LaTeX \if condition

Question

I am using \if condition in a custom command in my LaTeX document. I would like to know where I can find some good explanation about \if command. I followed a Wikibooks \if page but it's just an example; there are not any explanation.

Answer 1

A very good reference for conditionals is contained in Chapter 13 Conditionals of TeX by Topic. Specially, conditionals in TeX have one of the following two forms

\if...<test tokens><true text>\fi
\if...<test tokens><true text>\else<false text>\fi

where \if here is used generically to reference a whole host of conditionals (\if, \ifcat, \ifx, \ifdim, \ifnum, ...).

Let's use your example:

\documentclass{minimal}
\begin{document}

\ifx\mycmd\undefined
undefed
\else
  \if\mycmd1
  defed, 1
  \else
  defed
  \fi
\fi

\def\mycmd{1}

\ifx\mycmd\undefined
undefed
\else
  \if\mycmd1
  defed, 1
  \else
  defed
  \fi
\fi

\def\mycmd{0}

\ifx\mycmd\undefined
undefed
\else
  \if\mycmd1
  defed, 1
  \else
  defed
  \fi
\fi

\end{document}

It uses a nesting of \ifx and \if conditionals. Let's look at the outer \ifx definition first. From section 13.2 Character and control sequence tests (specifically section 13.2.3 \ifx),

Equality of tokens is tested in a stronger sense [...] by

\ifx<token1><token2>

[...]
Control sequences are [...] equal if they are macros with the same parameter text and replacement text, and the same status with respect to \outer and \long. For example,

\def\a{z} \def\b{z} \def\c1{z} \def\d{\a}
\ifx\a\b %is true
\ifx\a\c %is false
\ifx\a\d %is false

Tokens following this test are not expanded.

So, in your example, we start with \ifx\mycmd\undefined. This is true since both \mycmd and \undefined do not exist, hence the output undefed. Subsequent tests \ifx\mycmd\undefined are both false, as \mycmd has been \defined to be something (and is there not undefined and different from \undefined).

Now look at the \if tests. From TeX by Topic (specifically section 13.2.1 \if),

After \if TeX will expand until two unexpandable tokens are obtained, so it is necessary to prefix expandable control sequences and active characters with \noexpand when testing them with \if.

We can ignore the first since \mycmd was undefined. Before the second test, \def\mycmd{1} is executed, so \if\mycmd1 expands \mycmd to 1, leaving you with \if11. This tests the character codes of 1 against 1, and they are equal (of course), hence the output defed, 1.

Lastly, after \def\mycmd{0}, we end up with \if01 (after expansion of \mycmd in \if\mycmd1), which is false as 0 is not 1. Hence, the output defed.