具有无序子表达式的模式 [英] Patterns with Orderless subexpressions
问题描述
我需要处理像 f[{a,b}]=...
这样的模式,其中 a
和 b
应该是无序
I need to deal with patterns like f[{a,b}]=...
where a
and b
are supposed to be orderless
到目前为止,我已经通过在每次定义或评估 f
时对子表达式使用默认的 Sort[]
来实现这一点.
So far I've implemented this by using default Sort[]
on subexpressions every time f
is defined or evaluated.
我的问题是
- 这是否与
Orderless
一样强大? - 有更好的方法吗?
PS:一个示例应用程序是树分解,您可以递归地构建数量,例如 subtree[bag1->bag2],其中 bag1 和 bag2 是无序顶点集
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
答案更新
Michael Pilat 的回答展示了如何定义规则来自动对 f 的子表达式进行排序.另一种解决方案是定义一个像 Bag
这样具有 Orderless 属性的自定义头部,并将该头部用于任何无序子列表
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
推荐答案
在我回答这个问题
我咨询了几位同事,他们一致认为以下确实是处理此问题的最佳/典型方法:
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}}]
或者,您可以使用具有 Orderless
属性的自定义头部符号替换内部 List
头部,如果格式真的很重要,您可以使用各种格式技术最近在这里讨论过 =)
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 =)
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