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I have a little example that exhibits a successful pattern match in ReplaceAll that Cases misses and I wonder if the assembled sages would be so kind as to offer me an explanation?

This is chopped from an original Prolog emulator by Renan Cabrera circa 2000.

ClearAll[k,s,l,m,j,f,w,g,a,b]

Consider an unordered sequence of statements s stored in a knowledge base k, each being a predicate l about some terms m, f, j, and w:

k=s[l[m,f],
    l[m,w],
    l[j,w],
    l[j,m]
];

We wish to search this knowledge base for any predicates such that l[m,X_] and l[j,X_], where X_ matches the same term in both predicates. Our first cut is something like

k /. s[l[m,X_],l[j,X_]] -> a[X]
(* Out[42]:= s[l[m,f],l[m,w],l[j,w],l[j,m]] *)

but this doesn't work until we make s flat and orderless (associative and commutative):

SetAttributes[s,{Flat,Orderless}]

and now

k /. s[l[m,X_],l[j,X_]] -> a[X]
(* Out[43]:= s[a[w],l[j,m],l[m,f]] *)

This is, of course, just the same as ReplaceAll[k, s[l[m,X_],l[j,X_]] -> a[X]]. (notice the canonical reordering in the output), and now I can extract my answer with something like

Cases[k /. s[ l[m, X_], l[j, X_] ]->a[X],a[Y_]->Y]
(* Out[44]:= {w} *)

The surprise and the question is why doesn't this work?

Cases[k, s[ l[m, X_], l[j, X_] ] -> a[X]]
(* Out[45]:= {} *)

or even this

MatchQ[k, s[ l[m, X_], l[j, X_] ]]
(* Out[46]:= False *)

It's as though ReplaceAll, Cases, and MatchQ are using different pattern matchers.

Will be grateful for advice, clues, discussion, etc.

share|improve this question
2  
To me the interesting, and odd, result is not Cases or MatchQ but ReplaceAll. The only hint that I can find is that "ReplaceAll looks at each part of expr, tries all the rules on it, and then goes on to the next part of expr. The first rule that applies to a particular part is used; no further rules are tried on that part, or on any of its subparts." My guess is that this translates into somehow looking at all combinations of s with two arguments (due to Flat), one of them matches and is replaced by a and the whole thing is returned. –  tkott May 2 '12 at 13:43
2  
I think Cases simply does not take into account Flat and Orderless. It just looks at the elements of the input expression one by one, unlike ReplaceAll which does consider Flat and Orderless too. I realize this doesn't explain what happens, it merely re-states it. –  Szabolcs May 2 '12 at 14:25
2  
@Szabolcs I thought that too. But try this Cases[Hold[3 + 4 + 5], X___ + 3 :> X, 2] –  belisarius May 2 '12 at 14:50
1  
@belisarius You're right, even more convincing: Cases[Hold[3 + 4 + 5], X___ + 4 :> X]. You could make an answer out of that, I think. –  Szabolcs May 2 '12 at 14:59
1  
I think this is probably a particular behaviour of ReplaceAll (or perhaps of those built-ins that don't search depth-first if there's any other). The title should probably be changed to ReplaceAll. Try SetAttributes[f, Flat];replace=Replace[#1, #2, {0, Infinity}]&;Through@{replace, ReplaceAll}[f[1, 2, 3], f[2, 3]:>8]. Can't test thoroughly right now –  Rojo May 2 '12 at 16:42

2 Answers 2

up vote 9 down vote accepted

MatchQ looks to see if the whole expression matches the pattern, so you need to add a blank to allow for the other elements of s:

MatchQ[k, s[l[m, X_], l[j, X_], ___]]

Cases takes a list and checks each element in turn, so I think the pattern matcher only gets to see each l[a,b] in isolation. The best I could come up with is:

Cases[Subsets[k, {2}], s[l[m, X_], l[j, X_]] -> a[X]]

Further investigation

Here are a couple of questions about Cases I have tried to answer experimentally. I don't know if this helps anyone else much, but it was useful for me to go through it and I think I now understand how Cases behaves, even if the why is a mystery.

Q1. Does Cases take into account Orderless ?

In[1]:= SetAttributes[a,{Orderless}]
In[2]:= Cases[a[x,y],a[y,_]->0,{0}]
Out[2]= {0}

A1. Yes, it does. The pattern matcher understands that a[x,y] === a[y,x] which matches the pattern.

Q2. Does Cases take into account Flat ?

