Random numbers in TeX

Recent versions of pdfTeX contain primitives for generating random integers.

• \pdfuniformdeviate num generates a uniformly distributed random integer in the range [0, num).
• \pdfnormaldeviate generates a normally distributed random integer with mean 0 and “a unit of 65536”. (I've never seen unit used that way, so I'm not sure exactly what the manual means.)

These are both expandable and can be used if you need random numbers for some reason. Here's one toy example that generates coinflips with a biased coin.

\def\coinflip#1{%
        \ifnum#1>\pdfuniformdeviate1000
                H%
        \else
                T%
        \fi
}
\tt
\parindent=0pt
\raggedright
\newcount\n \n=0
\newcount\heads \heads=0
\newcount\tails \tails=0
\loop\ifnum\n<1000
        \if\coinflip{327}H%
                \advance\heads by 1
                H
        \else
                \advance\tails by 1
                T
        \fi
        \advance\n by 1
\repeat

\vskip\baselineskip
\rm
$p=0.327$\par
Heads: \number\heads\par
Tails: \number\tails
\bye

This generates 1000 coin flips with a bias of p = 0.327. It prints the results of each coin flip as well as counting the number of heads and tails.