Selecting Primes from list of list

Question

I want to use Select or Pick for selecting primes in all sublists.

list = {{2, 3, 4, 5, 7}, {8, 9, 11}, {16, 17, 18}}

I can do it by

Table[Select[list[[k]], PrimeQ], {k, 3}]

or

Map[If[PrimeQ[#], #] &] /@ list /. Null -> Sequence[]

resulting in both cases in

{{2, 3, 5, 7}, {11}, {17}}

But I want to use Select or Pick for unknown values of k.

Henrik Schumacher · Accepted Answer · 2019-11-30 13:46:19Z

11

You may try this:

Select[PrimeQ] /@ list

answered Nov 30, 2019 at 13:46

Henrik Schumacher

101k7 gold badges171 silver badges290 bronze badges

$\begingroup$ Thanks! Very short and good! $\endgroup$
– user57467
Nov 30, 2019 at 13:58

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kglr · Accepted Answer · 2019-11-30 17:35:47Z

10

You can also use Pick with PrimeQ @ list as the selector array:

Pick[#, PrimeQ @ #] & @ list

{{2, 3, 5, 7}, {11}, {17}}

Cases[_?PrimeQ] /@ list

{{2, 3, 5, 7}, {11}, {17}}

answered Nov 30, 2019 at 17:02

kglr

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$\begingroup$ Your first solution works fine, but to be honest: i don't understand the syntax! Your second solution worked for Cases[list, _?PrimeQ] on one dimesional list $\endgroup$
– user57467
Dec 1, 2019 at 8:51
$\begingroup$ @user57467, re the first method, Pick[#, PrimeQ @ #] & @ list is the same as Pick[list, PrimeQ @ list]. Cases[_?PrimeQ] /@ list should work for list of lists in versions 10+. $\endgroup$
– kglr
Dec 1, 2019 at 9:48

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2 Answers 2