[Octave-bug-tracker] [bug #54241] Lookup documentation

[Michael Leitner]

URL:
  <http://savannah.gnu.org/bugs/?54241>

                 Summary: Lookup documentation
                 Project: GNU Octave
            Submitted by: mleitner
            Submitted on: Tue 03 Jul 2018 09:25:10 AM UTC
                Category: Documentation
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Documentation
                  Status: None
             Assigned to: None
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
                 Release: dev
        Operating System: Any

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Details:

The documentation of lookup is incorrect, instead of


@code{table(idx(i)) <= y(i) < table(idx(i+1))}


it should read


@code{table(idx(i)) <= y(i) < table(idx(i)+1)}


While we are at it, I would also propose to describe its behaviour in natural
speech a bit better, as I find that I always have to try it with a simple
example in order to understand its syntax, and whenever I indicate to someone
that the function would be useful for their particular problem, they cannot
understand from the documentation what it's about. So I propose "For
increasing table, the function returns the indices of the last elements in
@var{table} that are smaller than or equal to the entries in @var{y}, that is,
...". 

By the way, I do not get at all the purpose of the options 'l' and 'r',
because, unless I am very much mistaken, we have that lookup(table,y,'l') is
always equal to lookup(table(2:end),y)+1, and lookup(table,y,'r') is even
equal to just lookup(table(1:end-1),y).  And the standard answer that this is
for compatibility with Matlab does not apply here, as lookup is not provided
in Matlab. 

On the other hand, I would find an option "rows", corresponding to
lexicographic comparisons in analogy to unique, useful. As I see it, computing
this efficiently at present would require the following evil trickery:


table=unique(round(3*rand(10,3)),"rows");
y=round(3*rand(10,3));

[yd,j]=sortrows(y);
[~,i]=sortrows([table; yd]);
idx(j,1)=cumsum(i<=size(table,1))(i>size(table,1));





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