF { \IntCompare {\lTmpaTl} = {1} } { \IntCompare {\lTmpaTl} = {14} }
  { \Result {True} }
  { \Result {False} }

\IgnoreSpacesOff

\end{document}
0

You can inspire by the OpTeX trick 0019. Ful expandable conditionals with AND and OR combined by parentheses are described here. The solution is base only on TeX primitives, of course.

5
  • I don't understand how \ifx aa gets expanded into IF a=a. Is a a macro? Is the \ missing? Should that be \ifx \a\a instead? I'm confused…
    – Atcold
    Mar 22 at 17:02
  • The \ifx primitive gets two following tokens (unexpanded) and compares if they have the same meaning. The token a is equal to the token a, it means that they have the same meaning. So, the result of \ifx is true. See TeX in nutshell for summary of TeX primitives.
    – wipet
    Mar 22 at 17:11
  • The token a is always going to be equal to the token a. So, is this an always true check? Is one supposed to swap aa for something else, when using the construct?
    – Atcold
    Mar 22 at 17:22
  • 1
    The \ifx aa is an example which only shows that you can use arbitrary \if... construct. Keep the context of the whole OpTeX trick.
    – wipet
    Mar 22 at 17:29
  • I understand. Thanks.
    – Atcold
    Mar 23 at 17:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.