Partial Sums in Spreadsheets

[EulerRocks]

Hello all.   I am very excited to be a part of this user group as we are using the Nspires in our district heavily next year.   I had a quick question about partial sums in the spreadsheet page on the Nspire.

On another page, I have allowed the user to define the number of terms "n" in a sequence stored as a variable.  Then on a spreadsheet page, I have a sequence an generated recursively using the "SeqGen" command consisting of exactly "n" terms as previously defined.  In the next column I would like to find the partial sums of these terms, namely S1 = a1, S2 = a1 + a2, ... , Sn = a1 + a2 + ...  + an.

I could definitely do this iteratively by adding the new term to the previous partial sum, but then when the number of terms, n, changes, this will not work.  My question is how to does one construct a sequence of partial sums from a sequence whose number of terms may change as a variable?

Any help would be much appreciated.
Chris

[Jim Fullerenex]

I believe the correct function you should use is the built-in cumulative sum, seems to be cumultsum() (I don't have hh with me right now, so just pray my memory serves.) For its detail usage check up the catalogue. Hope can help. :)

2011/6/19 EulerRocks <bologne...@gmail.com>

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Cheers,

Jimmy Fullerenex

[EulerRocks]

Excellent.  Cumulativesum(list) did the trick!  Thanks.

[Nelson Sousa]

Interactively works, but it's a pain. Plus, as the list grows it only gets worse.

Here's an alternative: column A is stored as variable list; define column B as 

=seq(sum(left(list,i)),i,1,dim(list))

Warning: the complexity grows with n^2, so keep your lists small (don't go define list with 1000 entries)

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Nelson

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[Jim Fullerenex]

How's the performance of cumulativesum() compared to the user-defined alternative? Why create an additional function while built-in can solve the problem literally the same?  I believe built-ins should be more efficient in most cases.

Cheers,

Jimmy Fullerenex

2011/6/20 Nelson Sousa <nso...@gmail.com>

[Nelson Sousa]

because I didn't think of cumulativesum and didn't read your message
before posting mine. :)
Cumulative sum probably has the same complexity and is very likely to
be a similar algorithm.

Nelson