>>>I am the clipper programmer and I want to try with visual foxpro now but I don't want to change all of my database files and also index files but I have some problem about the index files, can Visual Foxpro use the existing Clipper Index files ?
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>>dBase index files - yes - Clipper NTX files - no. You will need to rebuild them in VFP. If you have a program in clipper that uses the xBase INDEX ON command - you can use that code in VFP to rebuild the indexes into a CDX file.
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>Can he use NTX and CDX together? Access and update at the same time a database with both a Foxpro app and a Clipper app?

Yes and no. Setting a structural CDX would change the header thus may cause Clipper not to read it. Better leave your Clipper table as a free nonCDX table. Create a database and set a view for your table. You can index the view as you would a table. Set your keyfield and all fields as updatable. Use view in fox app and base Clipper table in Clipper app. This approach is what I do with FP 2.x tables that would be accessed by FP DOS and VFP (of course I don't do that for CDX but something like transaction support).
CDX has more advantages over NTX. They're compact (app. 1/6 of NTX), opened with table implicitly and updated as needed (no need explicit order). They use B-tree+ balanced and faster than NTX both at search and reindex time. Any tag could contain a key up to 240 chars if collating sequence is machine. And AFAIK tag count is unlimited (at least didn't hit a limit for so many years and no limit in documentation depicted).
Cetin