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based on an earlier version of the Wolfram Language.
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Pick

Pick[list, sel]
picks out those elements of list for which the corresponding element of sel is True.
Pick[list, sel, patt]
picks out those elements of list for which the corresponding element of sel matches patt.
  • sel can be a nested list of any depth.
  • Pick[list, sel, patt] picks out those list[[i1, i2, ...]] for which sel[[i1, i2, ...]] matches patt.
  • Depending on the arrangement of elements matching patt in a nested list sel, Pick may return a ragged array.
  • The heads in list and sel do not have to be List.
Pick out elements wherever a 1 appears in the "selector" list:
In[1]:=
Click for copyable input
Out[1]=
 
Pick out elements from a 2D array:
In[1]:=
Click for copyable input
Out[1]=
New in 5.1