Home/ Questions/Q 3395790
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-18T04:19:49+00:00

I need to deal with patterns like f[{a,b}]=… where a and b are supposed

  • 0

I need to deal with patterns like f[{a,b}]=... where a and b are supposed to be orderless

So far I’ve implemented this by using default Sort[] on subexpressions every time f is defined or evaluated.

My questions are

  1. Is this as robust as Orderless?
  2. Is there a better way?

PS: An example application is tree decomposition where you recursively build up quantities like subtree[bag1->bag2] where bag1 and bag2 are orderless sets of vertices

answer update

Michael Pilat’s answer shows how to define a rule to automatically sort f’s subexpressions. Alternative solution is to define a custom head like Bag with Orderless attribute and use that head for any orderless sublists

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    After I answered this question I consulted with a few colleagues who agreed that the following is indeed the best / typical way to handle this problem:

    f[{a_, b_}] := 
 f[{Sort[a], Sort[b]}] /; Not[OrderedQ[a]] || Not[OrderedQ[b]]

In[99]:= f[{{1, 2, 3}, {5, 4, 3}}]

Out[99]= f[{{1, 2, 3}, {3, 4, 5}}]

    Alternately, you could replace the inner List heads with a custom head symbol that has the Orderless attribute, and if formatting really matters you could use the various formatting techniques that have recently been discussed here =)

    • 0