If you just want to put the tabular data in a spreadsheet, just generate a .CSV file that can be directly read in EXCEL.
If you want a full-featured spreadsheet with multiple sheets, formulas, formats, inserted objects, pictures, diagrams, etc., as far as I know, there is no solution for the iPhone, since Microsoft does not provide the equivalent of its ActiveX Excel component.
If your requirements lie between them, you can try some alternatives. Take a look at Google Docs or QuickOffice.
I do not recommend you try to generate full XLS from scratch if there is too much dificult.
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