RE: How to Return from within a Table[]-Command?

*To*: mathgroup at smc.vnet.net*Subject*: [mg33759] RE: [mg33728] How to Return from within a Table[]-Command?*From*: "Wolf, Hartmut" <Hartmut.Wolf at t-systems.com>*Date*: Thu, 11 Apr 2002 02:14:17 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

> -----Original Message----- > From: Detlef Mueller [mailto:dmueller at mathematik.uni-kassel.de] To: mathgroup at smc.vnet.net > Sent: Wednesday, April 10, 2002 6:49 AM > To: mathgroup at smc.vnet.net > Subject: [mg33759] [mg33728] How to Return from within a Table[]-Command? > > > Hello, > > I wanted to scan trough al List, building a new > List, but aborting, if one Element of the first > List violates a special Condition ... > say for example: > > Divide2[L_List] := Module[{}, > Table[ > If[IntegerQ[L[[i]]], > i/2, > Return["vector not valid."]; > ] > , {i, 1, Length[L]} > ] > > ... > > ] > > But this results: > > In[13]:= Divide2[{2,3,a}] > Out[13]= {1/2, 1, Return["vector not valid."]} > > Wich is of course not what I want. > > Greetings > > Detlef > Detlef, two suggestions: (1) Catch and Throw: In[1]:= Divide2a[L_List] := Catch[Module[{}, Table[If[IntegerQ[L[[i]]], i/2, Throw["vector not valid."];], {i, 1, Length[L]}] + 0]] In[2]:= Divide2a[{2, 3, a, 5}] Out[2]= "vector not valid." In[3]:= Divide2a[{2, 3, 4, 5}] Out[3]= {1/2, 1, 3/2, 2} (-) Issue an error Message: In[4]:= Divide2b[L_List] := Module[{}, Table[If[IntegerQ[L[[i]]], i/2, Message[Divide2b::invalid]], {i, 1, Length[L]}] + 0] In[5]:= Divide2b::"invalid" = "vector not valid."; In[6]:= Divide2b[{2, 3, a, 5}] >From In[6]:= Divide2b::"invalid": "vector not valid." Out[6]= {1/2, 1, Null, 2} In[7]:= Divide2b[{2, 3, 4, 5}] Out[7]= {1/2, 1, 3/2, 2} Although you got the message, you also got an undesired Result. Prevent this with (2) Check: In[8]:= Check[Divide2b[{2, 3, a, 5}], $Aborted, Divide2b::invalid] >From In[8]:= Divide2b::"invalid": "vector not valid." Out[8]= $Aborted In[9]:= Check[Divide2b[{2, 3, 4, 5}], Null, Divide2b::invalid] Out[9]= {1/2, 1, 3/2, 2} If Divide2b is buried in a comprehensive calculation, which you want to have aborted totally, then wrap that into Check. Of course, instead of $Aborted you may return any value, Null e.g., which will not be printed. -- Hartmut Wolf