Re: Finding position of an element in a list:

*To*: mathgroup at smc.vnet.net*Subject*: [mg84068] Re: Finding position of an element in a list:*From*: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>*Date*: Sat, 8 Dec 2007 05:48:05 -0500 (EST)*Organization*: The Open University, Milton Keynes, UK*References*: <fj8pfv$j3j$1@smc.vnet.net> <fjavso$s11$1@smc.vnet.net>

Kevin J. McCann wrote: > Nice one, Bob. Yet another new function (Nearest) that I didn't know about. > > I found the output of your function to be a bit cumbersome. Also, if > there are repeated nearest values, then you get the positions twice. I > modified your function by adding a Union and a final Flatten to produce > a single list: > > nearestPosition2[lst_List,val_?NumericQ]:= > Flatten[Union[Flatten[Position[lst,#]]&/@Nearest[lst,val]]]; > > > So that the output of > > xlis = {0, 1, 5.5, 10, 4, 5, 5.5, 10.25}; > > is {3,7} instead of {{{3},{7}},{{3},{7}}}. > > Cheers, > > Kevin We can even slightly improve this improved function by removing the inner Flatten as in, nearestPosition3[lst_List, val_?NumericQ] := Flatten[Union[Position[lst, #] & /@ Nearest[lst, val]]]; xlis = {0, 1, 5.5, 10, 4, 5, 5.5, 10.25}; nearestPosition3[xlis, 6] returns {3, 7} Just my $0.02 :-) Best regards, Jean-Marc > Bob Hanlon wrote: >> nearestPosition[xlis_List, val_?NumericQ] := >> Flatten[Position[xlis, #]] & /@ >> Nearest[xlis, val]; >> >> xlis = {0, 1, 3, 10, 4, 5, 5.5, 10.25}; >> >> nearestPosition[xlis, 6] >> >> {{7}} >> >> xlis = {0, 1, 6.5, 10, 4, 5, 5.5, 10.25}; >> >> nearestPosition[xlis, 6] >> >> {{3}, {7}} >> >> >> Bob Hanlon >> >> ---- Gopinath Venkatesan <gopinathv at ou.edu> wrote: >>> Hello Friends, >>> >>> (This is an easy problem, but I was unable to get it work in Mathematica, probably because of the precision - I tried setting precision too, still got stuck) >>> >>> Suppose we have a list, xlis, having some elements, and I have to find one nearest value I am interested in that list. In the below example, I wanted to find the position of a number belonging to list and close to 6 (which is 5.5 located at 7th). >>> >>> I can do this: >>> >>> xlis = {0, 1, 3, 10, 4, 5, 5.5, 10.25}; >>> dropOuterBraces[lst_List] := >>> StringTake[#1, {2, StringLength[#1] - 1}] &[ >>> ToString[lst]]; (* took from previous help posted by Peter Pein *) >>> n1 = Nearest[xlis, 6] >>> eval = dropOuterBraces[n1] >>> Position[xlis, eval] >>> Position[xlis, 5] >>> >>> As you see, the last but one command returns nothing, and we have to explicitly say the number to return the position. >>> >>> Or should some other command be used to extract the position of an arbitrary number in a list? >>> >>> Same with the case of MemberQ command, it gives True for some and False for some. For example, consider this list, >>> >>> xlis = Table[N[((1 - Cos[(j - 1)/9 \[Pi]])/2) 1.32], {j, 1, 10}]; >>> dropOuterBraces[lst_List] := >>> StringTake[#1, {2, StringLength[#1] - 1}] &[ToString[lst]] >>> >>> eval = Nearest[xlis, 0.32]; >>> xlis >>> elem = dropOuterBraces[eval] >>> MemberQ[xlis, 0.545392] >>> MemberQ[xlis, 0.33] >>> Position[xlis, 0.33] >>> Position[xlis, elem] >>> >>> Please hint where I am wrong. >>> >>> Thanks, >>> >>> Gopinath Venkatesan >>> Graduate Student >>> University of Oklahoma >>> >> >