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I have a list of rules,

rulelist ={$x1 -> 1, $x10 -> 15, $x11 -> 1, $x2 -> 6, $x3 -> 15, $x4 -> 15, $x5 -> 10, $x6 -> 60, $x7 -> 20, $x8 -> 15, $x9 -> 45}

Trying SortBy[rulelist, Greater] keeps the rulelist the way it is, and trying SortBy[rulelist, Last] gives 

{$x1 -> 1, $x11 -> 1, $x2 -> 6, $x5 -> 10, $x10 -> 15, $x3 -> 15, $x4 -> 15, $x8 -> 15, $x7 -> 20, $x9 -> 45, $x6 -> 60}

How can I order these rules in numerical instead of canonical order?

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  • $\begingroup$ SortBy[rulelist, Last]? $\endgroup$
    – march
    Oct 3, 2017 at 17:05
  • $\begingroup$ Would an answer that let you replace, say, $x1 with $x01 work for you? By renaming the variables in that way, a standard Sort would do the job. $\endgroup$
    – aardvark2012
    Oct 4, 2017 at 10:24
  • $\begingroup$ That would work. I'll try that and see. $\endgroup$
    – BJParks
    Oct 5, 2017 at 0:02

1 Answer 1

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You can define a better ordering function in your SortBy. Assuming that all your rules look like $xnnnn where n is a digit, the ToExpression@StringTake[ToString[#[[1]]], {3, -1}] & would work.

SortBy[rulelist, ToExpression@StringTake[ToString[#[[1]]], {3, -1}] &]
(* {$x1 -> 1, $x2 -> 6, $x3 -> 15, $x4 -> 15, $x5 -> 10, $x6 -> 
  60, $x7 -> 20, $x8 -> 15, $x9 -> 45, $x10 -> 15, $x11 -> 1} *)

That's very straightforward way, a more elegant way probably exists.

Brief explanation:

#[[1]] takes left side of your rule (you can replace it with First[#])

ToString makes a string from the expression

StringTake[ ,{3,-1}] takes all characters except first two (removes "$x")

ToExpression makes a number out of string, this number is used to order by.

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  • $\begingroup$ That's perfect! I'll definitely look at the documentation and see why this works too. Thank you! $\endgroup$
    – BJParks
    Oct 3, 2017 at 17:11
  • $\begingroup$ @BJParks I added some explanation. Consider upvoting/accepting the answer if you find it useful. $\endgroup$
    – BlacKow
    Oct 3, 2017 at 17:18
  • $\begingroup$ Thanks again for your help; the explanation is greatly appreciated! $\endgroup$
    – BJParks
    Oct 5, 2017 at 0:03

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