Compute means between time steps for given duration values in MATLAB

Question

For example, I have 5 time steps and a duration value:

TimeSteps = [4, 5, 6, 7, 8, 10, 11, 12, 14, 15, 16];
Duration = [5, 3, 3];

I want to automatically generate the means between the time steps for every duration. In this case, the result would look like this:

Result = [mean([4, 5, 6, 7, 8]), mean([10, 11, 12]), mean([14, 15, 16])];

This will lead to:

Result = [6, 11, 15];

I have a 474x1 duration vector, so I need to automatize this. If anyone can help, it would be great. Thank you.

score 5 · Accepted Answer · 2015-02-13T12:18:08.537

5

You need to apply the mean function to a partition of your TimeSteps that has the sizes in Duration:

Partition = mat2cell(TimeSteps, 1, Duration);
Result    = cellfun(@mean, Partition)

If you're a fan of one-liners:

Result = cellfun(@mean, mat2cell(TimeSteps, 1, Duration));

Note: Please take in account that this solution is posted for row vectors; change the mat2cell call for column vectors.

edited Feb 13 '15 at 12:18

answered Feb 13 '15 at 12:11

This is even better than the `accumarray`-one! – knedlsepp Feb 13 '15 at 12:26

Luis Mendo · Answer 2 · 2015-02-13T12:23:14.160

4

It can be done easily with accumarray. This works for row or column vectors.

ind = [];
ind(cumsum(Duration(end:-1:1))) = 1;
ind = cumsum(ind(end:-1:1)); %// generate grouping index for accumarray
Result = (accumarray(ind(:), TimeSteps(:))./Duration(:)).';

edited Feb 13 '15 at 12:23

answered Feb 13 '15 at 12:12

Luis Mendo

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Off-topic, but: have you seen [this](http://stackoverflow.com/q/28497857/2586922)? Can you make any sense out of it? – Luis Mendo Feb 13 '15 at 12:18
Hmm Let me see, I did have a cursory glance at that discussion, need to look into it in detail. – Divakar Feb 13 '15 at 12:19
Read the first lines of the question, got into `accumarray`-mode, scrolled down, my adrenaline levels are normal again. – knedlsepp Feb 13 '15 at 12:25
@knedlsepp What! Are you _ever_ out of `acumarray`-mode? – Luis Mendo Feb 13 '15 at 12:26
I guess that's the reason I was too late. ;-) – knedlsepp Feb 13 '15 at 12:27
@LuisMendo Well that other question doesn't make sense to me either. I think OP must have posted some sort of minimal code that could reproduce that error. – Divakar Feb 13 '15 at 12:33
@Divakar Thanks. The answer doesn't make any sense to me either (that imaginary part stuff). And it got accepted. Strange – Luis Mendo Feb 13 '15 at 12:36
Strangest thing in my one year experience on SO haha! :) – Divakar Feb 13 '15 at 12:37

Divakar · Answer 3 · 2015-02-13T12:27:24.890

3

Using bsxfun's masking capability -

%// Setup mask to be used to map Timesteps values onto a 2D matrix
mask = bsxfun(@ge,Duration(:).',[1:max(Duration)]') %//'

%// Initiliaze the 2D matrix where Timesteps values would be put with NaNs
vals = nan(size(mask))
%// Now, put those values
vals(mask) = TimeSteps

%// Find mean along columns for the final output
Result = nanmean(vals,1)

Output -

Result =
     6    11    15

edited Feb 13 '15 at 12:27

answered Feb 13 '15 at 12:06

Divakar

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Good approach, but may be memory-hungry – Luis Mendo Feb 13 '15 at 12:14
@LuisMendo Of course would depend on the Durations, I mean for performance. – Divakar Feb 13 '15 at 12:15

Compute means between time steps for given duration values in MATLAB

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