I have an expression of the form:


I would like to replace each of the variables n with a different variable, like:


The order doesn't matter, but it matters that each n have a different name.

I can find the positions of the n's simply with:


and this returns a list of positions {{1}, {2, 1}, {5, 1}} as expected. I would then like to run through this list and make replacements in those positions. However, I don't want to have to do this by hand. Clearly I could do:


but the real expression I have is many thousands of times larger.

I have tried using:

Apply[Part, Join[{expr}, pos[[3]]]]=n2

which would allow be to do this in a loop, or functionally, but this doesn't alter the element of the expression as I would like. I have tried playing with Hold in various places but it doesn't seem to work.

Anything simple that I am missing?

  • 5
    $\begingroup$ expr /. n :> Unique@n $\endgroup$ Jan 9, 2014 at 16:30
  • 1
    $\begingroup$ expr = x + y + n + Sin[n] + n^2; j = 0; expr /. n :> (j++; nn[j]) $\endgroup$ Jan 9, 2014 at 16:32
  • 1
    $\begingroup$ @DanielLichtblau the reason is that procedural code gets a penalty :D HNY BTW $\endgroup$
    – Yves Klett
    Jan 9, 2014 at 16:35
  • 1
    $\begingroup$ @Yves Klett I meant "beaten" in terms of "first to appear". But I agree his is the better method. Not faulting mine specifically for procedural usage but more because it has an unscoped global. $\endgroup$ Jan 9, 2014 at 16:40
  • 1
    $\begingroup$ @belisarius answer, answer, ANSWER! $\endgroup$
    – Yves Klett
    Jan 9, 2014 at 16:46

1 Answer 1


You may do something like:

expr = x + y + n + Sin[n] + n^2; 
{newExpr, {newVars}} = Reap[expr /. n :> Sow[Unique@n]];
{newExpr, newVars}
 {n$9881 + n$9882^2 + x + y + Sin[n$9883], {n$9881, n$9882, n$9883}}
  • 1
    $\begingroup$ Hey, but that thing is worth lots of $! $\endgroup$
    – Yves Klett
    Jan 9, 2014 at 17:19
  • $\begingroup$ @Yves Use Unique["n"] to fix that. $\endgroup$
    – Mr.Wizard
    Jan 9, 2014 at 17:26
  • $\begingroup$ @Mr.Wizard Why do you want to fix that? Don't you like "$"? $\endgroup$ Jan 9, 2014 at 17:29
  • $\begingroup$ Yves complained about "$" so I gave a solution. Also Unique["n"] gives results that match the OP's example. $\endgroup$
    – Mr.Wizard
    Jan 9, 2014 at 17:32
  • $\begingroup$ @Mr.Wizard Damn, my understanding was that Yves was trying to buy my answer. $\endgroup$ Jan 9, 2014 at 17:35

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