Search All of the Math Forum:
Views expressed in these public forums are not endorsed by
NCTM or The Math Forum.


Math Forum
»
Discussions
»
sci.math.*
»
sci.math
Topic:
Binary or Logical Selection : Given an array A and a binary orlogical vector S, how do I select the rows of A when S = 1 or True ?
Replies:
1
Last Post:
Sep 24, 2012 2:31 AM




Re: Binary or Logical Selection : Given an array A and a binary orlogical vector S, how do I select the rows of A when S = 1 or True ?
Posted:
Sep 24, 2012 2:31 AM


On Sun, 23 Sep 2012 22:00:15 +0200, clicliclic wrote: > Beau Webber schrieb: >> Given a 3 column array A of x,y,z numbers, defining a set of spatial >> coordinates, and a binary or logical selection vector S (with as many >> elements as rows in A) : >> >> How do I express the operation of selecting an array R with the set of >> those coordinates in A where S is 1 (or true) ? >> >> In Apl I write this as R < S /[1] A . >> >> But a referee of a paper is asking that I express this in standard >> maths, and I have not yet managed to find an appropriate mathematical >> expression. ... > I can imagine that referees don't like mathematical explanations in > terms of APL  unless yours is a paper about APL (does the language > still exist?). But I can't think of an elegant way to express this in > standard vector notation either. This newsgroup is only concerned with > the development and application of computeralgebra systems like Maxima > or Maple, I am therefore widening the audience to include sci.math. > > Can anybody help the original poster?
Perhaps write something like following, in which the form p_i_j stands for p_{i_j} and the parenthesized lists stand for vectors of points p_i.
Suppose A = (p_1, p_2, ... p_n). Let R = (p_i_1, p_i_2, ... p_i_k) where i_j < i_{j+1} for all the j, and i \in { i_j } iff S_i = 1.
 jiw



