Re: Bug/feature in matrix multiply

[steinhh(at)ulrik.uio.no (Stein Vidar Hagfors Haugan)]

> Mark Fardal (fardal@weka.phast.umass.edu) writes:
> 
> > is the following a bug or feature?  I don't understand why changing the
> > type of the array changes the dimensions of the result.  Then again,
> > it's late on Friday, so my brain might just be mush.
> > 
> > IDL> junk=fltarr(3)  
> > IDL> junk=reform(junk,3,1)
> > IDL> help,junk
> > JUNK            FLOAT     = Array[3, 1]
> > IDL> help,[3.,2.,1.]#junk
> > <Expression>    FLOAT     = Array[1]
> > IDL> help,[3.d0,2.d0,1.d0]#junk
> > <Expression>    DOUBLE    = Array[3, 3]
> 
> I don't know if it is a bug or a feature, but I 
> agree that it is strange. But so is this command:
> 
>    junk = reform(junk, 3, 1)
> 
> Do you mean this:
> 
>    junk = reform(junk, 1, 3)
> 
> The latter will make a column vector, which makes more
> sense when multiplied by a row vector. What kind of result
> were you expecting? From my reading of the # operator
> I think the result with the floating array is correct.
> I don't have a clue why the double expression does what
> it does. :-(

Hint:
  
  junk=reform(junk,3,1)
  help,junk,double(junk)

 8-)