Re: Index Mapping for VB6

Hi Matthew,

> I need some help with doing index mapping in NetCDF v. 3.5.0 using VB 
> 6.  I just want to transpose two indices in a NetCDF variable but it 
> won't work.
> I have spent a whole day of crashing VB and I haven't got anywhere.  My 
> netcdf api hasn't failed me until now.  I am trying to use the function
> 
> Public Declare Function nc_get_varm_double Lib "netcdf.dll" (ByVal ncid 
> As Long, ByVal varid As Long, ByRef startp As Long, ByRef countp As 
> Long, ByRef stridep As Long, ByRef imapp As Long, ByRef ip As Double) As 
> Long
> 
> Firstly, I am hoping the above is correct.  It is based on all my other 
> netcdf api conversions but to date I have only done read/write of netcdf 
> files with single value, array and subarray functions.  I have never 
> tried anything like this before.
> 
> My netcdf file has dimensions for lon, lat, and time.  I have coordinate 
> variables for each as well.  I also have a data variable declared as:
>      short water_u(time, lat, lon) ;
>         water_u:long_name = "Eastward Water Velocity" ;
>         water_u:units = "meters/second" ;
>         water_u:_FillValue = -10000s ;
>         water_u:scale_factor = 0.01f ;
>         water_u:add_offset = 0.f ;
> 
> I need to read this into a VB variable declared as:
>     Dim WaterU(1 To lNumLat, 1 To lNumLon) As Double
> 
> If it wasn't for the fact that the array indices must be in this order I 
> would be fine.  I could just transpose after read the
> NetCDF variable but it very ineligent and rather slow in VB so I prefer 
> using the NetCDF api.  This is what I am trying to do now:
> 
>       Dim dims(1 To 3) As Long, count(1 To 3) As Long, stride(1 To 3) As 
> Long, imap(1 To 3) As Long
> 
>       dims(1) = lCurrentTime - 1      'VB arrays are base 1 for me so I 
> must subtract 1 to get NetCDF array index
>       dims(2) = 0
>       dims(3) = 0
> 
>       count(1) = 1                           'Only 1 time step
>       count(2) = lNumLat                'All the latitudes and 
> longitudes please
>       count(3) = lNumLon
>     
>       stride(1) = 1
>       stride(2) = 1
>       stride(3) = 1
> 
>       imap(1) = 1                              '** I am guessing my 
> problem is here but no combination of numbers has worked for me
>       imap(2) = nLon
>       imap(3) = CLng(nLat * nLon)
>      
>       NcErr = nc_get_varm_double(fHandle, var_id, dims(1), count(1), 
> stride(1), imap(1), dblArray(1, 1))
>       If NcErr <> 0 Then GoTo NcErrOut

It doesn't look like you've gotten any answers yet to your question,
so I'll try an answer, although I don't have a VB installation and
don't know much about VB.

I assume from your question that VB arrays are similar to Fortran
arrays, in that the first dimension varies fastest (also called column
major order).

Try 

      imap(1) = NumLat*NumLon
      imap(2) = 1
      imap(3) = NumLat

although it may not matter what value imap(1) is set to in this case,
since I don't think it will be used.  imap(2)=1 means that the
distance between two different lat indices in your internal array is
just 1 value, and imap(3)=NumLat means that the distance between
successive lon values is NumLat values.

--Russ