In[3]:= SetAttributes[a,{Flat}]
In[4]:= Cases[a[x,y,z],a[onebit_,anotherbit_]:>{onebit+anotherbit},{0}]
Out[4]= {{a[x]+a[y,z]}}

A2. Yes, it does. The pattern matcher understands that a[x,y,z] === a[a[x],a[y,z]] which matches the pattern.

Q3. Given that Cases can use the above transformation, will it get a "hit" on the pattern a[x] ?

In[5]:= Cases[a[x,y,z],a[x]->b,{0,Infinity}]
Out[5]= {}

A3. No, even though the transformed expression a[a[x],a[y,z]] clearly contains a match for a[x] at level 1, this doesn't count. Cases appears to require the entire expression at level n to match the pattern. The logic appears to be:

  • Level 0: expression a[x,y,x] does not match a[x]
  • Level 0: transformed expression a[a[x],a[y,z]] does not match a[x]
  • Level 1: expression x does not match a[x]
  • Level 1: expression y does not match a[x]

etc...

This is in contrast to ReplaceAll, which does pick up the a[x] in the transformed expression:

In[6]:= a[x,y,z]/.a[x]->b
Out[6]= a[b,y,z]

So it seems like ReplaceAll applies Flat and Orderless transformations outermost, and then for each transformed expression it digs down into the subexpressions looking for a match. Whereas Cases digs down into the subexpressions of the untransformed original expression outermost, and for each subexpression it tries the various Flat and Orderless transformations looking for a match.

I realise this is probably a complete misrepresentation of how the pattern matcher actually works, but as a hand-waving mental picture it seems to explain the different behaviour of Cases and ReplaceAll

share|improve this answer
    
I think Cases is supposed to work with any head, not just List, as in Cases[foo[bar, baz, qux], baz] ~~> baz. That's part of what led me to think the original idea Cases[k,s[l[m,X_],l[j,X_]]->a[X]] would work. I like your proposal, and note on the side that Cases[k,s[l[m,X_],l[j,X_],___]->a[X]] does not work. Still waiting for more explanation of what's going on with Cases. –  Reb.Cabin May 2 '12 at 16:44
    
@Reb.Cabin, Cases does work with any head. In your example foo is the head and it checks the individual elements bar, baz and qux against the pattern. When you try Cases[k, ...] the head is s and it checks the individual elements l[m,f], l[m,w] etc against the pattern. There is no match because the pattern has head s but none of the individual elements under test have head s. Note that Cases[k,s[_]] also returns an empty list. –  Simon Woods May 2 '12 at 20:39
    
@SimonWoods This does not quite explain it, since first, you should have used Cases[k,s[__]], and second, the zero-level spec, when provided, solves the problem here: Cases[k, s[__], {0, Infinity}], but not in the original example. –  Leonid Shifrin May 2 '12 at 21:09
    
@LeonidShifrin, good point, thanks. I've done a bit of investigating and extended my answer. –  Simon Woods May 3 '12 at 12:47
3  
Simon, it is worth pointing out that your test SetAttributes[a,{Orderless}]; Cases[a[x,y],a[y,x]->0,{0}] doesn't actually tell us anything about Cases. In this instance all the work is done by Orderless before Cases even sees it. In fact because Orderless does all the work if we interfere with it we lose the match: Cases[a[x, y], HoldPattern[a[y, x]] -> 0, {0}]. On the other hand, when a _ is involved the pattern matcher does the work: Cases[a[x, y], HoldPattern[a[y, _]] -> 0, {0}]. IMHO you should update your example. –  Mr.Wizard Jul 9 '12 at 12:34

I suspect that the difference relates to the fact that Cases traverses depth-first, while ReplaceAll traverses breadth-first. This can be seen here:

k = s[l[m, f], l[m, w], l[j, w], l[j, m]];
test = (Print[##]; False) &;

k /. x__?test :> Null;
s[l[m,f],l[m,w],l[j,w],l[j,m]]
s
l[m,f]
l
m
f
l[m,w]
l
m
w
l[j,w]
l
j
w
l[j,m]
l
j
m
Cases[k, x__?test, {0, -1}, Heads -> True];
s
l
m
f
l[m,f]
l
m
w
l[m,w]
l
j
w
l[j,w]
l
j
m
l[j,m]
s[l[m,f],l[m,w],l[j,w],l[j,m]]
share|improve this answer
    
+1 I always forget the handiness of "_?" for testing –  belisarius May 2 '12 at 22:16
    
@belisarius incidentally, in this application I could have used ?Print instead of ?test but I did not for the sake of clarity. –  Mr.Wizard May 2 '12 at 23:42

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