Re: Conditional list indexing

*To*: mathgroup at smc.vnet.net*Subject*: [mg95869] Re: [mg95855] Conditional list indexing*From*: "David Park" <djmpark at comcast.net>*Date*: Thu, 29 Jan 2009 05:51:35 -0500 (EST)*References*: <11575212.1233143165868.JavaMail.root@m02>

Check out MapIndexed. a = Table[Prime[i], {i, 10}] {2,3,5,7,11,13,17,19,23,29} Here is MapIndexed with a generic function f. MapIndexed[f, a] {f[2, {1}], f[3, {2}], f[5, {3}], f[7, {4}], f[11, {5}], f[13, {6}], f[17, {7}], f[19, {8}], f[23, {9}], f[29, {10}]} We want f to be a function that will do the test on the first element, if it passes return the first part of the second element, otherwise return a blank Sequence. The blank Sequence must be unevaluated so it is not eliminated. MapIndexed[If[#1 > 15, First[#2], Unevaluated@Sequence[]] &, a] {7, 8, 9, 10} Look up Sequence. Notice that: If[x, y, Sequence[]] gives If[x, y] not what we want, but If[x, y, Unevaluated@Sequence[]] works properly. David Park djmpark at comcast.net http://home.comcast.net/~djmpark/ From: daniele [mailto:danielelinaro at gmail.com] Hello everybody, I'd like to know how I can get the indices of the elements in a list that satisfy a certain condition. Here's an example: suppose I have the list a = Table[Prime[i],{i,10}]; that contains {2,3,5,7,11,13,17,19,23,29}. If I execute the command b = Select[a,#>15&]; b will contain {17,19,23,29}. I'd like to find a way to obtain a third list, say c, that will contain {7,8,9,10}, i.e. the indices of the elements in list a that are greater than 15. Thank you, Daniele