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I was trying to find the order of highest derivative of expression. Using solution given in detect-highest-order-of-derivative-in-expression

I just do not understand why when removing Heads->True then Cases do not work.

Here is example. I want to find the order of all derivatives (then I can use Max to find the largest)

ClearAll[y,x,n]
expr=y''[x]+y'[x]+x+y[x] Sin[x];
Cases[expr,Derivative[n_]->n,Infinity,Heads->True]
(*  {1,2} *)

but

 Cases[expr,Derivative[n_]->n,Infinity]
 (* {} *)

From help ref/Cases it just says that this option will just include the Head in the result:

Mathematica graphics

But I do not see how and why this will made Cases find the pattern. I do not even see the Head returned back in the result. May the above documentation should say more about this.

What is the rule of the thumb as to when to use Heads->True in Cases? Since this seems to be very critical in making it work in this example, and I do not want to just try each time to see if it works or not.

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    $\begingroup$ y''[x] // FullForm gives Derivative[2][y][x]. That Derivative term is in the Head. You'll could get away with Heads->False here by doing Derivative[n_][_][__], but note that this may not work in general. $\endgroup$ – b3m2a1 Nov 22 '17 at 4:58
  • $\begingroup$ @b3m2a1 thanks. I know this. But I do not see how this made Cases work. Help says that Head->True just makes Cases also return the Head in the result. So I would have expected something like {Derivative,1,2} to be returned. $\endgroup$ – Nasser Nov 22 '17 at 5:00
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    $\begingroup$ Nah, Heads->True makes it (try to) match the Head too. The docs were just noting that since it was matching the _Symbol, it was going to return the Head. $\endgroup$ – b3m2a1 Nov 22 '17 at 5:01
  • $\begingroup$ @b3m2a1 Ok, but that is not what documentation says :). Mathematica helps sometimes is not really clear and very short in explaining things. $\endgroup$ – Nasser Nov 22 '17 at 5:03
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When Cases searches an expression, it assumes by default Heads -> False. This means that expression heads are not searched for matches. With Heads -> True, heads are searched.

By means of the visualization helper function frameMatches (defined at the bottom of this post), we can explore the implications of this option.

First, let's look at the full form of the expression in question:

expr // FullForm

fullform of expression

Now, let's highlight all of the subexpressions (i.e. anything that matches _), but ignoring heads:

frameMatches[expr, _, Infinity, Heads -> False]

all subexpressions, ignoring heads

This is the full set of possible matches for an unrestricted pattern. Note that Derivative[1], Derivative[2], Derivative[1][y], Derivative[2][y] and the other heads are not in this set -- even though the expressions that contain them are. So it is no surprise that it we try to match the pattern Derivative[_] we come up empty:

frameMatches[expr, Derivative[_], Infinity, Heads -> False]

fullform of expression

Cases[expr, Derivative[_], Infinity, Heads -> False]
(* {} *)

Now, let's include heads when matching. Again, with the unrestricted pattern _:

frameMatches[expr, _, Infinity, Heads -> True]

all subexpressions, including heads

This time around, Derivative[1] and Derivative[2] are included in the search. And the search even descends into the heads of those expressions, to include Derivative.

This is why we now get the results we want using the desired pattern:

frameMatches[expr, Derivative[_], Infinity, Heads -> True]

matched Derivative forms

Cases[expr, Derivative[_], Infinity, Heads -> True]
(* {Derivative[1], Derivative[2]} *)

Matching Heads with Heads -> False

Incidentally, the option Heads -> True is not the only way to match heads within an expression. We can write explicit patterns that will extract the heads from matched non-head parts. For example:

Cases[expr, (d : Derivative[_])[_][_] :> d, Infinity]
(* {Derivative[1], Derivative[2]} *)

Is All This Documented?

Yes, but as usual it is terse. The documentation for Heads says:

Heads is an option for functions which use level specifications that specifies whether heads of expressions should be included.

Heads->True treats heads just like other elements of expressions for the purpose of levels.

Heads->False never includes heads as part of any level of an expression.


frameMatches

caveat: this function works for the examples at hand, but is not intended to be fully general

frameMatches[expr_, patt_, args___] :=
  Replace[expr // FullForm, e:patt :> Framed[e], args] // expandFrames

expandFrames[e_] :=
  MakeBoxes[e] //
  Replace[#, RowBox[{"Framed", "[", x_, "]"}] :> FrameBox[x], Infinity]& //
  Replace[RowBox[{FrameBox["FullForm", ___],"[", x_, "]"}] :> x] //
  RawBoxes
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