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We have the following list

data = {{1,2,3,4,5}, {6,7,8,9,10}, {11,12,13,-1,14}}

If I want to create a new list containing the first and the fourth elements of the first list we use

data2 = data[[All, {1, 4}]];

Now, what if I want the first and the Log10() of the fourth element? Log10(-1) does not exist so in the new list there should be only two sublists.

Any ideas?

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  • $\begingroup$ This seems like an extremely localized problem, but would Cases[data[[All, {1, 4}]], {_?Positive, _?Positive}] help you along? $\endgroup$
    – MarcoB
    Oct 20 '15 at 16:33
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    $\begingroup$ Cases[data[[All, {1, 4}]], {x_, y_} :> {x, Log[10, y]} /; y > 0] $\endgroup$
    – Dr. belisarius
    Oct 20 '15 at 16:37
  • $\begingroup$ @belisariusisforth Nice work! Please write a short answer so as to approve it. $\endgroup$
    – Vaggelis_Z
    Oct 20 '15 at 16:41
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data = {{1,2,3,4,5}, {6,7,8,9,10}, {11,12,13,-1,14}}
Cases[data[[All, {1, 4}]], {x_, y_} :> {x, Log[10, y]} /; y > 0]
(* {{1, Log[4]/Log[10]}, {6, Log[9]/Log[10]}} *)

edit

Or if you want it working for expressions too:

data = {{1, 2, 3, 4, 5}, {6, 7, 8, -1, 10}, {11, 12, 13, a, 14}};
Cases[data[[All, {1, 4}]], {x_, y_} :> {x, Log[10, y]} /; Positive[y] =!= False]
(* {{1, Log[4]/Log[10]}, {11, Log[a]/Log[10]}} *)
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Here's my take:

Cases[data, {x_, Repeated[_, {2}], y_?Positive, __} :> {x, Log10[y]}]
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Using belisarius/J.M. but with the new operator form of Cases

Cases[{x_, _, _, y_?Positive, _} :> {x, Log10[y]} ] @ data
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