Extract[expr, Position[expr, patt], h]
- To: mathgroup at smc.vnet.net
- Subject: [mg114045] Extract[expr, Position[expr, patt], h]
- From: kj <no.email at please.post>
- Date: Mon, 22 Nov 2010 07:35:40 -0500 (EST)
I am surprised that Mathematica does not have the equivalent of these functions already: extractMatching[expr_, patt_] := Extract[expr, Position[expr, patt]] extractMatching[expr_, patt_, h_] := Extract[expr, Position[expr, patt], h] The first one extracts all the subexpressions in expr that match the pattern patt (including possibly expr itself). The second wraps the extracted patterns with the head h before evaluation. I find myself needing one or the other of these functions all the time. Is there a way to make them automatically available to all my Mathematica sessions? But maybe they already exist in Mathematica, and I just missed them. If so, please let me know. Alternatively, maybe my reliance on extractMatching is diagnostic of my having some thought habits that are counterproductive when doing rules-based programming (analogous to the habit developed through the practice of procedural programming of using for-loops and while-loops)... Just to be concrete, here's the latest task for which I needed this functionality: Unset all the defined expressions foo[i] where i is some positive integer. The solution I found (after many, many failed attempts!) turned out to be extractMatching[DownValues[foo], HoldPattern[foo[x_ /; x > 0]], Unset] (The HoldPattern was necessitated by the fact that some of the DownValues of foo recursively refer to foo; without the HoldPattern one gets an infinite recursion.) Is there a simpler solution to this problem in terms Mathematica built-in functions? TIA! ~kj