Random numbers and if statements

Question

I want to produce a random number and with this check an if statement.
But it does not work. I tried the following:
documentclass{minimal}
usepackage[first=0, last=1]{lcg}
newcommand{random}{randarabic{rand}}
begin{document}

defvar{random}

ifnumvar=1
We have a one.
fi

ifnumvar=0
We have a zero.
fi

end{document}`

I thought that the reason might be that the variable var is not of the same type as 0 or 1.
Can anybody help?

egreg · Answer

rand is not “expandable”. It is an instruction for setting a counter, namely rand to some value.
Thus ifnum doesn't find just a number, but a set of instructions to produce one.
You can avoid the issue by using an indirect method:
documentclass{article}
usepackage[first=0, last=1]{lcg}

newcommand{newrandomvar}[1]{%
  newcommand{#1}{}% be sure the name is not taken
  rand
  edef#1{arabic{rand}}% note edef to end up with the actual value
}

begin{document}

newrandomvar{var}

ifnumvar=1
We have a one.
fi

ifnumvar=0
We have a zero.
fi

end{document}

A “more modern” version.
documentclass{article}
usepackage{xparse}

ExplSyntaxOn
NewDocumentCommand{newrandomvar}{m}
 {
  tl_new:N #1
  tl_set:Nx #1 { int_rand:nn { 0 } { 1 } }
 }
NewExpandableDocumentCommand{vartest}{mmm}
 {
  int_compare:nTF { #1 = 0 } { #2 } { #3 }
 }
ExplSyntaxOff

begin{document}

newrandomvar{var}

ifnumvar=1
We have a one.
fi

ifnumvar=0
We have a zero.
fi

We have a vartest{var}{zero}{one}.

end{document}

wipet · Answer

A "more modern" version should look like this:

ifcase pdfuniformdeviate2 We have zeroor We have onefi

The pdfTeX primitive pdfuniformdeviate and TeX primitive ifcase are used here.

The pdfuniformdeviate <number> expands to a random number from 0 to <number>-1.

Random numbers and if statements

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