[Octave-bug-tracker] [bug #62968] idx_vector constructor m_len calculation is incorrect

[Markus Mützel]

Update of bug #62968 (project octave):

                  Status:                    None => Need Info              
        Operating System:               GNU/Linux => Any                    
                 Summary: idx_vector constructor m_len calculation is incorret
=> idx_vector constructor m_len calculation is incorrect

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Follow-up Comment #1:

Is `m_len` the number of elements in the idx_vector? (I think it is.) Is
`limit` included in the range? (Not sure if it is meant to be.)

Assuming both are true (I'm not sure if they are), let's check `idx_vector (0,
9, 1)` (corresponding to the index range `1:1:10` in an .m file?). That should
result in 10 elements IIUC. With your updated formula, that would result in
`(9 - 0 + 1 - 1) / 1` which is 9. (But it would also be 9 with the current
formula.)

Nevertheless, I can't quite figure out what that function is meant to do. And
there might be something wrong in its implementations.

Would the following change fix this?

diff -r 4c2b83516eb7 liboctave/array/idx-vector.cc
--- a/liboctave/array/idx-vector.cc     Fri Aug 26 22:30:43 2022 +0200
+++ b/liboctave/array/idx-vector.cc     Sat Aug 27 15:47:40 2022 +0200
@@ -103,7 +103,7 @@
                                             octave_idx_type limit,
                                             octave_idx_type step)
     : idx_base_rep (), m_start(start),
-      m_len (step ? std::max ((limit - start) / step,
+      m_len (step ? std::max ((limit - start) / step + 1,
                               static_cast<octave_idx_type> (0))
              : -1),
       m_step (step)


CC'ing jwe because he'll hopefully know what that function was meant to do.


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