[netcdfgroup] arrays of fixed-length strings

[John Caron]

Jeff Whitaker wrote:
> John Caron wrote:
>>
>>
>> Jeff Whitaker wrote:
>>> Ed Hartnett wrote:
>>>> Jeff Whitaker <jswhit at fastmail.fm> writes:
>>>>
>>>>  
>>>>> In netcdf-4.0, I don't see how to create variables which are arrays of
>>>>> strings with length > 1.  I see how to create arrays of
>>>>> single-characters, and arrays of variable-length strings, but not
>>>>> strings of a specified length.
>>>>>
>>>>> Am I missing something, or is this not supported by HDF5?
>>>>>
>>>>>     
>>>>
>>>> Howdy Jeff!
>>>>
>>>> Strings are variable length by their nature.
>>>>
>>>> How about a two dimensional array of NC_CHAR?
>>>>
>>>> Thanks,
>>>>
>>>> Ed
>>>>
>>>>
>>>>   
>>>
>>>
>>> Ed:  I use arrays of fixed length strings, padded with spaces, quite 
>>> a bit.  This simplifies the memory management issues associated with 
>>> arrays of variable length strings (which have no counterpart in 
>>> Fortran 90/95, although they are allowed in Fortran 2003).  Below is 
>>> an excerpt from my previous reply which explains why I don't like 
>>> using 2-D arrays of characters to represent 1-D arrays of 
>>> fixed-length strings:
>>>
>>> Russ: I realize you can use a array of shape ndim,8 to store an array 
>>> of ndim 8 character strings.  Thats the way I've done it with 
>>> netcdf-3 - it just feels clunky.  A typical use case for me is 
>>> station data, where you want to store the name of the station. I end 
>>> up the with an array of characters shaped (nstations,ncars) -  in 
>>> fortran I read it into an  (nstations,nchars) character(len=1) array 
>>> (after first finding out what both nstations and nchars are), then 
>>> reshape it into a (nstations) character(len=nchars) array.  I'd 
>>> rather just read it into a character(len=nchars) array straight off.  
>>> Not a show stopper for sure, but it would be more convenient.  I 
>>> realize that specifying the data type would be tricky,  instead of 
>>> NC_CHAR, do you have a bunch of new types NC_CHAR1, NC_CHAR2, ... 
>>> NC_CHAR120?  Or a new function datatype = nc_set_chartype(nchars)?  
>>> However, I bet it would get used a lot more than the esoteric 
>>> datatypes you have in netcdf-4 already (enums and opaque for example).
>>
>> Hi Jeff:
>>
>> Suppose we stuck with fixed length char arrays for this case, but 
>> added a convenience method in the API that did the work for you. What 
>> would that convenience method look like?
>>
>> John
> 
> John:  A convenience method won't really help much - the thing I'd most 
> like to avoid is defining another dimension to hold the number of 
> characters in each string.  Essentially, I'd like to have that 
> information transferred to the datatype.

Hi Jeff:

It seems unlikely that we'd want to add a multitude of datatypes for this purpose. The extra dimension seems the right thing if you really want to specify that all the Strings are of the same length. 

John