# mutable considered harmful, Range edition

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## mutable considered harmful, Range edition

 Administrator When using --traditional, I noticed that the result of    zeros (1, 0) was [](0x0) instead of [](1x0). This error is related to the use of a mutable cache value in the Range data type.  Here's how: Constant row vectors (like those produced by zeros and ones) are stored as ranges with the number of elemnts set to the number of columns and the increment set to zero. But using --traditional implies "disable_range (true)" so the Range::matrix_value method is called is called in the octave_value constructor and that appears to fail.  If I understand correctly, the matrix_value method returns the incorrect result because it is declared const but also modifies and returns a mutable value (the cached Matrix value).  The compiler appears to be choosing to return the cache value before it is modified (the method is const, so normally it wouldn't change any member variables). Is it possible to make the mutable cache value reliable in situations like this?  If not, then this appears to be another example of why we need to eliminate mutable in most places in Octave. Separately, I see that other than the Range::matrix_value method, we set the cache value in the operators, like this:    Range operator + (const Range& r, double x)    {      Range result (r.base () + x, r.limit () + x, r.inc (), r.numel ());      if (result.m_numel < 0)        result.m_cache = r.matrix_value () + x;      return result;    } As I recall, setting the cache in these functions (and not just the matrix_value method) is done so that, for example, adding a constant to a range and then converting to a matrix will produce exactly the same result as converting a range to a matrix and then adding a constant to the matrix (in psuedo code):    matrix (r) + c == matrix (r + c) Using the cache this way does avoid the cost of any repeated conversions to a matrix value, but it also forces the cache to be created for any operation on a range, not just the result.  So it largely defeats the purpose of the efficient range object storage, and I'm wondering whether it is worth having a special range data type at all?  What do we really gain for the additional complexity? jwe
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## Re: mutable considered harmful, Range edition

 Administrator On 6/9/20 12:11 AM, I wrote: > Separately, I see that other than the Range::matrix_value method, we set > the cache value in the operators, like this: > >    Range operator + (const Range& r, double x) >    { >      Range result (r.base () + x, r.limit () + x, r.inc (), r.numel ()); >      if (result.m_numel < 0) >        result.m_cache = r.matrix_value () + x; > >      return result; >    } > > As I recall, setting the cache in these functions (and not just the > matrix_value method) is done so that, for example, adding a constant to > a range and then converting to a matrix will produce exactly the same > result as converting a range to a matrix and then adding a constant to > the matrix (in psuedo code): > >    matrix (r) + c == matrix (r + c) > > Using the cache this way does avoid the cost of any repeated conversions > to a matrix value, but it also forces the cache to be created for any > operation on a range, not just the result.  So it largely defeats the > purpose of the efficient range object storage, and I'm wondering whether > it is worth having a special range data type at all?  What do we really > gain for the additional complexity? I see now that there are limited cases where result.m_numel will be negative, so the cache is not updated for every operation.  However, the problems with the mutable cache remain, as do the issues with operations on ranges not being identical to the operations on the equivalent matrices.  Here is a simple example:    r0 = 1:0.1:10;    r1 = r0 + 2.3;   # range + scalar    r2 = [r0] + 2.3; # matrix + scalar    all (r1 == r2)   # returns false for me    d = r1 - r2;     # show elements with differences    idx = find (d)    d(idx) I understand the arguments about Octave being a numerical tool and not expecting exact results for floating point operations, but I'm still wondering whether the complexity of these range operations is justified.   If we do want to support operations that avoid immediate conversion to Matrix data, maybe we should only do so when we can guarantee that    matrix (r) OP val == matrix (r OP val) is true?  We should be able to do this when VAL and all elements of R are integers and will remain so after the operation.  Other cases might be possible as well, but harder to detect.  And maybe the cache should be eliminated and this test handled in the octave_value class hierarchy? jwe