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*Subject*: **Re: [office-comment] office-formula: FFT functions?**

*From*:**robert_weir@us.ibm.com***To*: stevenj@math.mit.edu*Date*: Mon, 4 Feb 2008 14:18:06 -0500

Hi Steven, This is an interesting idea. In practice,do you think this makes sense as spreadsheet array functions which take an array and return an array? Or as an application comment that prompts the user to select an input and output range,along the lines of what Excel does with its Analysis Toolkit? My experience is that most users are not even aware of array functions in Excel and how they use. On the other hand, most users are not aware of FFT's either. If done as a spreadsheet function, the FFT would obviously need to be recalculated every time one of the cells in the input range changed. At O(N log N) this isn't so bad. Do you know if there are any ways to calculate an FFT with incremental changes? So I calculate once in O(N log N) and then change only some of the input cells, a small portion of them -- is there a much faster way of calculating the new results? Note also that complex number support in spreadsheets is rather poor in general, and only a handful of functions work with them. What functions would you recommend? Maybe FFT() and the inverse IFFT(). Each one might have a parameter that specifies the normalization convention. Maybe also a Power() function? I could imagine in the time domain CrossCorrelation()and AutoCorrelation() would be useful as well. These are interesting for fields beyond signal processing, including economics. Note however that it is always a debatable point on whether a particular feature request should be placed in the standard, or whether it is best done as an extension. I'd be more encouraged to add features like this to the ODF standard if we had one or more vendors committed to supporting this feature in their product. If it were purely up to me, I'd love to create the ultimate engineer's/scientist's spreadsheet application. There are a lot of things we could do, like adding units support, integration with R, etc. But we would need a set of volunteers dedicated to implementing this in OpenOffice or Gnumeric or someplace. Regards, -Rob ___________________________ Rob Weir Software Architect Workplace, Portal and Collaboration Software IBM Software Group email: robert_weir@us.ibm.com phone: 1-978-399-7122 blog: http://www.robweir.com/blog/ "Steven G. Johnson" <stevenj@fftw.org> 01/28/2008 05:58 PM Please respond to stevenj@math.mit.edu To office-comment@lists.oasis-open.org cc Subject [office-comment] office-formula: FFT functions? Hi, I was wondering if there was any interest in including a discrete Fourier transform function in OpenFormula (and, based on that, one can easily implement convolutions, correlations, autocorrelations, etcetera as desired). e.g. Excel has some FFT functionality via the data analysis toolpack, as I understand it, but I didn't see anything in the OpenFormula draft. I'm co-author of FFTW (www.fftw.org), a popular free-software FFT implementation (used e.g. in Matlab, GNU Octave, etc.), and I wanted to send you a note in case you wanted any help with this. I would be happy to provide any advice on specification, a reference implementation, etcetera that you might need. I'm not a spreadsheet expert, but I hope that my knowledge of FFTs and their applications would be helpful. We get a sizeable amount of email at fftw.org from people who just want to transform some data in a spreadsheet, and so I get the impression that that there is a fair demand for this kind of capability; also, if you google "excel fft" you will get a lot of pages. FFTs themselves are kind of hard for a user to implement in a spreadsheet language, but once you have them there are all sorts of things that can be done directly in the formula language (windowing, filtering, correlations, etcetera). Regards, Steven G. Johnson [stevenj at math dot mit dot edu] PS. If you do specify FFT functions, you don't want to make the same mistakes that Excel did, e.g. you don't want to artificially limit support to power-of-two sizes (a very inconvenient restriction for analysis of user-generated data). Given a power-of-two FFT, you can implement support for arbitrary sizes with a couple dozen more lines of code (via Bluestein's algorithm), so there's really no excuse not to support any size with O(n log n) complexity. This publicly archived list offers a means to provide input to the OASIS Open Document Format for Office Applications (OpenDocument) TC. In order to verify user consent to the Feedback License terms and to minimize spam in the list archive, subscription is required before posting. Subscribe: office-comment-subscribe@lists.oasis-open.org Unsubscribe: office-comment-unsubscribe@lists.oasis-open.org List help: office-comment-help@lists.oasis-open.org List archive: http://lists.oasis-open.org/archives/office-comment/ Feedback License: http://www.oasis-open.org/who/ipr/feedback_license.pdf List Guidelines: http://www.oasis-open.org/maillists/guidelines.php Committee: http://www.oasis-open.org/committees/tc_home.php?wg_abbrev=office

**Follow-Ups**:**Re: [office-comment] office-formula: FFT functions?***From:*"Steven G. Johnson" <stevenj@fftw.org>

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