Sunday, November 11, 2012

SAS XPT Files, IBM Floats, and C#

Well, I recently was asked if I could support XPT files, you know, the SAS transport format. Well, SAS transport formats are documented and fairly straight-forward to read so 'why not?'.

Got started coding, client provided a nice sample, all went well. Around 8-10 hours after starting, I was happily reading SAS transport files and writing them to a tab-delimited file.

However, things weren't happy on Big Rock Candy Mountain.Seems the numbers were off. A little bit of reading in the SAS XPORT Documentation indicated the problem:

"All floating-point numbers in the file are stored using the IBM mainframe representation. If
your application is to read from or write to transport files, it will be necessary to
convert native floating-point numbers to or from the transport representation."


Not a problem. Let's go track down a C# library that handles IBM 370 floats and does the conversion. One small problem...I couldn't find one. What I did find was that this was a nasty little problem. I like to dabble in CompSci stuff up until the point of pain and we were at a severe pain threshhold. Anyone wishing to read what is involved, check the SAS link above.

After playing with lots of things for several days, I settled on using John Hou's great article on CodeProject:

Transform Between IEEE, IBM, or VAX floating point

Fortunately, I had a license for Tangible Software  C++ to C# convertor. That did a lot of lift (80-90%) and left me to do final cleanup. Once I got that all down, next up was to get the endianness correct on the parms passing in and then suddenly I saw a correct value passed back.

Long weekend dealing with it but glad it is over.

The final C# code that creates the conversion is available on John's aoriginal article as an alternative and I will post it on Savian as well. I learned more about SEF, floats, 4/8 byte variances, and everythig else than I ever wanted to know.

However, tonight I sent my client a translated XPT file and it appears to be correct. And I now have a .NET dll for handling the issue. That made all of the difference in the world...

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